[Bug middle-end/54315] New: Unnecessary copy of return value

[hjl.tools at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54315

             Bug #: 54315
           Summary: Unnecessary copy of return value
    Classification: Unclassified
           Product: gcc
           Version: 4.7.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: hjl.to...@gmail.com
                CC: areg.melikadam...@gmail.com, ubiz...@gmail.com


On Linux/x86-64, I got

[hjl@gnu-6 pr20020]$ cat x.i
union S160
{
  long double a;
};
extern union S160 check160 (void);
extern void checkx160 (union S160);
void
test160 (void)
{
  checkx160 (check160 ());
}
[hjl@gnu-6 pr20020]$ gcc -S -O2 x.i
[hjl@gnu-6 pr20020]$ cat x.s
    .file    "x.i"
    .text
    .p2align 4,,15
    .globl    test160
    .type    test160, @function
test160:
.LFB0:
    .cfi_startproc
    subq    $72, %rsp
    .cfi_def_cfa_offset 80
    call    check160
    fstpt    16(%rsp)
    movq    16(%rsp), %rdx
    movq    24(%rsp), %rax
    movq    %rdx, 32(%rsp)
    movq    %rax, 40(%rsp)
    movq    %rdx, (%rsp)
    movq    %rax, 8(%rsp)
    call    checkx160
    addq    $72, %rsp
    .cfi_def_cfa_offset 8
    ret
    .cfi_endproc
.LFE0:
    .size    test160, .-test160
    .ident    "GCC: (GNU) 4.7.1 20120629 (Red Hat 4.7.1-1)"
    .section    .note.GNU-stack,"",@progbits
[hjl@gnu-6 pr20020]$ cat y.i
struct S160
{
  long double a;
};
extern struct S160 check160 (void);
extern void checkx160 (struct S160);
void
test160 (void)
{
  checkx160 (check160 ());
}
[hjl@gnu-6 pr20020]$ gcc -S -O2 y.i
[hjl@gnu-6 pr20020]$ cat y.s
    .file    "y.i"
    .text
    .p2align 4,,15
    .globl    test160
    .type    test160, @function
test160:
.LFB0:
    .cfi_startproc
    subq    $24, %rsp
    .cfi_def_cfa_offset 32
    call    check160
    fstpt    (%rsp)
    call    checkx160
    addq    $24, %rsp
    .cfi_def_cfa_offset 8
    ret
    .cfi_endproc
.LFE0:
    .size    test160, .-test160
    .ident    "GCC: (GNU) 4.7.1 20120629 (Red Hat 4.7.1-1)"
    .section    .note.GNU-stack,"",@progbits
[hjl@gnu-6 pr20020]$ 

union causes unnecessary copy of return value. I don't believe
ABI calls for it.