-fsched2-use-superblocks fails to order loop operations and loop jump
instruction correctly. The following example will show that
-fsched2-use-superblocks causes generation of wrong code.
Steps to reproduce:
Compile
>>>
#include <string.h>
#include <stdio.h>
int main()
{
double retval = 0.0;
const unsigned char *start = "\005\b\000";
unsigned int len = strlen(start);
const unsigned char *const end = start + len;
while (start < end) {
retval += *(start++);
}
printf("%lf\n", retval);
return 0;
}
<<<
with -O2 -fsched2-use-superblocks -> Observe that 8.000 is printed (incorrect)
and with -O2 -> Observe that 13.000 is printed (correct)
Examining the generated assemblies, it shows that the loop does not include
the add operation:
>>> (loop code for -O2 -fsched2-use-superblocks)
.L10:
fstp %st(0)
.L5:
movzbl (%edx), %eax
incl %edx
pushl %eax
fildl (%esp)
addl $4, %esp
cmpl %edx, %ecx
ja .L10 <- this seems one instruction to early
faddp %st, %st(1)
<<<
I'm not much into assembly, but i thought you might want to have a look at it.
--
Summary: -fsched2-use-superblock produces wrong code
Product: gcc
Version: 4.1.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: sg313d at gmail dot com
GCC build triplet: gcc-4.1.1/vanilla
GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28726
