[Bug rtl-optimization/27671] [4.2 Regression] optimization error on pentium4-Linux with %, regression from gcc-4.1.0

[pinskia at gcc dot gnu dot org]

--- Comment #6 from pinskia at gcc dot gnu dot org  2006-05-29 16:09 ---
*** Bug 27322 has been marked as a duplicate of this bug. ***


-- 

pinskia at gcc dot gnu dot org changed:

   What|Removed |Added

 CC||martin at mpa-garching dot
   ||mpg dot de


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27671

[Bug rtl-optimization/27671] [4.2 Regression] optimization error on pentium4-Linux with %, regression from gcc-4.1.0

[kazu at gcc dot gnu dot org]

--- Comment #4 from kazu at gcc dot gnu dot org  2006-05-21 15:13 ---
Subject: Bug 27671

Author: kazu
Date: Sun May 21 15:13:36 2006
New Revision: 113955

URL: http://gcc.gnu.org/viewcvs?root=gccview=revrev=113955
Log:
gcc/
PR rtl-optimization/27671
* simplify-rtx.c (simplify_relational_operation_1): Fix
simplifications of (eq/ne (xor x y) y) and
(eq/ne (xor x y) x).

gcc/testsuite/
PR rtl-optimization/27671
* gcc.c-torture/execute/pr27671-1.c: New.
* gcc.dg/pr27671-2.c: Likewise.

Added:
trunk/gcc/testsuite/gcc.c-torture/execute/pr27671-1.c
trunk/gcc/testsuite/gcc.dg/pr27671-2.c
Modified:
trunk/gcc/ChangeLog
trunk/gcc/simplify-rtx.c
trunk/gcc/testsuite/ChangeLog


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27671

[Bug rtl-optimization/27671] [4.2 Regression] optimization error on pentium4-Linux with %, regression from gcc-4.1.0

[kazu at gcc dot gnu dot org]

--- Comment #5 from kazu at gcc dot gnu dot org  2006-05-21 15:17 ---
Just checked in a patch.


-- 

kazu at gcc dot gnu dot org changed:

   What|Removed |Added

 Status|ASSIGNED|RESOLVED
 Resolution||FIXED


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27671

[Bug rtl-optimization/27671] [4.2 Regression] optimization error on pentium4-Linux with %, regression from gcc-4.1.0

[kazu at gcc dot gnu dot org]

--- Comment #3 from kazu at gcc dot gnu dot org  2006-05-20 19:52 ---
Posted a patch.


-- 

kazu at gcc dot gnu dot org changed:

   What|Removed |Added

URL||http://gcc.gnu.org/ml/gcc-
   ||patches/2006-
   ||05/msg01034.html


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27671

[Bug rtl-optimization/27671] [4.2 Regression] optimization error on pentium4-Linux with %, regression from gcc-4.1.0

[kazu at gcc dot gnu dot org]

--- Comment #2 from kazu at gcc dot gnu dot org  2006-05-20 04:57 ---
The whole problem is reduced to the combiner doing wrong simplification
of XOR.  Here are some details.

Consider:

extern void abort (void) __attribute__ ((noreturn));
extern void exit (int) __attribute__ ((noreturn));

static int __attribute__((noinline))
foo (int a, int b)
{
  int c = a ^ b;
  if (c == a)
abort ();
}

int
main (void)
{
  foo (0, 1);
  exit (0);
}

Note that

  c == a  ==  a ^ b == a
  ==  b == 0

So foo aborts if and only if b == 0.  In the above testcase, b gets 1,
so foo shouldn't abort, but it does.

The combiner tries to combine the following two insns.  (I'm omitting
clobbers for brevity.)

(set (reg:SI 61)
 (xor:SI (reg/v:SI 60 [ b ])
 (reg/v:SI 59 [ a ])))

(set (reg:CCZ 17 flags)
 (compare:CCZ (reg:SI 61)
  (reg/v:SI 59 [ a ])))

The combiner produces

(set (reg:CCZ 17 flags)
 (compare:CCZ (reg/v:SI 59 [ a ])
  (const_int 0 [0x0])))

Note that this is wrong.  What matters is whether b is 0 or not.

Once the problem is reduced to this level,
-march=pentium4 is no longer needed.
Is is just that with -march=pentium4, k % 2 is expanded using XOR.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27671

[Bug rtl-optimization/27671] [4.2 Regression] optimization error on pentium4-Linux with %, regression from gcc-4.1.0

[pinskia at gcc dot gnu dot org]

--- Comment #1 from pinskia at gcc dot gnu dot org  2006-05-18 21:19 ---
Combine is causing the code to turn into:
(insn 17 16 18 2 (set (reg:CCGOC 17 flags)
(compare:CCGOC (mem/c/i:SI (reg/f:SI 16 argp) [0 k+0 S4 A32])
(const_int 0 [0x0]))) 3 {*cmpsi_ccno_1} (nil)
(nil))


-- 

pinskia at gcc dot gnu dot org changed:

   What|Removed |Added

 CC||pinskia at gcc dot gnu dot
   ||org
 Status|UNCONFIRMED |NEW
  Component|regression  |rtl-optimization
 Ever Confirmed|0   |1
   Keywords||wrong-code
   Last reconfirmed|-00-00 00:00:00 |2006-05-18 21:19:13
   date||
Summary|optimization error on   |[4.2 Regression]
   |pentium4-Linux with %,  |optimization error on
   |regression from gcc-4.1.0   |pentium4-Linux with %,
   ||regression from gcc-4.1.0
   Target Milestone|--- |4.2.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27671

[Bug rtl-optimization/27671] [4.2 Regression] optimization error on pentium4-Linux with %, regression from gcc-4.1.0

[kazu at gcc dot gnu dot org]

-- 

kazu at gcc dot gnu dot org changed:

   What|Removed |Added

 AssignedTo|unassigned at gcc dot gnu   |kazu at gcc dot gnu dot org
   |dot org |
 Status|NEW |ASSIGNED


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27671