77 lines
3.2 KiB
Markdown
77 lines
3.2 KiB
Markdown
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# Register Allocation (x64 Backend)
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`HostLoc`s contain values. A `HostLoc` ("host value location") is either a host CPU register or a host spill location.
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Values once set cannot be changed. Values can however be moved by the register allocator between `HostLoc`s. This is
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handled by the register allocator itself and code that uses the register allocator need not and should not move values
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between registers.
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The register allocator is based on three concepts: `Use`, `Def` and `Scratch`.
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* `Use`: The use of a value.
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* `Define`: The definition of a value, this is the only time when a value is set.
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* `Scratch`: Allocate a register that can be freely modified as one wishes.
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Note that `Use`ing a value decrements its `use_count` by one. When the `use_count` reaches zero the value is discarded and no longer exists.
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The member functions on `RegAlloc` are just a combination of the above concepts.
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### `Scratch`
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Xbyak::Reg64 ScratchGpr(HostLocList desired_locations = any_gpr)
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Xbyak::Xmm ScratchXmm(HostLocList desired_locations = any_xmm)
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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.
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### Pure `Use`
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Xbyak::Reg64 UseGpr(Argument& arg);
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Xbyak::Xmm UseXmm(Argument& arg);
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OpArg UseOpArg(Argument& arg);
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void Use(Argument& arg, HostLoc host_loc);
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At runtime, the value corresponding to `arg` will be placed a register. The actual register is determined by
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which one of the above functions is called. `UseGpr` places it in an unused GPR, `UseXmm` places it
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in an unused XMM register, `UseOpArg` might be in a register or might be a memory location, and `Use` allows
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you to specify a specific register (GPR or XMM) to use.
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This register **must not** have it's value changed.
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### `UseScratch`
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Xbyak::Reg64 UseScratchGpr(Argument& arg);
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Xbyak::Xmm UseScratchXmm(Argument& arg);
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void UseScratch(Argument& arg, HostLoc host_loc);
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At runtime, the value corresponding to `arg` will be placed a register. The actual register is determined by
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which one of the above functions is called. `UseScratchGpr` places it in an unused GPR, `UseScratchXmm` places it
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in an unused XMM register, and `UseScratch` allows you to specify a specific register (GPR or XMM) to use.
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The return value is the register allocated to you.
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You are free to modify the value in the register. The register is discarded at the end of the allocation scope.
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### `Define` as register
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A `Define` is the defintion of a value. This is the only time when a value may be set.
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void DefineValue(IR::Inst* inst, const Xbyak::Reg& reg);
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By calling `DefineValue`, you are stating that you wish to define the value for `inst`, and you have written the
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value to the specified register `reg`.
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### `Define`ing as an alias of a different value
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Adding a `Define` to an existing value.
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void DefineValue(IR::Inst* inst, Argument& arg);
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You are declaring that the value for `inst` is the same as the value for `arg`. No host machine instructions are
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emitted.
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## When to use each?
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* Prefer `Use` to `UseScratch` where possible.
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* Prefer the `OpArg` variants where possible.
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* Prefer to **not** use the specific `HostLoc` variants where possible.
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