https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96793
Bug ID: 96793
Summary: __builtin_floor produces wrong result when rounding
direction is FE_DOWNWARD
Product: gcc
Version: 10.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: chfast at gmail dot com
Target Milestone: ---
Created attachment 49125
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=49125&action=edit
Preprocessed test code
If the rounding direction is set to FE_DOWNWARD by fesetround(),
the __builtin_floor() result is -0 where it should be +0. Eg.
__builtin_floor(0.25) == -0.
The outputs are done with GCC 10.2.0 (x86_64-linux-gnu) with -O2
-frounding-math -lm. Full gcc -v output in cli.log file. This is also
reproducible in GCC 9 and trunk.
The test code to reproduce the bug checks if the result's sign is 0 as expected
(attached as builtin_floor_test.c):
enum { FE_DOWNWARD = 0x400 };
extern int fesetround(int rounding_direction);
__attribute__((noinline))
float builtin_floorf(float value)
{
return __builtin_floorf(value);
}
int main()
{
fesetround(FE_DOWNWARD);
float result = builtin_floorf(0.25f);
return __builtin_signbitf(result) != 0;
}
The __builtin_floor() generates the following assembly (part of
builtin_floor_test.s):
movss .LC1(%rip), %xmm2
movss .LC0(%rip), %xmm4
movaps %xmm0, %xmm3
movaps %xmm0, %xmm1
andps %xmm2, %xmm3
ucomiss %xmm3, %xmm4
jbe .L2
cvttss2sil %xmm0, %eax
pxor %xmm3, %xmm3
andnps %xmm1, %xmm2
cvtsi2ssl %eax, %xmm3
movaps %xmm3, %xmm4
cmpnless %xmm0, %xmm4
movss .LC2(%rip), %xmm0
andps %xmm0, %xmm4
subss %xmm4, %xmm3
movaps %xmm3, %xmm0
orps %xmm2, %xmm0
.L2:
ret
