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Reactor Debug Info Generation

Introduction

Reactor produces Just In Time compiled dynamic executable code and can be used to JIT high performance functions specialized for runtime configurations, or to even build a compiler.

In order to debug executable code at a higher level than disassembly, source code files are required.

Reactor has two potential sources of source code:

The C++ source code of the program that calls into Reactor.
External source files read by the program and passed to Reactor.

While case (2) is preferable for implementing a compiler, this is currently not implemented.

Reactor implements case (1) and this can be used by GDB to single line step and inspect variables.

Supported Platforms

Currently:

Debug info generation is only supported on Linux with the LLVM 7 backend.
GDB is the only supported debugger.
The program must be compiled with debug info iteself.

Enabling

Debug generation is enabled with REACTOR_EMIT_DEBUG_INFO CMake flag (defaults to disabled).

Implementation details

Source Location

All Reactor functions begin with a call to RR_DEBUG_INFO_UPDATE_LOC(), which calls into rr::DebugInfo::EmitLocation().

rr::DebugInfo::EmitLocation() calls rr::DebugInfo::getCallerBacktrace(), which in turn uses libbacktrace to unwind the stack and find the file, function and line of the caller.

This information is passed to llvm::IRBuilder<>::SetCurrentDebugLocation to emit source line information for the next LLVM instructions to be built.

Variables

There are 3 aspects to generating variable debug information:

1. Variable names

Constructing a Reactor LValue:

rr::Int a = 1;

Will emit an LLVM alloca instruction to allocate the storage of the variable, and emit another to initialize it to the constant 1. While fluent, none of the Reactor calls see the name of the C++ local variable "a", and the LLVM alloca value gets a meaningless numerical value.

There are two potential ways that Reactor can obtain the variable name:

Use the running executable's own debug information to examine the local declaration and extract the local variable's name.
Use the backtrace information to parse the name from the source file.

While (1) is arguably a cleaner and more robust solution, (2) is easier to implement and can work for the majority of use cases.

(2) is the current solution implemented.

rr::DebugInfo::getOrParseFileTokens() scans a source file line by line, and uses a regular expression to look for patterns of <type> <name>. Matching is not precise, but is adequate to find locals constructed with and without assignment.

2. Variable binding

Given that we can find a variable name for a given source line, we need a way of binding the LLVM values to the name.

Given our trivial example:

rr::Int a = 1

The rr::Int constructor calls RR_DEBUG_INFO_EMIT_VAR() passing the storage value as single argument. RR_DEBUG_INFO_EMIT_VAR() performs the backtrace to find the source file and line and uses the token information produced by rr::DebugInfo::getOrParseFileTokens() to identify the variable name.

However, things get a bit more complicated when there are multiple variables being constructed on the same line.

Take for example:

rr::Int a = rr::Int(1) + rr::Int(2)

Here we have 3 calls to the rr::Int constructor, each calling down to RR_DEBUG_INFO_EMIT_VAR().

To disambiguate which of these should be bound to the variable name "a", rr::DebugInfo::EmitVariable() buffers the binding into scope.pending and the last binding for a given line is used by DebugInfo::emitPending(). For variable construction and assignment, C++ guarantees that the LHS is the last value to be constructed.

This solution is not perfect.

Multi-line expressions, multiple assignments on a single line, macro obfuscation can all break variable bindings - however the majority of typical cases work.

3. Variable scope

rr::DebugInfo maintains a stack of llvm::DIScopes and llvm::DILocations that mirrors the current backtrace for function being called.

A synthetic call stack is produced by chaining llvm::DILocations with InlinedAts.

For example, at the declaration of i:

void B()
{
    rr::Int i; // <- here
}

void A()
{
    B();
}

int main(int argc, const char* argv[])
{
    A();
}

The DIScope hierarchy would be:

                              DIFile: "foo.cpp"
rr::DebugInfo::diScope[0].di: ↳ DISubprogram: "main"
rr::DebugInfo::diScope[1].di: ↳ DISubprogram: "A"
rr::DebugInfo::diScope[2].di: ↳ DISubprogram: "B"

The DILocation hierarchy would be:

rr::DebugInfo::diRootLocation:      DILocation(DISubprogram: "ReactorFunction")
rr::DebugInfo::diScope[0].location: ↳ DILocation(DISubprogram: "main")
rr::DebugInfo::diScope[1].location:   ↳ DILocation(DISubprogram: "A")
rr::DebugInfo::diScope[2].location:     ↳ DILocation(DISubprogram: "B")

Where '↳' represents an InlinedAt.

rr::DebugInfo::diScope is updated by rr::DebugInfo::syncScope().

llvm::DIScopes typically do not nest - there is usually a separate llvm::DISubprogram for each function in the callstack. All local variables within a function will typically share the same scope, regardless of whether they are declared within a sub-block.

Loops and jumps within a function add complexity. Consider:

void B()
{
    rr::Int i = 0;
}

void A()
{
    for (int i = 0; i < 3; i++)
    {
        rr::Int x = 0;
    }
    B();
}

int main(int argc, const char* argv[])
{
    A();
}

In this particular example Reactor will not be aware of the for loop, and will attempt to create three variables called "x" in the same function scope for A(). Duplicate symbols in the same llvm::DIScope result in undefined behavior.

To solve this, rr::DebugInfo::syncScope() observes when a function jumps backwards, and forks the current llvm::DILexicalBlock for the function. This results in a number of llvm::DILexicalBlock chains, each declaring variables that shadow the previous block.

At the declaration of i, the DIScope hierarchy would be:

                              DIFile: "foo.cpp"
rr::DebugInfo::diScope[0].di: ↳ DISubprogram: "main"
                              ↳ DISubprogram: "A"
                              | ↳ DILexicalBlock: "A".1
rr::DebugInfo::diScope[1].di: |   ↳ DILexicalBlock: "A".2
rr::DebugInfo::diScope[2].di: ↳ DISubprogram: "B"