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844 lines
30 KiB
844 lines
30 KiB
//===- WholeProgramDevirt.cpp - Whole program virtual call optimization ---===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This pass implements whole program optimization of virtual calls in cases
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// where we know (via !type metadata) that the list of callees is fixed. This
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// includes the following:
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// - Single implementation devirtualization: if a virtual call has a single
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// possible callee, replace all calls with a direct call to that callee.
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// - Virtual constant propagation: if the virtual function's return type is an
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// integer <=64 bits and all possible callees are readnone, for each class and
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// each list of constant arguments: evaluate the function, store the return
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// value alongside the virtual table, and rewrite each virtual call as a load
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// from the virtual table.
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// - Uniform return value optimization: if the conditions for virtual constant
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// propagation hold and each function returns the same constant value, replace
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// each virtual call with that constant.
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// - Unique return value optimization for i1 return values: if the conditions
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// for virtual constant propagation hold and a single vtable's function
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// returns 0, or a single vtable's function returns 1, replace each virtual
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// call with a comparison of the vptr against that vtable's address.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/IPO/WholeProgramDevirt.h"
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#include "llvm/ADT/ArrayRef.h"
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#include "llvm/ADT/DenseSet.h"
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#include "llvm/ADT/MapVector.h"
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#include "llvm/Analysis/TypeMetadataUtils.h"
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#include "llvm/IR/CallSite.h"
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#include "llvm/IR/Constants.h"
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#include "llvm/IR/DataLayout.h"
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#include "llvm/IR/DiagnosticInfo.h"
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#include "llvm/IR/IRBuilder.h"
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#include "llvm/IR/Instructions.h"
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#include "llvm/IR/Intrinsics.h"
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#include "llvm/IR/Module.h"
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#include "llvm/Pass.h"
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#include "llvm/Support/raw_ostream.h"
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#include "llvm/Transforms/IPO.h"
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#include "llvm/Transforms/Utils/Evaluator.h"
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#include "llvm/Transforms/Utils/Local.h"
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#include <set>
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using namespace llvm;
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using namespace wholeprogramdevirt;
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#define DEBUG_TYPE "wholeprogramdevirt"
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// Find the minimum offset that we may store a value of size Size bits at. If
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// IsAfter is set, look for an offset before the object, otherwise look for an
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// offset after the object.
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uint64_t
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wholeprogramdevirt::findLowestOffset(ArrayRef<VirtualCallTarget> Targets,
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bool IsAfter, uint64_t Size) {
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// Find a minimum offset taking into account only vtable sizes.
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uint64_t MinByte = 0;
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for (const VirtualCallTarget &Target : Targets) {
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if (IsAfter)
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MinByte = std::max(MinByte, Target.minAfterBytes());
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else
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MinByte = std::max(MinByte, Target.minBeforeBytes());
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}
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// Build a vector of arrays of bytes covering, for each target, a slice of the
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// used region (see AccumBitVector::BytesUsed in
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// llvm/Transforms/IPO/WholeProgramDevirt.h) starting at MinByte. Effectively,
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// this aligns the used regions to start at MinByte.
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//
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// In this example, A, B and C are vtables, # is a byte already allocated for
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// a virtual function pointer, AAAA... (etc.) are the used regions for the
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// vtables and Offset(X) is the value computed for the Offset variable below
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// for X.
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//
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// Offset(A)
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// | |
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// |MinByte
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// A: ################AAAAAAAA|AAAAAAAA
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// B: ########BBBBBBBBBBBBBBBB|BBBB
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// C: ########################|CCCCCCCCCCCCCCCC
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// | Offset(B) |
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//
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// This code produces the slices of A, B and C that appear after the divider
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// at MinByte.
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std::vector<ArrayRef<uint8_t>> Used;
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for (const VirtualCallTarget &Target : Targets) {
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ArrayRef<uint8_t> VTUsed = IsAfter ? Target.TM->Bits->After.BytesUsed
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: Target.TM->Bits->Before.BytesUsed;
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uint64_t Offset = IsAfter ? MinByte - Target.minAfterBytes()
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: MinByte - Target.minBeforeBytes();
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// Disregard used regions that are smaller than Offset. These are
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// effectively all-free regions that do not need to be checked.
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if (VTUsed.size() > Offset)
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Used.push_back(VTUsed.slice(Offset));
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}
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if (Size == 1) {
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// Find a free bit in each member of Used.
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for (unsigned I = 0;; ++I) {
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uint8_t BitsUsed = 0;
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for (auto &&B : Used)
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if (I < B.size())
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BitsUsed |= B[I];
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if (BitsUsed != 0xff)
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return (MinByte + I) * 8 +
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countTrailingZeros(uint8_t(~BitsUsed), ZB_Undefined);
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}
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} else {
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// Find a free (Size/8) byte region in each member of Used.
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// FIXME: see if alignment helps.
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for (unsigned I = 0;; ++I) {
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for (auto &&B : Used) {
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unsigned Byte = 0;
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while ((I + Byte) < B.size() && Byte < (Size / 8)) {
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if (B[I + Byte])
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goto NextI;
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++Byte;
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}
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}
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return (MinByte + I) * 8;
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NextI:;
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}
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}
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}
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void wholeprogramdevirt::setBeforeReturnValues(
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MutableArrayRef<VirtualCallTarget> Targets, uint64_t AllocBefore,
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unsigned BitWidth, int64_t &OffsetByte, uint64_t &OffsetBit) {
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if (BitWidth == 1)
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OffsetByte = -(AllocBefore / 8 + 1);
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else
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OffsetByte = -((AllocBefore + 7) / 8 + (BitWidth + 7) / 8);
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OffsetBit = AllocBefore % 8;
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for (VirtualCallTarget &Target : Targets) {
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if (BitWidth == 1)
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Target.setBeforeBit(AllocBefore);
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else
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Target.setBeforeBytes(AllocBefore, (BitWidth + 7) / 8);
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}
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}
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void wholeprogramdevirt::setAfterReturnValues(
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MutableArrayRef<VirtualCallTarget> Targets, uint64_t AllocAfter,
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unsigned BitWidth, int64_t &OffsetByte, uint64_t &OffsetBit) {
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if (BitWidth == 1)
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OffsetByte = AllocAfter / 8;
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else
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OffsetByte = (AllocAfter + 7) / 8;
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OffsetBit = AllocAfter % 8;
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for (VirtualCallTarget &Target : Targets) {
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if (BitWidth == 1)
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Target.setAfterBit(AllocAfter);
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else
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Target.setAfterBytes(AllocAfter, (BitWidth + 7) / 8);
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}
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}
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VirtualCallTarget::VirtualCallTarget(Function *Fn, const TypeMemberInfo *TM)
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: Fn(Fn), TM(TM),
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IsBigEndian(Fn->getParent()->getDataLayout().isBigEndian()) {}
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namespace {
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// A slot in a set of virtual tables. The TypeID identifies the set of virtual
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// tables, and the ByteOffset is the offset in bytes from the address point to
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// the virtual function pointer.
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struct VTableSlot {
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Metadata *TypeID;
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uint64_t ByteOffset;
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};
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}
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namespace llvm {
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template <> struct DenseMapInfo<VTableSlot> {
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static VTableSlot getEmptyKey() {
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return {DenseMapInfo<Metadata *>::getEmptyKey(),
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DenseMapInfo<uint64_t>::getEmptyKey()};
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}
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static VTableSlot getTombstoneKey() {
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return {DenseMapInfo<Metadata *>::getTombstoneKey(),
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DenseMapInfo<uint64_t>::getTombstoneKey()};
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}
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static unsigned getHashValue(const VTableSlot &I) {
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return DenseMapInfo<Metadata *>::getHashValue(I.TypeID) ^
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DenseMapInfo<uint64_t>::getHashValue(I.ByteOffset);
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}
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static bool isEqual(const VTableSlot &LHS,
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const VTableSlot &RHS) {
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return LHS.TypeID == RHS.TypeID && LHS.ByteOffset == RHS.ByteOffset;
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}
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};
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}
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namespace {
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// A virtual call site. VTable is the loaded virtual table pointer, and CS is
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// the indirect virtual call.
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struct VirtualCallSite {
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Value *VTable;
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CallSite CS;
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// If non-null, this field points to the associated unsafe use count stored in
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// the DevirtModule::NumUnsafeUsesForTypeTest map below. See the description
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// of that field for details.
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unsigned *NumUnsafeUses;
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void emitRemark() {
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Function *F = CS.getCaller();
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emitOptimizationRemark(F->getContext(), DEBUG_TYPE, *F,
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CS.getInstruction()->getDebugLoc(),
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"devirtualized call");
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}
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void replaceAndErase(Value *New) {
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emitRemark();
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CS->replaceAllUsesWith(New);
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if (auto II = dyn_cast<InvokeInst>(CS.getInstruction())) {
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BranchInst::Create(II->getNormalDest(), CS.getInstruction());
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II->getUnwindDest()->removePredecessor(II->getParent());
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}
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CS->eraseFromParent();
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// This use is no longer unsafe.
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if (NumUnsafeUses)
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--*NumUnsafeUses;
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}
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};
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struct DevirtModule {
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Module &M;
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IntegerType *Int8Ty;
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PointerType *Int8PtrTy;
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IntegerType *Int32Ty;
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MapVector<VTableSlot, std::vector<VirtualCallSite>> CallSlots;
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// This map keeps track of the number of "unsafe" uses of a loaded function
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// pointer. The key is the associated llvm.type.test intrinsic call generated
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// by this pass. An unsafe use is one that calls the loaded function pointer
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// directly. Every time we eliminate an unsafe use (for example, by
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// devirtualizing it or by applying virtual constant propagation), we
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// decrement the value stored in this map. If a value reaches zero, we can
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// eliminate the type check by RAUWing the associated llvm.type.test call with
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// true.
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std::map<CallInst *, unsigned> NumUnsafeUsesForTypeTest;
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DevirtModule(Module &M)
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: M(M), Int8Ty(Type::getInt8Ty(M.getContext())),
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Int8PtrTy(Type::getInt8PtrTy(M.getContext())),
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Int32Ty(Type::getInt32Ty(M.getContext())) {}
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void scanTypeTestUsers(Function *TypeTestFunc, Function *AssumeFunc);
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void scanTypeCheckedLoadUsers(Function *TypeCheckedLoadFunc);
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void buildTypeIdentifierMap(
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std::vector<VTableBits> &Bits,
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DenseMap<Metadata *, std::set<TypeMemberInfo>> &TypeIdMap);
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bool
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tryFindVirtualCallTargets(std::vector<VirtualCallTarget> &TargetsForSlot,
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const std::set<TypeMemberInfo> &TypeMemberInfos,
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uint64_t ByteOffset);
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bool trySingleImplDevirt(ArrayRef<VirtualCallTarget> TargetsForSlot,
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MutableArrayRef<VirtualCallSite> CallSites);
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bool tryEvaluateFunctionsWithArgs(
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MutableArrayRef<VirtualCallTarget> TargetsForSlot,
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ArrayRef<ConstantInt *> Args);
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bool tryUniformRetValOpt(IntegerType *RetType,
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ArrayRef<VirtualCallTarget> TargetsForSlot,
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MutableArrayRef<VirtualCallSite> CallSites);
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bool tryUniqueRetValOpt(unsigned BitWidth,
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ArrayRef<VirtualCallTarget> TargetsForSlot,
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MutableArrayRef<VirtualCallSite> CallSites);
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bool tryVirtualConstProp(MutableArrayRef<VirtualCallTarget> TargetsForSlot,
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ArrayRef<VirtualCallSite> CallSites);
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void rebuildGlobal(VTableBits &B);
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bool run();
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};
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struct WholeProgramDevirt : public ModulePass {
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static char ID;
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WholeProgramDevirt() : ModulePass(ID) {
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initializeWholeProgramDevirtPass(*PassRegistry::getPassRegistry());
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}
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bool runOnModule(Module &M) {
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if (skipModule(M))
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return false;
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return DevirtModule(M).run();
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}
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};
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} // anonymous namespace
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INITIALIZE_PASS(WholeProgramDevirt, "wholeprogramdevirt",
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"Whole program devirtualization", false, false)
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char WholeProgramDevirt::ID = 0;
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ModulePass *llvm::createWholeProgramDevirtPass() {
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return new WholeProgramDevirt;
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}
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PreservedAnalyses WholeProgramDevirtPass::run(Module &M,
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ModuleAnalysisManager &) {
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if (!DevirtModule(M).run())
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return PreservedAnalyses::all();
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return PreservedAnalyses::none();
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}
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void DevirtModule::buildTypeIdentifierMap(
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std::vector<VTableBits> &Bits,
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DenseMap<Metadata *, std::set<TypeMemberInfo>> &TypeIdMap) {
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DenseMap<GlobalVariable *, VTableBits *> GVToBits;
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Bits.reserve(M.getGlobalList().size());
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SmallVector<MDNode *, 2> Types;
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for (GlobalVariable &GV : M.globals()) {
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Types.clear();
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GV.getMetadata(LLVMContext::MD_type, Types);
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if (Types.empty())
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continue;
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VTableBits *&BitsPtr = GVToBits[&GV];
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if (!BitsPtr) {
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Bits.emplace_back();
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Bits.back().GV = &GV;
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Bits.back().ObjectSize =
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M.getDataLayout().getTypeAllocSize(GV.getInitializer()->getType());
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BitsPtr = &Bits.back();
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}
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for (MDNode *Type : Types) {
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auto TypeID = Type->getOperand(1).get();
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uint64_t Offset =
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cast<ConstantInt>(
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cast<ConstantAsMetadata>(Type->getOperand(0))->getValue())
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->getZExtValue();
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TypeIdMap[TypeID].insert({BitsPtr, Offset});
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}
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}
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}
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bool DevirtModule::tryFindVirtualCallTargets(
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std::vector<VirtualCallTarget> &TargetsForSlot,
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const std::set<TypeMemberInfo> &TypeMemberInfos, uint64_t ByteOffset) {
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for (const TypeMemberInfo &TM : TypeMemberInfos) {
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if (!TM.Bits->GV->isConstant())
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return false;
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auto Init = dyn_cast<ConstantArray>(TM.Bits->GV->getInitializer());
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if (!Init)
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return false;
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ArrayType *VTableTy = Init->getType();
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uint64_t ElemSize =
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M.getDataLayout().getTypeAllocSize(VTableTy->getElementType());
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uint64_t GlobalSlotOffset = TM.Offset + ByteOffset;
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if (GlobalSlotOffset % ElemSize != 0)
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return false;
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unsigned Op = GlobalSlotOffset / ElemSize;
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if (Op >= Init->getNumOperands())
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return false;
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auto Fn = dyn_cast<Function>(Init->getOperand(Op)->stripPointerCasts());
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if (!Fn)
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return false;
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// We can disregard __cxa_pure_virtual as a possible call target, as
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// calls to pure virtuals are UB.
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if (Fn->getName() == "__cxa_pure_virtual")
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continue;
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TargetsForSlot.push_back({Fn, &TM});
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}
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// Give up if we couldn't find any targets.
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return !TargetsForSlot.empty();
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}
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bool DevirtModule::trySingleImplDevirt(
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ArrayRef<VirtualCallTarget> TargetsForSlot,
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MutableArrayRef<VirtualCallSite> CallSites) {
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// See if the program contains a single implementation of this virtual
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// function.
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Function *TheFn = TargetsForSlot[0].Fn;
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for (auto &&Target : TargetsForSlot)
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if (TheFn != Target.Fn)
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return false;
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// If so, update each call site to call that implementation directly.
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for (auto &&VCallSite : CallSites) {
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VCallSite.emitRemark();
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VCallSite.CS.setCalledFunction(ConstantExpr::getBitCast(
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TheFn, VCallSite.CS.getCalledValue()->getType()));
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// This use is no longer unsafe.
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if (VCallSite.NumUnsafeUses)
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--*VCallSite.NumUnsafeUses;
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}
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return true;
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}
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bool DevirtModule::tryEvaluateFunctionsWithArgs(
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MutableArrayRef<VirtualCallTarget> TargetsForSlot,
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ArrayRef<ConstantInt *> Args) {
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// Evaluate each function and store the result in each target's RetVal
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// field.
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for (VirtualCallTarget &Target : TargetsForSlot) {
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if (Target.Fn->arg_size() != Args.size() + 1)
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return false;
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for (unsigned I = 0; I != Args.size(); ++I)
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if (Target.Fn->getFunctionType()->getParamType(I + 1) !=
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Args[I]->getType())
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return false;
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|
|
Evaluator Eval(M.getDataLayout(), nullptr);
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SmallVector<Constant *, 2> EvalArgs;
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EvalArgs.push_back(
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Constant::getNullValue(Target.Fn->getFunctionType()->getParamType(0)));
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EvalArgs.insert(EvalArgs.end(), Args.begin(), Args.end());
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Constant *RetVal;
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if (!Eval.EvaluateFunction(Target.Fn, RetVal, EvalArgs) ||
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!isa<ConstantInt>(RetVal))
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return false;
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Target.RetVal = cast<ConstantInt>(RetVal)->getZExtValue();
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}
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return true;
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}
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|
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bool DevirtModule::tryUniformRetValOpt(
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IntegerType *RetType, ArrayRef<VirtualCallTarget> TargetsForSlot,
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MutableArrayRef<VirtualCallSite> CallSites) {
|
|
// Uniform return value optimization. If all functions return the same
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|
// constant, replace all calls with that constant.
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uint64_t TheRetVal = TargetsForSlot[0].RetVal;
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for (const VirtualCallTarget &Target : TargetsForSlot)
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if (Target.RetVal != TheRetVal)
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return false;
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auto TheRetValConst = ConstantInt::get(RetType, TheRetVal);
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for (auto Call : CallSites)
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Call.replaceAndErase(TheRetValConst);
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return true;
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}
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bool DevirtModule::tryUniqueRetValOpt(
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unsigned BitWidth, ArrayRef<VirtualCallTarget> TargetsForSlot,
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MutableArrayRef<VirtualCallSite> CallSites) {
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// IsOne controls whether we look for a 0 or a 1.
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|
auto tryUniqueRetValOptFor = [&](bool IsOne) {
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const TypeMemberInfo *UniqueMember = 0;
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for (const VirtualCallTarget &Target : TargetsForSlot) {
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if (Target.RetVal == (IsOne ? 1 : 0)) {
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if (UniqueMember)
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return false;
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UniqueMember = Target.TM;
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}
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}
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|
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// We should have found a unique member or bailed out by now. We already
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// checked for a uniform return value in tryUniformRetValOpt.
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assert(UniqueMember);
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|
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// Replace each call with the comparison.
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for (auto &&Call : CallSites) {
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IRBuilder<> B(Call.CS.getInstruction());
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Value *OneAddr = B.CreateBitCast(UniqueMember->Bits->GV, Int8PtrTy);
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OneAddr = B.CreateConstGEP1_64(OneAddr, UniqueMember->Offset);
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Value *Cmp = B.CreateICmp(IsOne ? ICmpInst::ICMP_EQ : ICmpInst::ICMP_NE,
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Call.VTable, OneAddr);
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Call.replaceAndErase(Cmp);
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|
}
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return true;
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};
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|
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if (BitWidth == 1) {
|
|
if (tryUniqueRetValOptFor(true))
|
|
return true;
|
|
if (tryUniqueRetValOptFor(false))
|
|
return true;
|
|
}
|
|
return false;
|
|
}
|
|
|
|
bool DevirtModule::tryVirtualConstProp(
|
|
MutableArrayRef<VirtualCallTarget> TargetsForSlot,
|
|
ArrayRef<VirtualCallSite> CallSites) {
|
|
// This only works if the function returns an integer.
|
|
auto RetType = dyn_cast<IntegerType>(TargetsForSlot[0].Fn->getReturnType());
|
|
if (!RetType)
|
|
return false;
|
|
unsigned BitWidth = RetType->getBitWidth();
|
|
if (BitWidth > 64)
|
|
return false;
|
|
|
|
// Make sure that each function does not access memory, takes at least one
|
|
// argument, does not use its first argument (which we assume is 'this'),
|
|
// and has the same return type.
|
|
for (VirtualCallTarget &Target : TargetsForSlot) {
|
|
if (!Target.Fn->doesNotAccessMemory() || Target.Fn->arg_empty() ||
|
|
!Target.Fn->arg_begin()->use_empty() ||
|
|
Target.Fn->getReturnType() != RetType)
|
|
return false;
|
|
}
|
|
|
|
// Group call sites by the list of constant arguments they pass.
|
|
// The comparator ensures deterministic ordering.
|
|
struct ByAPIntValue {
|
|
bool operator()(const std::vector<ConstantInt *> &A,
|
|
const std::vector<ConstantInt *> &B) const {
|
|
return std::lexicographical_compare(
|
|
A.begin(), A.end(), B.begin(), B.end(),
|
|
[](ConstantInt *AI, ConstantInt *BI) {
|
|
return AI->getValue().ult(BI->getValue());
|
|
});
|
|
}
|
|
};
|
|
std::map<std::vector<ConstantInt *>, std::vector<VirtualCallSite>,
|
|
ByAPIntValue>
|
|
VCallSitesByConstantArg;
|
|
for (auto &&VCallSite : CallSites) {
|
|
std::vector<ConstantInt *> Args;
|
|
if (VCallSite.CS.getType() != RetType)
|
|
continue;
|
|
for (auto &&Arg :
|
|
make_range(VCallSite.CS.arg_begin() + 1, VCallSite.CS.arg_end())) {
|
|
if (!isa<ConstantInt>(Arg))
|
|
break;
|
|
Args.push_back(cast<ConstantInt>(&Arg));
|
|
}
|
|
if (Args.size() + 1 != VCallSite.CS.arg_size())
|
|
continue;
|
|
|
|
VCallSitesByConstantArg[Args].push_back(VCallSite);
|
|
}
|
|
|
|
for (auto &&CSByConstantArg : VCallSitesByConstantArg) {
|
|
if (!tryEvaluateFunctionsWithArgs(TargetsForSlot, CSByConstantArg.first))
|
|
continue;
|
|
|
|
if (tryUniformRetValOpt(RetType, TargetsForSlot, CSByConstantArg.second))
|
|
continue;
|
|
|
|
if (tryUniqueRetValOpt(BitWidth, TargetsForSlot, CSByConstantArg.second))
|
|
continue;
|
|
|
|
// Find an allocation offset in bits in all vtables associated with the
|
|
// type.
|
|
uint64_t AllocBefore =
|
|
findLowestOffset(TargetsForSlot, /*IsAfter=*/false, BitWidth);
|
|
uint64_t AllocAfter =
|
|
findLowestOffset(TargetsForSlot, /*IsAfter=*/true, BitWidth);
|
|
|
|
// Calculate the total amount of padding needed to store a value at both
|
|
// ends of the object.
|
|
uint64_t TotalPaddingBefore = 0, TotalPaddingAfter = 0;
|
|
for (auto &&Target : TargetsForSlot) {
|
|
TotalPaddingBefore += std::max<int64_t>(
|
|
(AllocBefore + 7) / 8 - Target.allocatedBeforeBytes() - 1, 0);
|
|
TotalPaddingAfter += std::max<int64_t>(
|
|
(AllocAfter + 7) / 8 - Target.allocatedAfterBytes() - 1, 0);
|
|
}
|
|
|
|
// If the amount of padding is too large, give up.
|
|
// FIXME: do something smarter here.
|
|
if (std::min(TotalPaddingBefore, TotalPaddingAfter) > 128)
|
|
continue;
|
|
|
|
// Calculate the offset to the value as a (possibly negative) byte offset
|
|
// and (if applicable) a bit offset, and store the values in the targets.
|
|
int64_t OffsetByte;
|
|
uint64_t OffsetBit;
|
|
if (TotalPaddingBefore <= TotalPaddingAfter)
|
|
setBeforeReturnValues(TargetsForSlot, AllocBefore, BitWidth, OffsetByte,
|
|
OffsetBit);
|
|
else
|
|
setAfterReturnValues(TargetsForSlot, AllocAfter, BitWidth, OffsetByte,
|
|
OffsetBit);
|
|
|
|
// Rewrite each call to a load from OffsetByte/OffsetBit.
|
|
for (auto Call : CSByConstantArg.second) {
|
|
IRBuilder<> B(Call.CS.getInstruction());
|
|
Value *Addr = B.CreateConstGEP1_64(Call.VTable, OffsetByte);
|
|
if (BitWidth == 1) {
|
|
Value *Bits = B.CreateLoad(Addr);
|
|
Value *Bit = ConstantInt::get(Int8Ty, 1ULL << OffsetBit);
|
|
Value *BitsAndBit = B.CreateAnd(Bits, Bit);
|
|
auto IsBitSet = B.CreateICmpNE(BitsAndBit, ConstantInt::get(Int8Ty, 0));
|
|
Call.replaceAndErase(IsBitSet);
|
|
} else {
|
|
Value *ValAddr = B.CreateBitCast(Addr, RetType->getPointerTo());
|
|
Value *Val = B.CreateLoad(RetType, ValAddr);
|
|
Call.replaceAndErase(Val);
|
|
}
|
|
}
|
|
}
|
|
return true;
|
|
}
|
|
|
|
void DevirtModule::rebuildGlobal(VTableBits &B) {
|
|
if (B.Before.Bytes.empty() && B.After.Bytes.empty())
|
|
return;
|
|
|
|
// Align each byte array to pointer width.
|
|
unsigned PointerSize = M.getDataLayout().getPointerSize();
|
|
B.Before.Bytes.resize(alignTo(B.Before.Bytes.size(), PointerSize));
|
|
B.After.Bytes.resize(alignTo(B.After.Bytes.size(), PointerSize));
|
|
|
|
// Before was stored in reverse order; flip it now.
|
|
for (size_t I = 0, Size = B.Before.Bytes.size(); I != Size / 2; ++I)
|
|
std::swap(B.Before.Bytes[I], B.Before.Bytes[Size - 1 - I]);
|
|
|
|
// Build an anonymous global containing the before bytes, followed by the
|
|
// original initializer, followed by the after bytes.
|
|
auto NewInit = ConstantStruct::getAnon(
|
|
{ConstantDataArray::get(M.getContext(), B.Before.Bytes),
|
|
B.GV->getInitializer(),
|
|
ConstantDataArray::get(M.getContext(), B.After.Bytes)});
|
|
auto NewGV =
|
|
new GlobalVariable(M, NewInit->getType(), B.GV->isConstant(),
|
|
GlobalVariable::PrivateLinkage, NewInit, "", B.GV);
|
|
NewGV->setSection(B.GV->getSection());
|
|
NewGV->setComdat(B.GV->getComdat());
|
|
|
|
// Copy the original vtable's metadata to the anonymous global, adjusting
|
|
// offsets as required.
|
|
NewGV->copyMetadata(B.GV, B.Before.Bytes.size());
|
|
|
|
// Build an alias named after the original global, pointing at the second
|
|
// element (the original initializer).
|
|
auto Alias = GlobalAlias::create(
|
|
B.GV->getInitializer()->getType(), 0, B.GV->getLinkage(), "",
|
|
ConstantExpr::getGetElementPtr(
|
|
NewInit->getType(), NewGV,
|
|
ArrayRef<Constant *>{ConstantInt::get(Int32Ty, 0),
|
|
ConstantInt::get(Int32Ty, 1)}),
|
|
&M);
|
|
Alias->setVisibility(B.GV->getVisibility());
|
|
Alias->takeName(B.GV);
|
|
|
|
B.GV->replaceAllUsesWith(Alias);
|
|
B.GV->eraseFromParent();
|
|
}
|
|
|
|
void DevirtModule::scanTypeTestUsers(Function *TypeTestFunc,
|
|
Function *AssumeFunc) {
|
|
// Find all virtual calls via a virtual table pointer %p under an assumption
|
|
// of the form llvm.assume(llvm.type.test(%p, %md)). This indicates that %p
|
|
// points to a member of the type identifier %md. Group calls by (type ID,
|
|
// offset) pair (effectively the identity of the virtual function) and store
|
|
// to CallSlots.
|
|
DenseSet<Value *> SeenPtrs;
|
|
for (auto I = TypeTestFunc->use_begin(), E = TypeTestFunc->use_end();
|
|
I != E;) {
|
|
auto CI = dyn_cast<CallInst>(I->getUser());
|
|
++I;
|
|
if (!CI)
|
|
continue;
|
|
|
|
// Search for virtual calls based on %p and add them to DevirtCalls.
|
|
SmallVector<DevirtCallSite, 1> DevirtCalls;
|
|
SmallVector<CallInst *, 1> Assumes;
|
|
findDevirtualizableCallsForTypeTest(DevirtCalls, Assumes, CI);
|
|
|
|
// If we found any, add them to CallSlots. Only do this if we haven't seen
|
|
// the vtable pointer before, as it may have been CSE'd with pointers from
|
|
// other call sites, and we don't want to process call sites multiple times.
|
|
if (!Assumes.empty()) {
|
|
Metadata *TypeId =
|
|
cast<MetadataAsValue>(CI->getArgOperand(1))->getMetadata();
|
|
Value *Ptr = CI->getArgOperand(0)->stripPointerCasts();
|
|
if (SeenPtrs.insert(Ptr).second) {
|
|
for (DevirtCallSite Call : DevirtCalls) {
|
|
CallSlots[{TypeId, Call.Offset}].push_back(
|
|
{CI->getArgOperand(0), Call.CS, nullptr});
|
|
}
|
|
}
|
|
}
|
|
|
|
// We no longer need the assumes or the type test.
|
|
for (auto Assume : Assumes)
|
|
Assume->eraseFromParent();
|
|
// We can't use RecursivelyDeleteTriviallyDeadInstructions here because we
|
|
// may use the vtable argument later.
|
|
if (CI->use_empty())
|
|
CI->eraseFromParent();
|
|
}
|
|
}
|
|
|
|
void DevirtModule::scanTypeCheckedLoadUsers(Function *TypeCheckedLoadFunc) {
|
|
Function *TypeTestFunc = Intrinsic::getDeclaration(&M, Intrinsic::type_test);
|
|
|
|
for (auto I = TypeCheckedLoadFunc->use_begin(),
|
|
E = TypeCheckedLoadFunc->use_end();
|
|
I != E;) {
|
|
auto CI = dyn_cast<CallInst>(I->getUser());
|
|
++I;
|
|
if (!CI)
|
|
continue;
|
|
|
|
Value *Ptr = CI->getArgOperand(0);
|
|
Value *Offset = CI->getArgOperand(1);
|
|
Value *TypeIdValue = CI->getArgOperand(2);
|
|
Metadata *TypeId = cast<MetadataAsValue>(TypeIdValue)->getMetadata();
|
|
|
|
SmallVector<DevirtCallSite, 1> DevirtCalls;
|
|
SmallVector<Instruction *, 1> LoadedPtrs;
|
|
SmallVector<Instruction *, 1> Preds;
|
|
bool HasNonCallUses = false;
|
|
findDevirtualizableCallsForTypeCheckedLoad(DevirtCalls, LoadedPtrs, Preds,
|
|
HasNonCallUses, CI);
|
|
|
|
// Start by generating "pessimistic" code that explicitly loads the function
|
|
// pointer from the vtable and performs the type check. If possible, we will
|
|
// eliminate the load and the type check later.
|
|
|
|
// If possible, only generate the load at the point where it is used.
|
|
// This helps avoid unnecessary spills.
|
|
IRBuilder<> LoadB(
|
|
(LoadedPtrs.size() == 1 && !HasNonCallUses) ? LoadedPtrs[0] : CI);
|
|
Value *GEP = LoadB.CreateGEP(Int8Ty, Ptr, Offset);
|
|
Value *GEPPtr = LoadB.CreateBitCast(GEP, PointerType::getUnqual(Int8PtrTy));
|
|
Value *LoadedValue = LoadB.CreateLoad(Int8PtrTy, GEPPtr);
|
|
|
|
for (Instruction *LoadedPtr : LoadedPtrs) {
|
|
LoadedPtr->replaceAllUsesWith(LoadedValue);
|
|
LoadedPtr->eraseFromParent();
|
|
}
|
|
|
|
// Likewise for the type test.
|
|
IRBuilder<> CallB((Preds.size() == 1 && !HasNonCallUses) ? Preds[0] : CI);
|
|
CallInst *TypeTestCall = CallB.CreateCall(TypeTestFunc, {Ptr, TypeIdValue});
|
|
|
|
for (Instruction *Pred : Preds) {
|
|
Pred->replaceAllUsesWith(TypeTestCall);
|
|
Pred->eraseFromParent();
|
|
}
|
|
|
|
// We have already erased any extractvalue instructions that refer to the
|
|
// intrinsic call, but the intrinsic may have other non-extractvalue uses
|
|
// (although this is unlikely). In that case, explicitly build a pair and
|
|
// RAUW it.
|
|
if (!CI->use_empty()) {
|
|
Value *Pair = UndefValue::get(CI->getType());
|
|
IRBuilder<> B(CI);
|
|
Pair = B.CreateInsertValue(Pair, LoadedValue, {0});
|
|
Pair = B.CreateInsertValue(Pair, TypeTestCall, {1});
|
|
CI->replaceAllUsesWith(Pair);
|
|
}
|
|
|
|
// The number of unsafe uses is initially the number of uses.
|
|
auto &NumUnsafeUses = NumUnsafeUsesForTypeTest[TypeTestCall];
|
|
NumUnsafeUses = DevirtCalls.size();
|
|
|
|
// If the function pointer has a non-call user, we cannot eliminate the type
|
|
// check, as one of those users may eventually call the pointer. Increment
|
|
// the unsafe use count to make sure it cannot reach zero.
|
|
if (HasNonCallUses)
|
|
++NumUnsafeUses;
|
|
for (DevirtCallSite Call : DevirtCalls) {
|
|
CallSlots[{TypeId, Call.Offset}].push_back(
|
|
{Ptr, Call.CS, &NumUnsafeUses});
|
|
}
|
|
|
|
CI->eraseFromParent();
|
|
}
|
|
}
|
|
|
|
bool DevirtModule::run() {
|
|
Function *TypeTestFunc =
|
|
M.getFunction(Intrinsic::getName(Intrinsic::type_test));
|
|
Function *TypeCheckedLoadFunc =
|
|
M.getFunction(Intrinsic::getName(Intrinsic::type_checked_load));
|
|
Function *AssumeFunc = M.getFunction(Intrinsic::getName(Intrinsic::assume));
|
|
|
|
if ((!TypeTestFunc || TypeTestFunc->use_empty() || !AssumeFunc ||
|
|
AssumeFunc->use_empty()) &&
|
|
(!TypeCheckedLoadFunc || TypeCheckedLoadFunc->use_empty()))
|
|
return false;
|
|
|
|
if (TypeTestFunc && AssumeFunc)
|
|
scanTypeTestUsers(TypeTestFunc, AssumeFunc);
|
|
|
|
if (TypeCheckedLoadFunc)
|
|
scanTypeCheckedLoadUsers(TypeCheckedLoadFunc);
|
|
|
|
// Rebuild type metadata into a map for easy lookup.
|
|
std::vector<VTableBits> Bits;
|
|
DenseMap<Metadata *, std::set<TypeMemberInfo>> TypeIdMap;
|
|
buildTypeIdentifierMap(Bits, TypeIdMap);
|
|
if (TypeIdMap.empty())
|
|
return true;
|
|
|
|
// For each (type, offset) pair:
|
|
bool DidVirtualConstProp = false;
|
|
for (auto &S : CallSlots) {
|
|
// Search each of the members of the type identifier for the virtual
|
|
// function implementation at offset S.first.ByteOffset, and add to
|
|
// TargetsForSlot.
|
|
std::vector<VirtualCallTarget> TargetsForSlot;
|
|
if (!tryFindVirtualCallTargets(TargetsForSlot, TypeIdMap[S.first.TypeID],
|
|
S.first.ByteOffset))
|
|
continue;
|
|
|
|
if (trySingleImplDevirt(TargetsForSlot, S.second))
|
|
continue;
|
|
|
|
DidVirtualConstProp |= tryVirtualConstProp(TargetsForSlot, S.second);
|
|
}
|
|
|
|
// If we were able to eliminate all unsafe uses for a type checked load,
|
|
// eliminate the type test by replacing it with true.
|
|
if (TypeCheckedLoadFunc) {
|
|
auto True = ConstantInt::getTrue(M.getContext());
|
|
for (auto &&U : NumUnsafeUsesForTypeTest) {
|
|
if (U.second == 0) {
|
|
U.first->replaceAllUsesWith(True);
|
|
U.first->eraseFromParent();
|
|
}
|
|
}
|
|
}
|
|
|
|
// Rebuild each global we touched as part of virtual constant propagation to
|
|
// include the before and after bytes.
|
|
if (DidVirtualConstProp)
|
|
for (VTableBits &B : Bits)
|
|
rebuildGlobal(B);
|
|
|
|
return true;
|
|
}
|