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Getting Started with VIXL AArch64

This guide will show you how to use the VIXL framework for AArch64. 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 in the VIXL simulator.

The source code of the example developed in this guide can be found in the examples/aarch64 directory (examples/aarch64/getting-started.cc).

Creating the macro assembler and the simulator.

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

#include "aarch64/simulator-aarch64.h"
#include "aarch64/macro-assembler-aarch64.h"

All VIXL components are declared in the vixl::aarch64 namespace, so let's add this to the beginning of the file for convenience:

using namespace vixl::aarch64;

Creating a macro assembler is as simple as

MacroAssembler masm;

VIXL's assembler will generate some code at run-time, and this code needs to be stored in a buffer. By default the assembler will automatically manage the code buffer. However constructors are available that allow manual management of the code buffer.

We also need to set up the simulator. The simulator uses a Decoder object to read and decode the instructions from the code buffer. We need to create a decoder and bind our simulator to this decoder.

Decoder decoder;
Simulator simulator(&decoder);

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.

For instance, the hardware add instruction can only take a 12-bit immediate optionally shifted by 12, but the macro assembler can generate one or more instructions to handle any 64-bit immediate. For example, Add(x0, x0, -1) will be turned into Sub(x0, x0, 1).

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

#define __ masm->

It allows us to write __ Mov(x0, 42); instead of masm->Mov(x0, 42); to generate code.

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

void GenerateDemoFunction(MacroAssembler *masm) {
  __ Ldr(x1, 0x1122334455667788);
  __ And(x0, x0, x1);
  __ Ret();
}

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

uint64_t demo_function(uint64_t x);

This function doesn't perform any useful operation. It loads the value 0x1122334455667788 into x1 and performs a bitwise and operation with the function's argument (stored in x0). The result of this and operation is returned by the function in x0.

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 GenerateDemoFunction to generate the code.

Label demo_function;
masm.Bind(&demo_function);
GenerateDemoFunction(&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(x0, 42);
__ Bind(&foo);  // Actual address of foo is here.
__ Mov(x1, 0xc001);

If we run this code fragment the Mov(x0, 42) will never be executed since the first thing this code does is to jump to foo, which correspond to the Mov(x1, 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. There are two reasons for this:

They can't 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.
The B instruction does not branch to labels which are out of range of the branch. The B instruction has a range of 2^28 bytes, but other variants (such as conditional or CBZ-like branches) have smaller ranges. Confining them to local ranges doesn't mean that we won't hit these limits, but it makes the lifetime of the labels much shorter and eases the debugging of these kinds of issues.

Literal Pool

On ARMv8 instructions are 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(x0, 0x1122334455667788);

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 to efficiently generate the 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(x0, 0x1122334455667788);

The assembler will store the immediate value in a "literal pool", a set of constants embedded in the code. VIXL will emit literal pools after natural breaks in the control flow, such as unconditional branches or return instructions.

Literal pools are emitted regularly, such that they are within range of the instructions that refer to them. However, you can force a literal pool to be emitted using masm.EmitLiteralPool().

Running the code in the simulator.

Now we are going to see how to use the simulator to run the code that we generated previously.

Use the simulator to assign a value to the registers. Our previous code example uses the register x0 as an input, so let's set the value of this register.

simulator.WriteXRegister(0, 0x8899aabbccddeeff);

Now we can jump to the "entry" label to execute the code:

simulator.RunFrom(entry.target());

When the execution is finished and the simulator returned, you can inspect the value of the registers after the execution. For instance:

printf("x0 = %" PRIx64 "\n", simulator.ReadXRegister(0));

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/aarch64 directory showing more features of both the macro assembler and the ARMv8 architecture.

Extras

In addition to this document and the examples, you can find documentation and guides on various topics that may be helpful here.

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