[Bug target/28825] New: return (vector float) { a, a, b, b } generates unwanted MMX insns

["stuart at apple dot com" <gcc-bugzilla@gcc.gnu.org>]

X-Bugzilla-Reason: CC

+++ This bug was initially created as a clone of Bug #24073 +++

Take the following example:
#define vector __attribute__((vector_size(16)))

float a; float b;
vector float f(void) { return (vector float){ a, b, 0.0, 0.0}; }
---
Currently we get:
        subl    $12, %esp
        movss   _b, %xmm0
        movss   _a, %xmm1
        unpcklps        %xmm0, %xmm1
        movaps  %xmm1, %xmm0
        xorl    %eax, %eax
        xorl    %edx, %edx
        movl    %eax, (%esp)
        movl    %edx, 4(%esp)
        xorps   %xmm1, %xmm1
        movlhps %xmm1, %xmm0
        addl    $12, %esp

------
We should be able to produce:
movss _b, %xmm0
movss _a, %xmm1
shufps 60, /*[0, 3, 3, 0]*/, %xmm1, %xmm0 // _a, 0, 0, _b
shufps 201, /*[3, 0, 2, 1]*/, %xmm0, %xmm0 // _a, _b, 0, 0

This is from Nathan Begeman.
================================================================
 ------- Comment #4 From Uros Bizjak  2005-09-27 11:41   -------

I think that following example wins the contest:

vector float f(void) { return (vector float){ a, a, b, b}; }

gcc -O2 -msse -fomit-frame-pointer

        subl    $28, %esp
        movss   a, %xmm0
        movss   %xmm0, 4(%esp)
        movss   b, %xmm0
        movd    4(%esp), %mm0
        punpckldq       %mm0, %mm0
        movss   %xmm0, 4(%esp)
        movq    %mm0, 16(%esp)
        movd    4(%esp), %mm0
        punpckldq       %mm0, %mm0
        movq    %mm0, 8(%esp)
        movlps  16(%esp), %xmm1
        movhps  8(%esp), %xmm1
        addl    $28, %esp
        movaps  %xmm1, %xmm0
        ret

Note the usage of MMX registers.


------- Comment #5 From Andrew Pinski 2005-09-27 14:33 -------

(In reply to comment #4)
> I think that following example wins the contest:
> 
> vector float f(void) { return (vector float){ a, a, b, b}; }

For this, it is a different bug.  The issue with the above is that
ix86_expand_vector_init_duplicate check 
for mmx_okay is bad.
Currently, we have
      if (!mmx_ok && !TARGET_SSE)
but I if I change it to:
      if (!mmx_ok)
we get:
        movss   _a, %xmm0
        movss   _b, %xmm1
        unpcklps        %xmm0, %xmm0
        unpcklps        %xmm1, %xmm1
        movlhps %xmm1, %xmm0
Which looks ok to me.  That testcase should be opened into another bug as it is
obviously wrong.

=====================================================================
Cloned from 24073 to track the MMX insn issue; the original 24073 problem is a
performance issue.


-- 
           Summary: return (vector float) { a, a, b, b } generates unwanted
                    MMX insns
           Product: gcc
           Version: 4.2.0
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: target
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: stuart at apple dot com
GCC target triplet: i786-*-*


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28825