yuzu/externals/dynarmic/docs/RegisterAllocator.md

Register Allocation (x64 Backend)

HostLocs contain values. A HostLoc ("host value location") is either a host CPU register or a host spill location.

Values once set cannot be changed. Values can however be moved by the register allocator between HostLocs. This is handled by the register allocator itself and code that uses the register allocator need not and should not move values between registers.

The register allocator is based on three concepts: Use, Def and Scratch.

• Use: The use of a value.
• Define: The definition of a value, this is the only time when a value is set.
• Scratch: Allocate a register that can be freely modified as one wishes.

Note that Useing a value decrements its use_count by one. When the use_count reaches zero the value is discarded and no longer exists.

The member functions on RegAlloc are just a combination of the above concepts.

Scratch

Xbyak::Reg64 ScratchGpr(HostLocList desired_locations = any_gpr)
Xbyak::Xmm ScratchXmm(HostLocList desired_locations = any_xmm)

At runtime, allocate one of the registers in desired_locations. You are free to modify the register. The register is discarded at the end of the allocation scope.

Pure Use

Xbyak::Reg64 UseGpr(Argument& arg);
Xbyak::Xmm UseXmm(Argument& arg);
OpArg UseOpArg(Argument& arg);
void Use(Argument& arg, HostLoc host_loc);

At runtime, the value corresponding to arg will be placed a register. The actual register is determined by which one of the above functions is called. UseGpr places it in an unused GPR, UseXmm places it in an unused XMM register, UseOpArg might be in a register or might be a memory location, and Use allows you to specify a specific register (GPR or XMM) to use.

This register must not have it's value changed.

UseScratch

Xbyak::Reg64 UseScratchGpr(Argument& arg);
Xbyak::Xmm UseScratchXmm(Argument& arg);
void UseScratch(Argument& arg, HostLoc host_loc);

At runtime, the value corresponding to arg will be placed a register. The actual register is determined by which one of the above functions is called. UseScratchGpr places it in an unused GPR, UseScratchXmm places it in an unused XMM register, and UseScratch allows you to specify a specific register (GPR or XMM) to use.

The return value is the register allocated to you.

You are free to modify the value in the register. The register is discarded at the end of the allocation scope.

Define as register

A Define is the defintion of a value. This is the only time when a value may be set.

void DefineValue(IR::Inst* inst, const Xbyak::Reg& reg);

By calling DefineValue, you are stating that you wish to define the value for inst, and you have written the value to the specified register reg.

Defineing as an alias of a different value

Adding a Define to an existing value.

void DefineValue(IR::Inst* inst, Argument& arg);

You are declaring that the value for inst is the same as the value for arg. No host machine instructions are emitted.

When to use each?

• Prefer Use to UseScratch where possible.
• Prefer the OpArg variants where possible.
• Prefer to not use the specific HostLoc variants where possible.