[Bug target/95941] New: Passing a struct by value wastes 16 bytes on the stack

[josephcsible at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95941

            Bug ID: 95941
           Summary: Passing a struct by value wastes 16 bytes on the stack
           Product: gcc
           Version: 10.1.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: josephcsible at gmail dot com
  Target Milestone: ---
            Target: x86_64-linux-gnu

Consider this C code:

extern struct foo {
    long x, y, z;
} s;

void f(struct foo);

void g(void) {
    f(s);
}

At "-O3", it compiles to this:

g:
        subq    $16, %rsp
        pushq   s+16(%rip)
        pushq   s+8(%rip)
        pushq   s(%rip)
        call    f
        addq    $40, %rsp
        ret

There's no reason at all to use that extra 16 bytes of stack space. It should
have compiled to this instead:

g:
        pushq   s+16(%rip)
        pushq   s+8(%rip)
        pushq   s(%rip)
        call    f
        addq    $24, %rsp
        ret