Move dead submodules in-tree

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externals/dynarmic/docs/RegisterAllocator.md

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+# 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.
+* `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 `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(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`.
+
+### `Define`ing 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.