Re: GCC 4.3.2 bug (was: Illegal subtraction in tmp-dive_1.s)
On 2009-04-20 15:17:44 +0200, Vincent Lefevre wrote: On 2009-04-17 12:09:42 -0500, Gabriel Dos Reis wrote: At least, let's get it archived on GCC mailing lists. Is it a bug that has been identified? FYI, this has been fixed in the 4.3 branch in r143494. This was PR tree-optimization/36765. -- Vincent Lefèvre vinc...@vinc17.org - Web: http://www.vinc17.org/ 100% accessible validated (X)HTML - Blog: http://www.vinc17.org/blog/ Work: CR INRIA - computer arithmetic / Arenaire project (LIP, ENS-Lyon)
Re: GCC 4.3.2 bug (was: Illegal subtraction in tmp-dive_1.s)
2009/4/19 Jason Mancini jayrus...@hotmail.com: Vincent Lefevre writes: while ((*(q++))-- == 0) ; Is that defined and legal?? Is q incremented before or after *q is decremented? They are both post operators! Jason Mancini It's defined and legal (so long as q != q, which might well be guaranteed by the type system for an incrementable q -- it's late, and I might be missing a counterexample to that). The order of the increment/decrement makes no difference except in the pathological case where they attempt to change the same object (and in that case, the behavior is undefined). Note: the decrement is done to *initial_value_of_q, as q++ evaluates to a copy of q's initial value. q could even be incremented before that, so long as the decrement still applies to *initial_value_of_q. All of this assumes the absence of volatile, of course. -- James
Re: GCC 4.3.2 bug (was: Illegal subtraction in tmp-dive_1.s)
On 2009-04-17 12:09:42 -0500, Gabriel Dos Reis wrote:
At least, let's get it archived on GCC mailing lists.
Is it a bug that has been identified? If not, perhaps this should
be added to the regression tests.
The program without the quotes:
/* With GCC 4.3.2 and -O2 option: output value is 1 instead of 0.
* If -fno-strict-aliasing is added, this bug disappears.
*/
#include stdio.h
#include stdlib.h
int test (int n)
{
unsigned long *p, *q;
int i;
q = p = malloc (n * sizeof (unsigned long));
if (p == NULL)
return 2;
for (i = 0; i n - 1; i++)
p[i] = 0;
p[n - 1] = 1;
while ((*(q++))-- == 0) ;
return p[n - 1] == 1;
}
int main (void)
{
int r;
r = test (17);
printf (%d\n, r);
return r;
}
--
Vincent Lefèvre vinc...@vinc17.org - Web: http://www.vinc17.org/
100% accessible validated (X)HTML - Blog: http://www.vinc17.org/blog/
Work: CR INRIA - computer arithmetic / Arenaire project (LIP, ENS-Lyon)
Re: GCC 4.3.2 bug (was: Illegal subtraction in tmp-dive_1.s)?
On 2009-04-20 00:30:21 -0700, James Dennett wrote: 2009/4/19 Jason Mancini jayrus...@hotmail.com: Vincent Lefevre writes: while ((*(q++))-- == 0) ; Is that defined and legal?? Is q incremented before or after *q is decremented? They are both post operators! It's defined and legal (so long as q != q, which might well be [...] Yes. BTW, I wondered if this could be due to a pathological case such as a[a[i]] = ..., which is undefined when a[i] == i, even though the code looks correct. But this seems to be OK. As the bug occurs only when malloc is in the tested function, I also wondered whether the failure was due to the use of uninitialized data or a buffer overflow (note that in particular, the GMP code was much more complex than my testcase), but again, this is OK. -- Vincent Lefèvre vinc...@vinc17.org - Web: http://www.vinc17.org/ 100% accessible validated (X)HTML - Blog: http://www.vinc17.org/blog/ Work: CR INRIA - computer arithmetic / Arenaire project (LIP, ENS-Lyon)
Re: GCC 4.3.2 bug (was: Illegal subtraction in tmp-dive_1.s)?
As the bug occurs only when malloc is in the tested function, Note that gcc 'knows' that memory obtained by malloc does not alias other memory. You can use a differently named wrapper function for malloc, or use the malloc attribute for another function, to experiment how this affects code generation.
Re: GCC 4.3.2 bug (was: Illegal subtraction in tmp-dive_1.s)?
On 2009-04-20 10:04:00 -0400, Joern Rennecke wrote: As the bug occurs only when malloc is in the tested function, Note that gcc 'knows' that memory obtained by malloc does not alias other memory. Yes, in the case of GMP, this was a GMP internal function, not malloc, but this function is declared with __attribute__ ((malloc)), and the bug disappears if I remove this attribute. I noticed that when I tried to simplify the testcase. You can use a differently named wrapper function for malloc, or use the malloc attribute for another function, to experiment how this affects code generation. I did a test with a wrapper function my_malloc (that just calls malloc and returns its value), but the bug was still visible, perhaps due to optimization. -- Vincent Lefèvre vinc...@vinc17.org - Web: http://www.vinc17.org/ 100% accessible validated (X)HTML - Blog: http://www.vinc17.org/blog/ Work: CR INRIA - computer arithmetic / Arenaire project (LIP, ENS-Lyon)
Re: GCC 4.3.2 bug (was: Illegal subtraction in tmp-dive_1.s)
Vincent Lefevre writes: while ((*(q++))-- == 0) ; Is that defined and legal?? Is q incremented before or after *q is decremented? They are both post operators! Jason Mancini _ Rediscover Hotmail®: Get e-mail storage that grows with you. http://windowslive.com/RediscoverHotmail?ocid=TXT_TAGLM_WL_HM_Rediscover_Storage2_042009
Re: GCC 4.3.2 bug (was: Illegal subtraction in tmp-dive_1.s)
At least, let's get it archived on GCC mailing lists.
On Fri, Apr 17, 2009 at 11:25 AM, Torbjorn Granlund t...@gmplib.org wrote:
Vincent Lefevre vinc...@vinc17.org writes:
FYI, here's a simple testcase:
/* With GCC 4.3.2 and -O2 option: output value is 1 instead of 0.
* If -fno-strict-aliasing is added, this bug disappears.
*/
#include stdio.h
#include stdlib.h
int test (int n)
{
unsigned long *p, *q;
int i;
q = p = malloc (n * sizeof (unsigned long));
if (p == NULL)
return 2;
for (i = 0; i n - 1; i++)
p[i] = 0;
p[n - 1] = 1;
while ((*(q++))-- == 0) ;
return p[n - 1] == 1;
}
int main (void)
{
int r;
r = test (17);
printf (%d\n, r);
return r;
}
You may want to use it in configure to detect the bug there (as not
all users run make check). Possibly add -fno-strict-aliasing if
the bug is detected.
Nice. I think that test should go onto the GCC testsuite.
I suppose a problem is that the code wrt GMP need to be *executed*;
configure feature tests should work also when cross-compiling, I think.
--
Torbjörn
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