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From: "josephcsible at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/105932] New: Small structures returned incorrectly in
 i386 Microsoft ABI
Date: Sun, 12 Jun 2022 01:32:55 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D105932

            Bug ID: 105932
           Summary: Small structures returned incorrectly in i386
                    Microsoft ABI
           Product: gcc
           Version: 12.1.0
            Status: UNCONFIRMED
          Keywords: ABI, wrong-code
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: josephcsible at gmail dot com
  Target Milestone: ---

Consider this C code:

struct foo {
    int x, y;
};

extern int x, y;

struct foo f(void) {
    struct foo rv;
    rv.x =3D x;
    rv.y =3D y;
    return rv;
}

When compiled with "-O2 -m32 -mabi=3Dms", it compiles to this:

f:
        movd    x, %xmm0
        movl    4(%esp), %eax
        movd    y, %xmm1
        punpckldq       %xmm1, %xmm0
        movq    %xmm0, (%eax)
        ret

Which expects to be passed a hidden parameter to hold the address of the re=
turn
value. But in the i386 Microsoft ABI, that's not how returns work for POD t=
ypes
that are 64 bits or smaller. Here's what it should compile to instead:

f:
        movd    x, %eax
        movd    y, %edx
        ret=