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From: "josephcsible at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/95941] New: Passing a struct by value wastes 16 bytes on
 the stack
Date: Sat, 27 Jun 2020 16:44:28 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: target
X-Bugzilla-Version: 10.1.0
X-Bugzilla-Keywords: missed-optimization
X-Bugzilla-Severity: normal
X-Bugzilla-Who: josephcsible at gmail dot com
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X-Bugzilla-Priority: P3
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D95941

            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 shou=
ld
have compiled to this instead:

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