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Getting Started with VIXL for AArch32

This guide will show you how to use the VIXL framework for AArch32. We will see how to set up the VIXL assembler and generate some code. We will also go into details on a few useful features provided by VIXL and see how to run the generated code.

The source code of the example developed in this guide can be found in the examples/aarch32 directory (examples/aarch32/getting-started.cc).

Creating the macro assembler.

First of all you need to make sure that the header files for the assembler are included. You should have the following lines at the beginning of your source file:

// You may use <cstdint> if using C++11 or later.
extern "C" {
#include <stdint.h>
}

#include <cstdio>
#include <string>
#include "aarch32/constants-aarch32.h"
#include "aarch32/instructions-aarch32.h"
#include "aarch32/macro-assembler-aarch32.h"

In our case, those files are included by "examples.h".

All VIXL components are declared in the vixl::aarch32 namespace, so let's add this to the beginning of the file for convenience (once again, done in "examples.h"):

using namespace vixl::aarch32;

Now we are ready to create and initialise the different components.

First of all we need to create a macro assembler object.

MacroAssembler masm;

Generating some code.

We are now ready to generate some code. The macro assembler provides methods for all the instructions that you can use. As it's a macro assembler, the instructions that you tell it to generate may not directly map to a single hardware instruction. Instead, it can produce a short sequence of instructions that has the same effect.

Before looking at how to generate some code, let's introduce a simple but handy macro:

#define __ masm->

It allows us to write __ Mov(r0, 42); instead of masm->Mov(r0, 42); to generate code.

Now we are going to write a C++ function to generate our first assembly code fragment.

void GenerateDemo(MacroAssembler *masm) {
  __ Ldr(r1, 0x12345678);
  __ And(r0, r0, r1);
  __ Bx(lr);
}

The generated code corresponds to a function with the following C prototype:

uint32_t demo(uint32_t x);

This function doesn't perform any useful operation. It loads the value 0x12345678 into r1 and performs a bitwise and operation with the function's argument (stored in r0). The result of this and operation is returned by the function in r0.

Now in our program main function, we only need to create a label to represent the entry point of the assembly function and to call GenerateDemo to generate the code.

Label demo;
masm.Bind(&demo);
GenerateDemo(&masm);
masm.Finalize();

Now we are going to learn a bit more on a couple of interesting VIXL features which are used in this example.

Label

VIXL's assembler provides a mechanism to represent labels with Label objects. They are easy to use: simply create the C++ object and bind it to a location in the generated instruction stream.

Creating a label is easy, since you only need to define the variable and bind it to a location using the macro assembler.

Label my_label;      // Create the label object.
__ Bind(&my_label);  // Bind it to the current location.

The target of a branch using a label will be the address to which it has been bound. For example, let's consider the following code fragment:

Label foo;

__ B(&foo);     // Branch to foo.
__ Mov(r0, 42);
__ Bind(&foo);  // Actual address of foo is here.
__ Mov(r1, 0xc001);

If we run this code fragment the Mov(r0, 42) will never be executed since the first thing this code does is to jump to foo, which correspond to the Mov(r1, 0xc001) instruction.

When working with labels you need to know that they are only to be used for local branches, and should be passed around with care. The major reason is that they cannot safely be passed or returned by value because this can trigger multiple constructor and destructor calls. The destructor has assertions to check that we don't try to branch to a label that hasn't been bound.

Literal Pool

On AArch32 instructions are 16 or 32 bits long, thus immediate values encoded in the instructions have limited size. If you want to load a constant bigger than this limit you have two possibilities:

Use multiple instructions to load the constant in multiple steps. This solution is already handled in VIXL. For instance you can write:

__ Mov(r0, 0x12345678);

The previous instruction would not be legal since the immediate value is too big. However, VIXL's macro assembler will automatically rewrite this line into multiple instructions efficiently generate the value, ultimately setting 'r0' with the correct value.

Store the constant in memory and load this value from the memory. The value needs to be written near the code that will load it since we use a PC-relative offset to indicate the address of this value. This solution has the advantage of making the value easily modifiable at run-time; since it does not reside in the instruction stream, it doesn't require cache maintenance when updated.

VIXL also provides a way to do this:

__ Ldr(r0, 0x12345678);

The assembler will store the immediate value in a "literal pool", a set of constants embedded in the code. VIXL will emit the literal pool when needed.

The literal pool is emitted regularly, such that they are within range of the instructions that refer to it. However, you can force the literal pool to be emitted using masm.EmitLiteralPool(). It generates a branch to skip the pool.

Running the code.

We first need to run a few operations to get executable code. The ExecutableMemory helper takes care of it:

byte* code = masm.GetBuffer().GetBuffer();
uint32_t code_size = masm.GetBuffer().GetSizeInBytes();
ExecutableMemory memory(code, code_size);

Then we compute a pointer to the function we just generated and copy:

uint32_t (*demo_function)(uint32_t) =
    memory.GetOffsetAddress<uint32_t (*)(uint32_t)>(0);

Now, we can call this function pointer exactly as if it were a pointer on a C function:

uint32_t input_value = 0x89abcdef;
uint32_t output_value = (*demo_function)(input_value);

A little trace:

printf("native: abs(%08x) = %08x\n", input_value, output_value);

The example shown in this tutorial is very simple, because the goal was to demonstrate the basics of the VIXL framework. There are more complex code examples in the VIXL examples/aarch32 directory showing more features of both the macro assembler and the AArch32 architecture.

Disassembling the generated code.

Once you have generated something with the macro-assembler, you may want to disassemble it.

First, you must include iostream.

#include <iostream>

And the disassembler header file:

#include "aarch32/disasm-aarch32.h"

Then you have to define the pc used to disassemble (the one which is used to display the addresses not the location of the instructions):

uint32_t display_pc = 0x1000;

Or, if you running on a 32 bit host, you can use the real address:

uint32_t display_pc = static_cast<uintptr_t>(masm.GetBuffer().GetBuffer());

Then you can disassemble the macro assembler's buffer:

PrintDisassembler disasm(std::cout, display_pc);
disasm.DisassembleA32Buffer(
    masm.GetBuffer().GetOffsetAddress<uint32_t*>(0), masm.GetCursorOffset());

If you generated T32 code instead of A32 code, you must use DisassembleT32Buffer. Warning: if your buffer contains some data or contains mixed T32 and A32 code, the result won't be accurate (everything will be disassembled as T32 or A32 code).

Example of disassembly:

0x00001000  e30f0fff	mov r0, #65535
0x00001004  e34f0fff	movt r0, #65535
0x00001008  e3041567	mov r1, #17767
0x0000100c  e3401123	movt r1, #291
0x00001010  e3a02000	mov r2, #0
0x00001014  e7c2001f	bfc r0, #0, #3
0x00001018  e7d4081f	bfc r0, #16, #5
0x0000101c  e7c72011	bfi r2, r1, #0, #8
0x00001020  e7df2811	bfi r2, r1, #16, #16
0x00001024  e1000070	hlt 0

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