85 lines
4.8 KiB
Markdown
85 lines
4.8 KiB
Markdown
|
# Register Allocation (x64 Backend)
|
||
|
|
|
||
|
|
`HostLoc`s 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 `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.
|
||
|
|
|
||
|
|
The register allocator is based on three concepts: `Use`, `Def` and `Scratch`.
|
||
|
|
|
||
|
|
* `Use`: The use of a value.
|
||
|
|
* `Def`: 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 `Use`ing 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(IR::Value use_value, HostLocList desired_locations = any_gpr);
|
||
|
|
Xbyak::Xmm UseXmm(IR::Value use_value, HostLocList desired_locations = any_xmm);
|
||
|
|
OpArg UseOpArg(IR::Value use_value, HostLocList desired_locations);
|
||
|
|
|
||
|
|
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.
|
||
|
|
|
||
|
|
This register **must not** have it's value changed.
|
||
|
|
|
||
|
|
* `UseGpr`: The location is a GPR.
|
||
|
|
* `UseXmm`: The location is an XMM register.
|
||
|
|
* `UseOpArg`: The location may be one of the locations specified by `desired_locations`, but may also be a host memory reference.
|
||
|
|
|
||
|
|
### `UseScratch`
|
||
|
|
|
||
|
|
Xbyak::Reg64 UseScratchGpr(IR::Value use_value, HostLocList desired_locations = any_gpr)
|
||
|
|
Xbyak::Xmm UseScratchXmm(IR::Value use_value, HostLocList desired_locations = any_xmm)
|
||
|
|
|
||
|
|
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 are free to modify the register. The register is discarded at the end of the allocation scope.
|
||
|
|
|
||
|
|
### `Def`
|
||
|
|
|
||
|
|
A `Def` is the defintion of a value. This is the only time when a value may be set.
|
||
|
|
|
||
|
|
Xbyak::Xmm DefXmm(IR::Inst* def_inst, HostLocList desired_locations = any_xmm)
|
||
|
|
Xbyak::Reg64 DefGpr(IR::Inst* def_inst, HostLocList desired_locations = any_gpr)
|
||
|
|
|
||
|
|
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.
|
||
|
|
|
||
|
|
### `AddDef`
|
||
|
|
|
||
|
|
Adding a `Def` to an existing value.
|
||
|
|
|
||
|
|
void RegisterAddDef(IR::Inst* def_inst, const IR::Value& use_inst);
|
||
|
|
|
||
|
|
You are declaring that the value for `def_inst` is the same as the value for `use_inst`. No host machine instructions are emitted.
|
||
|
|
|
||
|
|
### `UseDef`
|
||
|
|
|
||
|
|
Xbyak::Reg64 UseDefGpr(IR::Value use_value, IR::Inst* def_inst, HostLocList desired_locations = any_gpr)
|
||
|
|
Xbyak::Xmm UseDefXmm(IR::Value use_value, IR::Inst* def_inst, HostLocList desired_locations = any_xmm)
|
||
|
|
|
||
|
|
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.
|
||
|
|
|
||
|
|
### `UseDef` (OpArg variant)
|
||
|
|
|
||
|
|
std::tuple<OpArg, Xbyak::Reg64> UseDefOpArgGpr(IR::Value use_value, IR::Inst* def_inst, HostLocList desired_locations = any_gpr)
|
||
|
|
std::tuple<OpArg, Xbyak::Xmm> UseDefOpArgXmm(IR::Value use_value, IR::Inst* def_inst, HostLocList desired_locations = any_xmm)
|
||
|
|
|
||
|
|
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.
|
||
|
|
|
||
|
|
## When to use each?
|
||
|
|
|
||
|
|
The variety of different ways to `Use` and `Def` values are for performance reasons.
|
||
|
|
|
||
|
|
* `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.
|
||
|
|
* `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.
|
||
|
|
* `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`.
|
||
|
|
* OpArg variants: Save host code-cache by merging memory loads into other instructions instead of the register allocator having to emit a `mov`.
|