https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105513
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=haswell -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 = as_int(x);
if (__builtin_expect(i > 99, 0)) return 0;
if (i*2u < 77) if (i==2) return 0;
return y*x;
}
GCC moves 'x' to 'i' via stack and then reloads from stack again when computing
'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?
