[Bug tree-optimization/20922] New: missed always false conditional

[Bug tree-optimization/20922] New: missed always false conditional

[pinskia at gcc dot gnu dot org]

The following function should just "return 1":
int f(int i)
{
  int i1 = i -2;
  if (i1 > i)
    return 0;
  return 1;
}
We miss this on the tree level, I found this while look into the following fortran code:
SUBROUTINE d ( a, b,n)
IMPLICIT NONE
INTEGER :: n
REAL,DIMENSION(n) :: a
REAL,DIMENSION(n) :: b
b(n-2:n) = sqrt(a(n-2:n))
END SUBROUTINE d


In final_cleanup, we have:
  D.679 = *n;
  S.2 = D.679 - 2;
  if (D.679 < S.2) goto L.1; else goto <L7>;

-- 
           Summary: missed always false conditional
           Product: gcc
           Version: 4.1.0
            Status: UNCONFIRMED
          Keywords: missed-optimization, TREE
          Severity: enhancement
          Priority: P2
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: pinskia at gcc dot gnu dot org
                CC: gcc-bugs at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20922

[Bug tree-optimization/20922] missed always false conditional

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-04-10 21:02 -------
Reduced testcase showing fold does not do it:
int f(int i)
{
  return (i - 2) > i;
}

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20922

[Bug tree-optimization/20922] missed always false conditional

[pinskia at gcc dot gnu dot org]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
OtherBugsDependingO|                            |19986
              nThis|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20922

[Bug tree-optimization/20922] missed always false conditional

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-04-10 21:06 -------
This is not caught untill RTL combine.

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20922

[Bug tree-optimization/20922] missed always false conditional

[kazu at cs dot umass dot edu]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |kazu at cs dot umass dot edu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20922

[Bug tree-optimization/20922] missed always false conditional

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-04-10 23:43 -------
Note this is only true with -fno-wrapv (which is default for C and C++ and I think Fortran too but not 
Java).

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20922

[Bug tree-optimization/20922] missed always false conditional

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-04-11 19:49 -------
Confirmed, the code which does this on the RTL level is in simplify_const_relational_operation.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|                            |1
   Last reconfirmed|0000-00-00 00:00:00         |2005-04-11 19:49:49
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20922

[Bug tree-optimization/20922] missed always false conditional

[phython at gcc dot gnu dot org]

------- Additional Comments From phython at gcc dot gnu dot org  2005-04-16 17:12 -------
Created an attachment (id=8654)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=8654&action=view)
Fold stuff


-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|unassigned at gcc dot gnu   |phython at gcc dot gnu dot
                   |dot org                     |org
             Status|NEW                         |ASSIGNED


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20922

[Bug tree-optimization/20922] missed always false conditional

[cvs-commit at gcc dot gnu dot org]

------- Additional Comments From cvs-commit at gcc dot gnu dot org  2005-04-18 15:18 -------
Subject: Bug 20922

CVSROOT:	/cvs/gcc
Module name:	gcc
Changes by:	phython@gcc.gnu.org	2005-04-18 15:18:22

Modified files:
	gcc            : ChangeLog fold-const.c 
	gcc/testsuite  : ChangeLog 
Added files:
	gcc/testsuite/gcc.dg: pr20922-1.c pr20922-2.c pr20922-3.c 
	                      pr20922-4.c pr20922-5.c pr20922-6.c 

Log message:
	2005-04-18  James A. Morrison  <phython@gcc.gnu.org>
	
	PR tree-optimization/20922
	* fold-const.c (fold_binary): Fold X - c > X and X + c < X to false.
	Fold X + c >= X and fold X - c <= X to true.

Patches:
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/ChangeLog.diff?cvsroot=gcc&r1=2.8345&r2=2.8346
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/fold-const.c.diff?cvsroot=gcc&r1=1.562&r2=1.563
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/testsuite/ChangeLog.diff?cvsroot=gcc&r1=1.5364&r2=1.5365
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/testsuite/gcc.dg/pr20922-1.c.diff?cvsroot=gcc&r1=NONE&r2=1.1
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/testsuite/gcc.dg/pr20922-2.c.diff?cvsroot=gcc&r1=NONE&r2=1.1
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/testsuite/gcc.dg/pr20922-3.c.diff?cvsroot=gcc&r1=NONE&r2=1.1
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/testsuite/gcc.dg/pr20922-4.c.diff?cvsroot=gcc&r1=NONE&r2=1.1
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/testsuite/gcc.dg/pr20922-5.c.diff?cvsroot=gcc&r1=NONE&r2=1.1
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/testsuite/gcc.dg/pr20922-6.c.diff?cvsroot=gcc&r1=NONE&r2=1.1



-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20922

[Bug tree-optimization/20922] missed always false conditional

[phython at gcc dot gnu dot org]

------- Additional Comments From phython at gcc dot gnu dot org  2005-04-18 15:20 -------
Fixed in the last commit.

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20922

[Bug tree-optimization/20922] missed always false conditional

[pinskia at gcc dot gnu dot org]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |4.1.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20922

[Bug tree-optimization/20922] missed always false conditional

[markus at oberhumer dot com]

------- Additional Comments From markus at oberhumer dot com  2005-04-25 00:38 -------
Is this optimization really legal in C/C++? Could some language lawyer look at
this (and check the output when compiling with -O3):

#include <limits.h>

int f1(int i) {  return (i - 2) > i; }
int f2(int i) {  return (i + 2) > i; }

int g1(void)  { return f1(INT_MIN); }
int g2(void)  { return f2(INT_MAX); }


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20922

[Bug tree-optimization/20922] missed always false conditional

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-04-25 00:40 -------
(In reply to comment #15)
> Is this optimization really legal in C/C++? Could some language lawyer look at
> this (and check the output when compiling with -O3):
Overflow/Underflow of signed operands in C/C++ is undefined, now in Java it is not.

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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20922

[Bug tree-optimization/20922] missed always false conditional

[jim dot morrison at gmail dot com]

------- Additional Comments From jim dot morrison at gmail dot com  2005-04-25 00:42 -------
Subject: Re:  missed always false conditional

On 25 Apr 2005 00:39:23 -0000, markus at oberhumer dot com
<gcc-bugzilla@gcc.gnu.org> wrote:
> 
> 
> --
>            What    |Removed                     |Added
> ----------------------------------------------------------------------------
>                  CC|                            |markus at oberhumer dot com
> 
> 
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20922

 The overflow semantics are undefined for signed types in C/C++.
Try with -ftrapv or -fwrapv if you need defined semantics for C/C++.



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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20922

[Bug tree-optimization/20922] missed always false conditional

[markus at oberhumer dot com]

------- Additional Comments From markus at oberhumer dot com  2005-04-25 01:01 -------
Ah, many thanks for the clarification. Still it's somewhat confusing that f1()
and g1() don't agree after inlining, but "undefined" probably means exactly that...

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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20922