[Bug target/110247] New: suboptimal code about `no_caller_saved_registers` on caller side

[lh_mouse at 126 dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110247

            Bug ID: 110247
           Summary: suboptimal code about `no_caller_saved_registers` on
                    caller side
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: lh_mouse at 126 dot com
  Target Milestone: ---

Given:
(https://gcc.godbolt.org/z/xevzx56Y5)

```
int complex(int x, int y)
  __attribute__((__no_caller_saved_registers__));

int test(int x, int y, int z)
  {
    return complex(x, y) + complex(y, z) + complex(z, x);
  }
```


My understanding is that `__no_caller_saved_registers__` says no register will
be clobbered by `complex`, so it is not necessary for GCC to establish a stack
frame and push arguments there. This is gonna help a lot if the function will
be inlined.


Clang generates much better assembly but I wonder whether it is valid to assume
that arguments registers are also preserved, like

```
test:
  push rcx            ; align %rsp
                      ; %edi := x, %esi = y, %edx = z
  call complex        ;
  mov ecx, eax        ; %ecx = complex(x, y)
  xchg edi, esi       ; %edi = y
  xchg esi, edx       ; %esi = z, %edx = x
  call complex        ;
  add ecx, eax        ; %ecx += complex(y, z)
  mov edi, esi        ; %edi = z
  mov esi, edx        ; %esi = x
  call complex        ;
  add eax, ecx        ; %eax = %ecx + complex(z, x)
  pop rcx
  ret
```