https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94863
Bug ID: 94863
Summary: Failure to use blendps over mov when possible
Product: gcc
Version: 10.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: gabravier at gmail dot com
Target Milestone: ---
typedef double v2df __attribute__((vector_size(16)));
v2df move_sd(v2df a, v2df b)
{
v2df result = a;
result[0] = b[0];
return result;
}
LLVM -O3 compiles this as such :
move_sd(double __vector(2), double __vector(2)): # @move_sd(double __vector(2),
double __vector(2))
blendps xmm0, xmm1, 3 # xmm0 = xmm1[0,1],xmm0[2,3]
ret
GCC gives this :
move_sd(double __vector(2), double __vector(2)):
movsd xmm0, xmm1
ret
Using `blendps` here should be a worthy tradeoff. Here is a table of
throughputs for various CPU architectures formatted as "arch-name:
blendps-throughput, movsd-throughput" :
Wolfdale: 1, 0.33
Nehalem: 1, 1
Westmere: 1, 1
Sandy Bridge: 0.5, 1
Ivy Bridge: 0.5, 1
Haswell: 0.33, 1
Broadwell: 0.33, 1
Skylake: 0.33, 1
Skylake-X: 0.33, 1
Kaby Lake: 0.33, 1
Coffee Lake: 0.33, 1
Cannon Lake: 0.33, 0.33
Ice Lake: 0.33, 0.33
Zen+: 0.5, 0.25
Zen 2: 0.33, 0.25
Unless there is an important factor other than thoughput that could affect
this, this should improve performance or keep it identical on every
architecture except Zen+
