https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83350
Bug ID: 83350
Summary: Missed optimization in math expression: missing cube
of the sum formula
Product: gcc
Version: 8.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: zamazan4ik at tut dot by
Target Milestone: ---
gcc(trunk) with '--std=c++17 -O3 -march=native -ffast-math' flags for this
code:
double test(double a, double b)
{
return a*a*a + 3.0*a*a*b + 3.0*a*b*b + b*b*b;
}
generates this assembly:
test(double, double):
vmovsd xmm2, QWORD PTR .LC0[rip]
vmulsd xmm3, xmm0, xmm0
vmovapd xmm4, xmm0
vfmadd132sd xmm4, xmm1, xmm2
vmulsd xmm4, xmm4, xmm1
vfmadd132sd xmm2, xmm4, xmm3
vmulsd xmm1, xmm2, xmm1
vfmadd132sd xmm0, xmm1, xmm3
ret
.LC0:
.long 0
.long 1074266112
But there is formula: a*a*a + 3.0*a*a*b + 3.0*a*b*b + b*b*b == (a + b)^3. And
it can be compiled in faster way.
