[Bug target/55752] New: __builtin_ia32_ldmxcsr / __builtin_ia32_stmxcsr are not scheduling barriers

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55752



             Bug #: 55752

           Summary: __builtin_ia32_ldmxcsr / __builtin_ia32_stmxcsr are

                    not scheduling barriers

    Classification: Unclassified

           Product: gcc

           Version: 4.8.0

            Status: UNCONFIRMED

          Keywords: wrong-code

          Severity: normal

          Priority: P3

         Component: target

        AssignedTo: unassig...@gcc.gnu.org

        ReportedBy: rgue...@gcc.gnu.org

            Target: x86_64-*-*





float foo (float x, float f32)

{

  unsigned int mxscr_stat;

  mxscr_stat = __builtin_ia32_stmxcsr ();

  __builtin_ia32_ldmxcsr (mxscr_stat | 0x00000800);

  f32 = (x + f32) - f32;

  mxscr_stat = mxscr_stat & 0xffffffc0;

  __builtin_ia32_ldmxcsr (mxscr_stat);

  return f32;

}



Compiled at O2 yields:



foo:

.LFB0:

        .cfi_startproc

        stmxcsr -4(%rsp)

        movl    -4(%rsp), %eax

        movl    %eax, %edx

        orb     $8, %dh

        movl    %edx, -4(%rsp)

        ldmxcsr -4(%rsp)

        addss   %xmm1, %xmm0

        andl    $-64, %eax

        movl    %eax, -4(%rsp)

        ldmxcsr -4(%rsp)

        subss   %xmm1, %xmm0

        ret



note how the subss is scheduled after the ldmxcsr call.



It's ok (by pure luck of course) at the GIMPLE level.