https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107520
Bug ID: 107520
Summary: Optimize std::lerp(d, d, 0.5)
Product: gcc
Version: 13.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: enhancement
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: glisse at gcc dot gnu.org
Target Milestone: ---
In some C++ code I have, it would be convenient if the compiler, possibly with
the help of the standard library, could make the following function cheap,
ideally just the identity. I'll probably end up wrapping lerp with a function
that first checks with __builtin_constant_p if the 2 bounds are equal, but I'll
post this in case people have ideas how to improve things.
#include <cmath>
double f(double d){
return std::lerp(d, d, .5);
}
Currently, with -O3, we generate
movapd %xmm0, %xmm1
pxor %xmm0, %xmm0
comisd %xmm1, %xmm0
jnb .L7
comisd %xmm0, %xmm1
jb .L6
.L7:
pxor %xmm0, %xmm0
ucomisd %xmm0, %xmm1
jp .L6
je .L11
.L6:
movapd %xmm1, %xmm0
subsd %xmm1, %xmm0
mulsd .LC1(%rip), %xmm0
addsd %xmm1, %xmm0
maxsd %xmm1, %xmm0
ret
.p2align 4,,10
.p2align 3
.L11:
mulsd .LC1(%rip), %xmm1
movapd %xmm1, %xmm0
addsd %xmm1, %xmm0
ret
(clang is better at avoiding the redundant comparison)
With -fno-trapping-math to help a bit, I see at the beginning
if (d_2(D) == 0.0)
goto <bb 3>; [34.00%]
else
goto <bb 4>; [66.00%]
<bb 3> [local count: 475287355]:
_7 = d_2(D) * 5.0e-1;
_10 = _7 * 2.0e+0;
I think that even with the default -fsigned-zeros, simplifying to _10 = d_2(D)
is valid.
Adding -fno-signed-zeros
<bb 2> [local count: 1073741824]:
if (d_2(D) == 0.0)
goto <bb 5>; [34.00%]
else
goto <bb 3>; [66.00%]
<bb 3> [local count: 598454470]:
_13 = d_2(D) - d_2(D);
_14 = _13 * 5.0e-1;
__x_15 = d_2(D) + _14;
if (d_2(D) u>= __x_15)
goto <bb 5>; [50.00%]
else
goto <bb 4>; [50.00%]
<bb 4> [local count: 299227235]:
<bb 5> [local count: 1073741825]:
# _12 = PHI <d_2(D)(2), __x_15(4), d_2(D)(3)>
return _12;
_13 is 0 or NaN, which doesn't change for _14, and __x_15 is just d_2, so we
always return d_2.
