RE: optimization/10362: Optimization with constant shifts and com pares fails on MIPS

RE: optimization/10362: Optimization with constant shifts and com pares fails on MIPS

[Wolfgang Bangerth]

The following reply was made to PR optimization/10362; it has been noted by GNATS.

From: Wolfgang Bangerth <bangerth@ices.utexas.edu>
To: Berger David-MGI2063 <dberger@motorola.com>
Cc: "'gcc-bugs@gcc.gnu.org'" <gcc-bugs@gcc.gnu.org>,
   "'gcc-gnats@gcc.gnu.org'" <gcc-gnats@gcc.gnu.org>
Subject: RE: optimization/10362: Optimization with constant shifts and com
 pares fails on MIPS
Date: Wed, 9 Apr 2003 18:07:54 -0500 (CDT)

 > volatile unsigned long i;
 > 
 > void test(void)
 > {
 >   i = (((i & 0xf0) >> 4) >= 14); /* This one generates incorrect code with -O2 */
 >   i = ((i & 0xf0) >= (14 << 4)); /* This is what the optimizer is trying to do */
 > }
 
 I think what the compiler actually does, is to transform it to 
   i = ((i & 0xf0) > (14<<4-1));
 However, since the lowest 4 bits of the left hand side of the comparison 
 are provably zero, this is equivalent to
   i = ((i & 0xf0) > (13<<4));
 
 Do you have a value for "i", where you actually get a wrong comparison?
 
 W.
 
 -------------------------------------------------------------------------
 Wolfgang Bangerth              email:            bangerth@ices.utexas.edu
                                www: http://www.ices.utexas.edu/~bangerth/
 
 

Re: optimization/10362: Optimization with constant shifts and com pares fails on MIPS

[Falk Hueffner]

The following reply was made to PR optimization/10362; it has been noted by GNATS.

From: Falk Hueffner <falk.hueffner@student.uni-tuebingen.de>
To: Berger David-MGI2063 <dberger@motorola.com>
Cc: "'bangerth@dealii.org'" <bangerth@dealii.org>,
   "'gcc-bugs@gcc.gnu.org'" <gcc-bugs@gcc.gnu.org>,
   "'gcc-prs@gcc.gnu.org'" <gcc-prs@gcc.gnu.org>,
   "'nobody@gcc.gnu.org'" <nobody@gcc.gnu.org>,
   "'gcc-gnats@gcc.gnu.org'" <gcc-gnats@gcc.gnu.org>
Subject: Re: optimization/10362: Optimization with constant shifts and com pares fails on MIPS
Date: 10 Apr 2003 01:07:44 +0200

 Berger David-MGI2063 <dberger@motorola.com> writes:
 
 > volatile unsigned long i;
 > 
 > void test(void)
 > {
 >   i = (((i & 0xf0) >> 4) >= 14); /* This one generates incorrect code with -O2 */
 >   i = ((i & 0xf0) >= (14 << 4)); /* This is what the optimizer is trying to do */
 > }
 
 The optimization seems valid to me. Can you give an example where it
 fails?
 
 -- 
 	Falk

RE: optimization/10362: Optimization with constant shifts and com pares fails on MIPS

[Berger David-MGI2063]

The following reply was made to PR optimization/10362; it has been noted by GNATS.

From: Berger David-MGI2063 <dberger@motorola.com>
To: "'bangerth@dealii.org'" <bangerth@dealii.org>,
   "'gcc-bugs@gcc.gnu.org'" <gcc-bugs@gcc.gnu.org>,
   "'gcc-prs@gcc.gnu.org'"
	 <gcc-prs@gcc.gnu.org>,
   "'nobody@gcc.gnu.org'" <nobody@gcc.gnu.org>,
   "'gcc-gnats@gcc.gnu.org'" <gcc-gnats@gcc.gnu.org>
Cc:  
Subject: RE: optimization/10362: Optimization with constant shifts and com
	pares fails on MIPS
Date: Wed, 9 Apr 2003 16:02:37 -0700 

 Copy code below to junk.c and compile as follows:  gcc -o junk.s -O2 -S junk.c
 
 volatile unsigned long i;
 
 void test(void)
 {
   i = (((i & 0xf0) >> 4) >= 14); /* This one generates incorrect code with -O2 */
   i = ((i & 0xf0) >= (14 << 4)); /* This is what the optimizer is trying to do */ }
 
 
 -----Original Message-----
 From: bangerth@dealii.org [mailto:bangerth@dealii.org] 
 Sent: Wednesday, April 09, 2003 3:45 PM
 To: David Berger; gcc-bugs@gcc.gnu.org; gcc-prs@gcc.gnu.org; nobody@gcc.gnu.org
 Subject: Re: optimization/10362: Optimization with constant shifts and compares fails on MIPS
 
 
 Synopsis: Optimization with constant shifts and compares fails on MIPS
 
 State-Changed-From-To: open->feedback
 State-Changed-By: bangerth
 State-Changed-When: Wed Apr  9 22:44:59 2003
 State-Changed-Why:
     The attachment got lost. Can you please re-send it as a 
     reply to this mail?
     
     Thanks
       Wolfgang
 
 http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=10362
 

RE: optimization/10362: Optimization with constant shifts and com pares fails on MIPS

[Berger David-MGI2063]

The following reply was made to PR optimization/10362; it has been noted by GNATS.

From: Berger David-MGI2063 <dberger@motorola.com>
To: "'bangerth@dealii.org'" <bangerth@dealii.org>,
   Berger David-MGI2063
	 <dberger@motorola.com>,
   "'gcc-bugs@gcc.gnu.org'" <gcc-bugs@gcc.gnu.org>,
   "'gcc-prs@gcc.gnu.org'" <gcc-prs@gcc.gnu.org>,
   "'nobody@gcc.gnu.org'"
	 <nobody@gcc.gnu.org>,
   "'gcc-gnats@gcc.gnu.org'" <gcc-gnats@gcc.gnu.org>
Cc:  
Subject: RE: optimization/10362: Optimization with constant shifts and com
	pares fails on MIPS
Date: Wed, 9 Apr 2003 16:01:12 -0700 

 volatile unsigned long i;
 
 void test(void)
 {
   i = (((i & 0xf0) >> 4) >= 14); /* This one generates incorrect code with -O2 */
   i = ((i & 0xf0) >= (14 << 4)); /* This is what the optimizer is trying to do */
 }
 
 
 -----Original Message-----
 From: bangerth@dealii.org [mailto:bangerth@dealii.org] 
 Sent: Wednesday, April 09, 2003 3:45 PM
 To: David Berger; gcc-bugs@gcc.gnu.org; gcc-prs@gcc.gnu.org; nobody@gcc.gnu.org
 Subject: Re: optimization/10362: Optimization with constant shifts and compares fails on MIPS
 
 
 Synopsis: Optimization with constant shifts and compares fails on MIPS
 
 State-Changed-From-To: open->feedback
 State-Changed-By: bangerth
 State-Changed-When: Wed Apr  9 22:44:59 2003
 State-Changed-Why:
     The attachment got lost. Can you please re-send it as a 
     reply to this mail?
     
     Thanks
       Wolfgang
 
 http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=10362