https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81376
Bug ID: 81376
Summary: unnecessary cast before comparison
Product: gcc
Version: 8.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: drepper.fsp+rhbz at gmail dot com
Target Milestone: ---
Take the following code:
typedef double c_t;
typedef int a_t;
int f(a_t a1, a_t a2) {
return (c_t) a1 < (c_t) a2;
}
With IEEE 754 double we have a 52 bits mantissa which is wide enough to
represent all of the 'int' values exactly. No possibility for imprecision and
especially not for Inf or NaN.
Still gcc (trunk as of today but likely older versions as well) generate the
code for the conversion. This is for x86-64 with -O2:
f:
vxorpd %xmm0, %xmm0, %xmm0
vxorpd %xmm1, %xmm1, %xmm1
xorl %eax, %eax
vcvtsi2sd %edi, %xmm0, %xmm0
vcvtsi2sd %esi, %xmm1, %xmm1
vucomisd %xmm0, %xmm1
seta %al
ret
A simple
xorl %eax, %eax
cmpl %esi, %edi
setl %al
ret
is sufficient, just as if c_t above would be defined as 'int'.
