https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107294
Bug ID: 107294
Summary: Missed optimization: multiplying real with complex
number in Fortran (only)
Product: gcc
Version: 11.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: fortran
Assignee: unassigned at gcc dot gnu.org
Reporter: bartoldeman at users dot sourceforge.net
Target Milestone: ---
This code:
complex function csmul(a, b)
real, value :: a
complex, value :: b
csmul = a * b
end function csmul
produces this assembly on x86-64 (11.3, -O2)
0: 66 0f d6 4c 24 f8 movq %xmm1,-0x8(%rsp)
6: f3 0f 10 64 24 fc movss -0x4(%rsp),%xmm4
c: f3 0f 10 4c 24 f8 movss -0x8(%rsp),%xmm1
12: 0f 28 d0 movaps %xmm0,%xmm2
15: 66 0f ef db pxor %xmm3,%xmm3 # xmm3 = 0
19: f3 0f 59 d1 mulss %xmm1,%xmm2
1d: 0f 28 ec movaps %xmm4,%xmm5
20: f3 0f 59 eb mulss %xmm3,%xmm5 # xmm5 = 0
24: f3 0f 59 c4 mulss %xmm4,%xmm0
28: f3 0f 59 cb mulss %xmm3,%xmm1 # xmm1 = 0
2c: f3 0f 5c d5 subss %xmm5,%xmm2 # xmm2 unchanged
30: f3 0f 58 c1 addss %xmm1,%xmm0 # xmm0 unchanged
34: f3 0f 11 54 24 f0 movss %xmm2,-0x10(%rsp)
3a: f3 0f 11 44 24 f4 movss %xmm0,-0xc(%rsp)
40: f3 0f 7e 44 24 f0 movq -0x10(%rsp),%xmm0
46: c3 retq
here xmm3 (imaginary part of a, promoted to complex) is set to 0 but this is
not exploited in the remainder.
On the other hand the assembly for the corresponding C code looks good, with
two mul instructions, as expected:
float _Complex csmul(float a, float _Complex b)
{
return a * b;
}
0000000000000000 <csmul>:
0: 66 0f d6 4c 24 f8 movq %xmm1,-0x8(%rsp)
6: f3 0f 10 4c 24 f8 movss -0x8(%rsp),%xmm1
c: f3 0f 59 c8 mulss %xmm0,%xmm1
10: f3 0f 59 44 24 fc mulss -0x4(%rsp),%xmm0
16: f3 0f 11 4c 24 f0 movss %xmm1,-0x10(%rsp)
1c: f3 0f 11 44 24 f4 movss %xmm0,-0xc(%rsp)
22: f3 0f 7e 44 24 f0 movq -0x10(%rsp),%xmm0
28: c3 retq
The same issue is still present in trunk, according to godbolt.org.
