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From: "amonakov at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug rtl-optimization/105513] New: [9/10/11/12/13 Regression]
 Unnecessary SSE spill
Date: Sat, 07 May 2022 08:24:29 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D105513

            Bug ID: 105513
           Summary: [9/10/11/12/13 Regression] Unnecessary SSE spill
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Keywords: missed-optimization, ra
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: amonakov at gcc dot gnu.org
  Target Milestone: ---
            Target: x86_64-*-* i?86-*-*

Minimized from PR 105504.

Compile with -O2 -mtune=3Dhaswell -mavx (other -mtune variants are affected=
 too):

static int as_int(float x)
{
    return (union{float x; int i;}){x}.i;
}

float f(double y, float x)
{
    int i =3D as_int(x);
    if (__builtin_expect(i > 99, 0)) return 0;
    if (i*2u < 77) if (i=3D=3D2) return 0;
    return y*x;
}

GCC moves 'x' to 'i' via stack and then reloads from stack again when compu=
ting
'y*x':

f:
        vmovss  DWORD PTR [rsp-4], xmm1
        mov     eax, DWORD PTR [rsp-4]
        cmp     eax, 99
        jg      .L5
        lea     edx, [rax+rax]
        cmp     edx, 76
        ja      .L6
        cmp     eax, 2
        je      .L5
.L6:
        vcvtss2sd       xmm1, xmm1, DWORD PTR [rsp-4]
        vmulsd  xmm0, xmm1, xmm0
        vcvtsd2ss       xmm0, xmm0, xmm0
        ret
.L5:
        vxorps  xmm0, xmm0, xmm0
        ret

This is a regression relative to gcc-8. Interestingly, flipping '0' to '1' =
in
__builtin_expect (i.e. making early exit likely) results in good code, so
perhaps RA costing takes block probabilities backwards somewhere?=