https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70777
Bug ID: 70777
Summary: x*x pessimised to pow(x,2) with -Og -ffast-math
Product: gcc
Version: 6.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: suckfish at ihug dot co.nz
Target Milestone: ---
Compiling the function
double foo(double x) { return x*x; }
on x86_64 with "gcc -Og -ffast-math" converts the multiplication to a call to
pow():
foo:
.LFB0:
.cfi_startproc
subq $8, %rsp
.cfi_def_cfa_offset 16
movsd .LC0(%rip), %xmm1
call pow
addq $8, %rsp
.cfi_def_cfa_offset 8
ret
Removing the -ffast-math or using -O/-O2/-O3 with -ffast-math, the
multiplication compiles to a multiply instruction.
Observed with gcc 4.8.3, 5.4.1 and 6.0.
