85 lines
No EOL
4.8 KiB
Markdown
85 lines
No EOL
4.8 KiB
Markdown
# 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 handled by the register allocator itself and code that uses the register allocator need not and should not move values 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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* `Def`: 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(IR::Value use_value, HostLocList desired_locations = any_gpr);
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Xbyak::Xmm UseXmm(IR::Value use_value, HostLocList desired_locations = any_xmm);
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OpArg UseOpArg(IR::Value use_value, HostLocList desired_locations);
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At runtime, the value corresponding to `use_value` will be placed into one of the `HostLoc`s specified by `desired_locations`. The return value is the actual location.
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This register **must not** have it's value changed.
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* `UseGpr`: The location is a GPR.
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* `UseXmm`: The location is an XMM register.
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* `UseOpArg`: The location may be one of the locations specified by `desired_locations`, but may also be a host memory reference.
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### `UseScratch`
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Xbyak::Reg64 UseScratchGpr(IR::Value use_value, HostLocList desired_locations = any_gpr)
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Xbyak::Xmm UseScratchXmm(IR::Value use_value, HostLocList desired_locations = any_xmm)
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At runtime, the value corresponding to `use_value` will be placed into one of the `HostLoc`s specified by `desired_locations`. The return value is the actual location.
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You are free to modify the register. The register is discarded at the end of the allocation scope.
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### `Def`
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A `Def` is the defintion of a value. This is the only time when a value may be set.
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Xbyak::Xmm DefXmm(IR::Inst* def_inst, HostLocList desired_locations = any_xmm)
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Xbyak::Reg64 DefGpr(IR::Inst* def_inst, HostLocList desired_locations = any_gpr)
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By calling `DefXmm` or `DefGpr`, you are stating that you wish to define the value for `def_inst`, and you wish to write the value to one of the `HostLoc`s specified by `desired_locations`. You must write the value to the register returned.
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### `AddDef`
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Adding a `Def` to an existing value.
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void RegisterAddDef(IR::Inst* def_inst, const IR::Value& use_inst);
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You are declaring that the value for `def_inst` is the same as the value for `use_inst`. No host machine instructions are emitted.
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### `UseDef`
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Xbyak::Reg64 UseDefGpr(IR::Value use_value, IR::Inst* def_inst, HostLocList desired_locations = any_gpr)
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Xbyak::Xmm UseDefXmm(IR::Value use_value, IR::Inst* def_inst, HostLocList desired_locations = any_xmm)
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At runtime, the value corresponding to `use_value` will be placed into one of the `HostLoc`s specified by `desired_locations`. The return value is the actual location. You must write the value correponding to `def_inst` by the end of the allocation scope.
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### `UseDef` (OpArg variant)
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std::tuple<OpArg, Xbyak::Reg64> UseDefOpArgGpr(IR::Value use_value, IR::Inst* def_inst, HostLocList desired_locations = any_gpr)
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std::tuple<OpArg, Xbyak::Xmm> UseDefOpArgXmm(IR::Value use_value, IR::Inst* def_inst, HostLocList desired_locations = any_xmm)
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These have the same semantics as `UseDefGpr` and `UseDefXmm` except `use_value` may not be present in the register, and may actually be in a host memory location.
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## When to use each?
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The variety of different ways to `Use` and `Def` values are for performance reasons.
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* `UseDef`: Instead of performing a `Use` and a `Def`, `UseDef` uses one less register in the case when this `Use` is the last `Use` of a value.
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* `UseScratch`: Instead of performing a `Use` and a `Scratch`, `UseScratch` uses one less register in the case when this `Use` is the last `Use` of a value.
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* `AddDef`: This drastically reduces the number of registers required when it can be used. It can be used when values are truncations of other values. For example, if `u8_value` contains the truncation of `u32_value`, `AddDef(u8_value, u32_value)` is a valid definition of `u8_value`.
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* OpArg variants: Save host code-cache by merging memory loads into other instructions instead of the register allocator having to emit a `mov`. |