[Bug c/23109] New: compiler generates wrong code leading to spurious division by zero

[Bug c/23109] New: compiler generates wrong code leading to spurious division by zero

[p dot van-hoof at qub dot ac dot uk]

The attached code crashes on a division by zero when compiled as follows:

dogbert> gcc -O1 -funsafe-math-optimizations -ftrapping-math b.c
dogbert> a.out
Floating exception
dogbert> gcc -v
Using built-in specs.
Target: i686-pc-linux-gnu
Configured with: ../gcc/configure --prefix=/usr/local/gcc410
--enable-languages=c,c++,f95
Thread model: posix
gcc version 4.1.0 20050725 (experimental)

I don't believe that -funsafe-math-optimizations enables -fno-trapping-math, but
to be really sure I added the -ftrapping-math command line option. It makes no
difference.

Taken at face value it looks like the optimizer lifts the division by d outside
the conditional on line 19, even though that could lead to a spurious division
by zero and -ftrapping-math is in effect.

This behavior only shows up on the mainline, and not on the 3.4 and 4.0
branches. A slightly modified version (to enable FP traps on Suns) also crashes
with gcc 4.1.0 on the target sparc-sun-solaris2.9.

-- 
           Summary: compiler generates wrong code leading to spurious
                    division by zero
           Product: gcc
           Version: 4.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: p dot van-hoof at qub dot ac dot uk
                CC: gcc-bugs at gcc dot gnu dot org
GCC target triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23109

[Bug c/23109] compiler generates wrong code leading to spurious division by zero

[p dot van-hoof at qub dot ac dot uk]

------- Additional Comments From p dot van-hoof at qub dot ac dot uk  2005-07-28 10:42 -------
Created an attachment (id=9377)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=9377&action=view)
code that causes the crash


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23109

[Bug c/23109] compiler generates wrong code leading to spurious division by zero

[rguenth at gcc dot gnu dot org]

------- Additional Comments From rguenth at gcc dot gnu dot org  2005-07-28 14:47 -------
Indeed unsafe-math-optimization is not the only precondition for this
transformation.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|                            |1
   Last reconfirmed|0000-00-00 00:00:00         |2005-07-28 14:47:37
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23109

[Bug c/23109] compiler generates wrong code leading to spurious division by zero

[rguenth at gcc dot gnu dot org]

------- Additional Comments From rguenth at gcc dot gnu dot org  2005-07-28 15:01 -------
I have a patch.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|unassigned at gcc dot gnu   |rguenth at gcc dot gnu dot
                   |dot org                     |org
             Status|NEW                         |ASSIGNED
   Last reconfirmed|2005-07-28 14:47:37         |2005-07-28 15:01:58
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23109

[Bug tree-optimization/23109] [4.1 Regression] compiler generates wrong code leading to spurious division by zero with -funsafe-math-optimizations (instead of -ftrapping-math)

[pinskia at gcc dot gnu dot org]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |minor
          Component|c                           |tree-optimization
            Summary|compiler generates wrong    |[4.1 Regression] compiler
                   |code leading to spurious    |generates wrong code leading
                   |division by zero            |to spurious division by zero
                   |                            |with -funsafe-math-
                   |                            |optimizations (instead of -
                   |                            |ftrapping-math)
   Target Milestone|---                         |4.1.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23109

[Bug tree-optimization/23109] [4.1 Regression] compiler generates wrong code leading to spurious division by zero with -funsafe-math-optimizations (instead of -ftrapping-math)

[cvs-commit at gcc dot gnu dot org]

------- Additional Comments From cvs-commit at gcc dot gnu dot org  2005-08-01 08:53 -------
Subject: Bug 23109

CVSROOT:	/cvs/gcc
Module name:	gcc
Changes by:	rguenth@gcc.gnu.org	2005-08-01 08:53:00

Modified files:
	gcc            : ChangeLog tree-ssa-loop-im.c 
	                 tree-ssa-math-opts.c 

Log message:
	2005-08-01  Richard Guenther  <rguenther@suse.de>
	
	PR tree-optimization/23109
	* tree-ssa-math-opts.c (execute_cse_reciprocals_1):
	If trapping math is in effect, use post-dominator information
	to check if we'd in any case reach a trapping point before
	doing the reciprocal insertion.
	(execute_cse_reciprocals): Compute post-dominators, if necessary.
	* tree-ssa-loop-im.c (determine_invariantness_stmt): RDIV
	expressions are invariant only if trapping math is not in effect.

Patches:
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/ChangeLog.diff?cvsroot=gcc&r1=2.9622&r2=2.9623
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/tree-ssa-loop-im.c.diff?cvsroot=gcc&r1=2.48&r2=2.49
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/tree-ssa-math-opts.c.diff?cvsroot=gcc&r1=2.2&r2=2.3



-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23109

[Bug tree-optimization/23109] [4.1 Regression] compiler generates wrong code leading to spurious division by zero with -funsafe-math-optimizations (instead of -ftrapping-math)

[rguenth at gcc dot gnu dot org]

------- Additional Comments From rguenth at gcc dot gnu dot org  2005-08-01 08:54 -------
Fixed.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|                            |FIXED


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23109