[all-commits] [llvm/llvm-project] 26f97e: [AArch64] Stop reserved registers from being saved...

[Nuko Y. via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 26f97ee9aa413db240c397f96ddd5b0553a57d30
      https://github.com/llvm/llvm-project/commit/26f97ee9aa413db240c397f96ddd5b0553a57d30
  Author: Nuko Y. <or at dmc.chat>
  Date:   2025-05-12 (Mon, 12 May 2025)

  Changed paths:
    M llvm/lib/Target/AArch64/AArch64FrameLowering.cpp
    M llvm/lib/Target/AArch64/AArch64RegisterInfo.cpp
    M llvm/lib/Target/AArch64/AArch64RegisterInfo.h
    A llvm/test/CodeGen/AArch64/reserveXreg-for-regalloc.ll
    M llvm/test/CodeGen/AArch64/reserveXreg.ll

  Log Message:
  -----------
  [AArch64] Stop reserved registers from being saved in prolog/epilog (#138448)

[GCC's
documentation](https://gcc.gnu.org/onlinedocs/gcc-15.1.0/gcc/Code-Gen-Options.html)
is clear on how -ffixed-reg must behave:
```
  Treat the register named reg as a fixed register; generated
  code should never refer to it (except perhaps as a stack pointer,
  frame pointer or in some other fixed role).
```

This implies prolog/epilog code also must not save/restore explicitly
fixed registers, even when it is callee-saved. Some projects rely on
this (GCC's) behavior.

For example,
```
void f() {
  register uint64_t x28 asm("x28") = 0xee;
  asm volatile("" : "+r"(x28)); // avoid mov being eliminated
}
```
should not touch x28 outside of `mov w28,#0xee`.

For riscv64, clang behaves the same as GCC, so I am inclined to believe
this is indeed a bug.

Fixes #111379.



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