[dumael]

> It's pretty cool that clang uses this when it knows the value in the first argument is byte-sized.

Clang is using the 8 bit subregister due to how it legalizes types.

When LLVM-IR is compiled for a target, it undergoes a process called "legalization" where invalid operations for the target are Expanded (replace the invalid operation with a semantically equivalent but legal series of operations), or Promoted (e.g. promote operations on boolean types to character types), Libcall (call out to the likes of libgcc.a) or Legal where the target directly supports the operation.

Since X86(_64) supports 8, 16, 32 (and 64) bit register accesses and operations, operations on variables of those sizes will be matched to the corresponding operation and register sizes.

If you were to compile that code for the likes of MIPS, ARM or PowerPC you'd see fully 32 bit code.