https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85050
Bug ID: 85050
Summary: Vectorized function - suboptimal gather
Product: gcc
Version: 7.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: marcin.krotkiewski at gmail dot com
Target Milestone: ---
I compile the following function with gcc 7.2 and 8.0.1, with -march=broadwell
-O3 -ftree-vectorize -ffast-math -fopenmp
#pragma omp declare simd notinbranch
double testfun(double arg)
{
static const double A[11] =
{5.53,5.49,5.46,5.43,5.40,5.25,5.00,4.69,4.48,4.16,3.85};
int iidx = int(arg);
return A[iidx];
}
Also here https://godbolt.org/g/wo7pdv
For some reason GCC's 4-wide vectorized function contains asm that works on two
2-wide vectors instead of a single 4-wide vector:
_ZGVdN4v__Z7testfund:
[...]
vmovapd %ymm0, -32(%rsp)
vmovapd .LC0(%rip), %xmm2
vinsertf128 $0x1, -16(%rsp), %ymm0, %ymm0
vmovapd %xmm2, %xmm4
vcvttpd2dqy %ymm0, %xmm0
vgatherdpd %xmm4, (%rax,%xmm0,8), %xmm3
vpshufd $238, %xmm0, %xmm0
vgatherdpd %xmm2, (%rax,%xmm0,8), %xmm1
vmovaps %xmm3, -64(%rsp)
vmovaps %xmm1, -48(%rsp)
vmovapd -64(%rsp), %ymm0
[...]
Code generated using ICC looks like expected:
_ZGVYN4v__Z7testfund:
vcvttpd2dq xmm1, ymm0 #11.18
vpcmpeqd ymm2, ymm2, ymm2 #12.10
vxorpd ymm0, ymm0, ymm0 #12.10
vgatherdpd ymm0, QWORD PTR [A.5.0.1+xmm1*8], ymm2 #12.10
I don't see anything wrong with my compiler options. Is this behaviour in GCC
expected, and a result of a different vectorization cost model?
