[Bug target/56186] New: [4.8 regression] function return ABI change for 128-bit types on Win64

[ebotcazou at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56186

             Bug #: 56186
           Summary: [4.8 regression] function return ABI change for
                    128-bit types on Win64
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: major
          Priority: P3
         Component: target
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: ebotcazou@gcc.gnu.org
            Target: x86_64-*-mingw


Some 128-bit wide types are returned differently with 4.8 than with 4.7 because
of the introduction of MAX_FIXED_MODE_SIZE in the x86 back-end.  For example:

struct S {
  long long int l1, l2;
};

struct S foo (void)
{
  struct S s = { 0, 0 };
  return s;
}

with 4.7.x:

        movq    %rcx, 16(%rbp)
        movq    $0, -16(%rbp)
        movq    $0, -8(%rbp)
        movq    16(%rbp), %rax
        movq    -16(%rbp), %rdx
        movq    %rdx, (%rax)
        movq    -8(%rbp), %rdx
        movq    %rdx, 8(%rax)
        movq    16(%rbp), %rax
        addq    $16, %rsp
        popq    %rbp
        ret

with 4.8.0:

        movq    $0, -16(%rbp)
        movq    $0, -8(%rbp)
        movq    -16(%rbp), %rax
        movq    -8(%rbp), %rdx
        movq    %rax, -32(%rbp)
        movq    %rdx, -24(%rbp)
        movdqa  -32(%rbp), %xmm0
        addq    $32, %rsp
        popq    %rbp
        ret

The implementation in function_value_ms_64 is too fragile and only looks at
modes instead of looking at types first.