bug?

[regehr]

Bugzilla Link	2271
Resolution	FIXED
Resolved on	May 01, 2008 14:14
Version	unspecified
OS	Linux
CC	@sunfishcode

Extended Description

Assuming integer types are 2's complement, the attached code appears to have well-defined behavior. If I am misunderstanding something about C arithmetic then I apologize profusely for wasting peoples' time. The source should be small enough that someone can check this out relatively quickly.

With LLVM from recent svn on x86-32 I get the following:

[regehr@babel tmp1]$ llvm-gcc -O0 -fwrapv small.c ; ./a.out
foo
[regehr@babel tmp1]$ llvm-gcc -O1 -fwrapv small.c ; ./a.out
bar

For what it's worth "foo" is what gcc prints at all optimization levels.

[regehr]

Oops, attachment failed. Here's the program:

#include <stdio.h>

static inline unsigned int
mod_rhs (int rhs)
{
if (rhs == 0)
return 1;
return rhs;
}

int func_11 (unsigned int p_12);
int func_11 (unsigned int p_12)
{
return p_12;
}

int main (void)
{
int g_516 = -9;
int g_511 = 1;
int g_357 = 1;
unsigned int g_296 = -5;
unsigned char g_3 = -5;

if (
(
(g_3 % mod_rhs (
(func_11 (95) - 5) | -5) || 0)) %
mod_rhs (95) & g_511 < (mod_rhs (33))
)
{
printf ("foo\n");
}
else
{
printf ("bar\n");
}
return 0;
}

[lattner]

would it be possible for you to reduce down the testcase a bit more? thanks

[regehr]

Sure try this one out:

#include <stdio.h>

int func_11 (unsigned int p_12);
int func_11 (unsigned int p_12)
{
return p_12;
}

int main (void)
{
unsigned int x = (func_11 (95) - 5) | -5;

// printf ("%d\n", x);

if ((unsigned char)-5 % x)
{
printf ("foo\n");
}
else
{
printf ("bar\n");
}
return 0;
}

[regehr]

Or perhaps this is a bit better. Something funny is going on-- if you uncomment the line that prints x, the behavior changes.

#include <stdio.h>

int func_11 (unsigned int p_12);
int func_11 (unsigned int p_12)
{
return p_12;
}

int main (void)
{
unsigned char y = -5;
unsigned int x = (func_11 (95) - 5) | -5;

//printf ("%d\n", x);
//printf ("%d\n", y);
//printf ("%d\n", (func_11 (95) - 5) | -5);

if (y % x)
{
printf ("foo\n");
}
else
{
printf ("bar\n");
}
return 0;
}

[sunfishcode]

It's a bug in LLVM (instcombine). I'm working on a fix.

[sunfishcode]

This is fixed here:
http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20080428/061860.html