https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101958
Bug ID: 101958
Summary: compiling with "-O2 -fselective-scheduling2
-fno-tree-ch" produce bad result
Product: gcc
Version: 11.2.0
Status: UNCONFIRMED
Keywords: missed-optimization, wrong-code
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: channhuang at 163 dot com
CC: jakub at gcc dot gnu.org
Target Milestone: ---
Host: x86_64-pc-linux-gnu
Target: x86_64-pc-linux-gnu
Build: x86_64-pc-linux-gnu
gcc-11.2.0, x86_64-pc-linux-gnu, compiling with the following options:
"-O2 -fselective-scheduling2 -fno-tree-ch -lm"
The source code is for counting the number of primes up to 10000.
#include <stdio.h>
#include <math.h>
unsigned long max_prime =10000;
int cpu_execute_event()
{
unsigned long c,l;
int n = 0;
double t;
for(c = 2; c < max_prime; c++)
{
t = sqrt((double)c);
for(l = 2; l <= t; l++)
if (c % l == 0)
break;
if (l > t )
n++;
}
return n;
}
int main ()
{
int n;
n = cpu_execute_event();
printf("n:%d\n",n);
return 0;
}
The real return value of cpu_execute_event() is 2 where it should be 1229.
It works correctly when I compiled with "-O2 -fselective-scheduling2" or "-O2
-fno-tree-ch".I suspect that instruction scheduling changes the meaning of the
code.And gcc10.2 also produces the same phenomenon.
The command line is the following:
gcc-11.2 cpu_execute_event.c -O2 -fselective-scheduling2 -fno-tree-ch -lm
There is no output message, no error, no warning even compiled with the options
"-Wall -Wextra".
I use gcc 11.2.0, configured with:
../gcc-11.2.0/configure --prefix=/home/chann/gcc/gcc11_2 --enable-threads=posix
--disable-checking --disable-multilib --enable-languages=c,c++
