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From: "josephcsible at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/95783] New: Inefficient use of the stack when a function
 takes the address of its argument
Date: Sat, 20 Jun 2020 06:34:56 +0000
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X-Bugzilla-Component: target
X-Bugzilla-Version: 10.1.0
X-Bugzilla-Keywords: missed-optimization
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D95783

            Bug ID: 95783
           Summary: Inefficient use of the stack when a function takes the
                    address of its argument
           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:

void g(long *);
long f(long x) {
    g(&x);
    return x;
}

At either "-O3" or "-Os", it results in this assembly:

f:
        subq    $24, %rsp
        movq    %rdi, 8(%rsp)
        leaq    8(%rsp), %rdi
        call    g
        movq    8(%rsp), %rax
        addq    $24, %rsp
        ret

There are two problems with this: it's unnecessarily complicated with extra
instructions, and it wastes 16 bytes of stack space. I'd rather see this
assembly instead:

f:
        pushq   %rdi
        movq    %rsp, %rdi
        call    g
        popq    %rax
        ret

https://godbolt.org/z/PuNB6Y=