https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93682
Bug ID: 93682
Summary: Wrong optimization: on x87 -fexcess-precision=standard
is incompatible with -mpc64
Product: gcc
Version: 10.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: ch3root at openwall dot com
Target Milestone: ---
Spotted by Alexander Monakov in bug 85957, comment 10...
Not sure if -mpc64 is supposed to be compatible with
-fexcess-precision=standard (and -std=c11 which enables it) but I don't see any
warnings in the description of these options.
-mpc64 changes hardware precision but the optimizer seems to be unaffected
which leads to discrepancies. For example, AIUI -fexcess-precision=standard is
supposed to give predictable results but the results of the following testcase
depend on the optimization level:
----------------------------------------------------------------------
#include <stdio.h>
int main()
{
int x = 1;
printf("%a\n", 0x1p-60 + x - 1);
}
----------------------------------------------------------------------
$ gcc -std=c11 -pedantic -Wall -Wextra -m32 -march=i686 -mpc64 test.c &&
./a.out
0x0p+0
$ gcc -std=c11 -pedantic -Wall -Wextra -m32 -march=i686 -mpc64 -O3 test.c &&
./a.out
0x1p-60
----------------------------------------------------------------------
gcc x86-64 version: gcc (GCC) 10.0.1 20200211 (experimental)
----------------------------------------------------------------------
