gcc 3.3 -03 produces bad code

gcc 3.3 -03 produces bad code

[Ronald Kukuck]

[-- Attachment #1: Type: text/plain, Size: 620 bytes --]

Hello,

I'm using:

gcc version 3.3 20030226 (prerelease) (SuSE Linux)

A large application (~25MB), which worked fine with the gcc included
in Suse 8.1, is now crashing in some modules if I compile it with "-O3".

I have tried to make the bug reproducible in a small file (test1.C).
To see it working OK use:

g++ -O -o test test1.C

to see it fail use:

g++ -O3 -funroll-loops -mcpu=pentiumpro -o test test1.C

or use simply:

g++ -O3 -o test test1.C

which is producing different results then above one.


Is there a workaround for this problem or is there a place where I can 
download a new binary?

Thanks

Ronald

[-- Attachment #2: test1.C --]
[-- Type: text/plain, Size: 1826 bytes --]

//
// g++  -O -o test test1.C          
// OK
//
// g++ -O3 -funroll-loops -Wuninitialized -mcpu=pentiumpro -Wno-deprecated -o test test1.C
// BUG

#include <cstring>
#include <cstdlib>
#include <iostream>

using namespace std;

#define NIL 1

short s_in[12];


class Cq_rab {
   public:
   int first;
   int last;
   int akt;
   int pred;
   int succ;
   int next;
   Cq_rab() {
      first=NIL;
      last=NIL;
      akt=NIL;
      pred=NIL;
      succ=NIL;
      next=NIL;
   };
};

inline int ReadDbRecHead(int *pred,int *succ,int *next, short *infopa, const int rec_no, const int dbunit) {
      int *adress, *iptr;
      
     // DbsFindReadAdr(dbunit, rec_no);
      adress = (int*)&s_in;
      iptr=(int*)infopa;
      *pred=*adress++;
      *succ=*adress++;
      *next=*adress++;
      *iptr++=*adress++;
      *iptr=*adress;
      return(*adress);
   }
   

inline void Gthdpa(int *objide, int *objqcn, int *objiln, int *objcln, Cq_rab *rab, int rcunit) {
      short infopa[4];
      
      ReadDbRecHead(&rab->pred,&rab->succ,&rab->next,infopa,rab->akt,rcunit);
      *objide = infopa[0];
      *objqcn = infopa[1];
      *objcln = infopa[2];
      *objiln = infopa[3];
      return;
   }


int main() {
   int      objcln, objide, objiln, objqcn;
   int   unit = 81;
   Cq_rab  rab;       
   s_in[6]=11111;
   s_in[7]=12222;
   s_in[8]=13333;
   s_in[9]=14444;
   //
   // If you uncomment the following lines, only the second line(value) is
   // wrong 
   //
   /* 
   short s_out[4];
   int   out[4];
   memcpy(s_out,s_in,8);
   out[0] = s_out[0];
   out[1] = s_out[1];
   out[2] = s_out[2];
   out[3] = s_out[3];
   */
   Gthdpa(&objide,&objqcn,&objiln,&objcln,&rab,unit);
   
   cout << objide << endl;
   cout << objqcn << endl;
   cout << objcln << endl;
   cout << objiln << endl;   
   return(0);
}   

Re: gcc 3.3 -03 produces bad code

[Daniel Jacobowitz]

On Thu, Apr 17, 2003 at 09:32:23PM +0200, Ronald Kukuck wrote:
> Hello,
> 
> I'm using:
> 
> gcc version 3.3 20030226 (prerelease) (SuSE Linux)
> 
> A large application (~25MB), which worked fine with the gcc included
> in Suse 8.1, is now crashing in some modules if I compile it with "-O3".
> 
> I have tried to make the bug reproducible in a small file (test1.C).
> To see it working OK use:
> 
> g++ -O -o test test1.C
> 
> to see it fail use:
> 
> g++ -O3 -funroll-loops -mcpu=pentiumpro -o test test1.C
> 
> or use simply:
> 
> g++ -O3 -o test test1.C
> 
> which is producing different results then above one.
> 
> 
> Is there a workaround for this problem or is there a place where I can 
> download a new binary?

> inline int ReadDbRecHead(int *pred,int *succ,int *next, short *infopa, const int rec_no, const int dbunit) {
>       int *adress, *iptr;
>       
>      // DbsFindReadAdr(dbunit, rec_no);
>       adress = (int*)&s_in;
>       iptr=(int*)infopa;
>       *pred=*adress++;
>       *succ=*adress++;
>       *next=*adress++;
>       *iptr++=*adress++;
>       *iptr=*adress;
>       return(*adress);
>    }

You can't cast a short * to an int * that way.  I recommend searching
for information on type aliasing, or seeing the effect of
-fno-strict-aliasing on your code.


-- 
Daniel Jacobowitz
MontaVista Software                         Debian GNU/Linux Developer

Re: type aliasing (was: gcc 3.3 -03 produces bad code)

[Tolga Dalman]

hello, 

i missed to reply to the group, only Daniel got the message. sorry about
that.

On Thu, 17 Apr 2003 18:25:24 -0400 Daniel Jacobowitz <drow@mvista.com> wrote:

> On Fri, Apr 18, 2003 at 12:21:54AM +0200, Tolga Dalman wrote:
> > hello, 
> > 
> > On Thu, 17 Apr 2003 16:04:24 -0400 Daniel Jacobowitz <drow@mvista.com>
> > wrote:
> > 
> > > On Thu, Apr 17, 2003 at 09:32:23PM +0200, Ronald Kukuck wrote:
> > > > Hello,
> > > > 
> > > > I'm using:
> > > > 
> > > > gcc version 3.3 20030226 (prerelease) (SuSE Linux)
> > > > 
> > > > A large application (~25MB), which worked fine with the gcc included
> > > > in Suse 8.1, is now crashing in some modules if I compile it with "-O3".
> > > > 
> > > > I have tried to make the bug reproducible in a small file (test1.C).
> > > > To see it working OK use:
> > > > 
> > > > g++ -O -o test test1.C
> > > > 
> > > > to see it fail use:
> > > > 
> > > > g++ -O3 -funroll-loops -mcpu=pentiumpro -o test test1.C
> > > > 
> > > > or use simply:
> > > > 
> > > > g++ -O3 -o test test1.C
> > > > 
> > > > which is producing different results then above one.
> > > > 
> > > > 
> > > > Is there a workaround for this problem or is there a place where I can 
> > > > download a new binary?
> > > 
> > > > inline int ReadDbRecHead(int *pred,int *succ,int *next, short *infopa,
> > > > const int rec_no, const int dbunit) {
> > > >       int *adress, *iptr;
> > > >       
> > > >      // DbsFindReadAdr(dbunit, rec_no);
> > > >       adress = (int*)&s_in;
> > > >       iptr=(int*)infopa;
> > > >       *pred=*adress++;
> > > >       *succ=*adress++;
> > > >       *next=*adress++;
> > > >       *iptr++=*adress++;
> > > >       *iptr=*adress;
> > > >       return(*adress);
> > > >    }
> > > 
> > > You can't cast a short * to an int * that way.  I recommend searching
> > > for information on type aliasing, or seeing the effect of
> > > -fno-strict-aliasing on your code.
> > 
> > well, i was wondering about type aliasing and the effect of
> > -fno-strict-aliasing for a long time. could you give a brief explaination
> > about aliasing? the example in the man page was not really enlightening to
> > me :( when "should" i use -fno-strict-aliasing and when not?
> 
> You should never use it - fix the code.  I really can't explain it any
> better than that example.

can't there be a case, where it can't be avoided to violate aliasing rules?
i haven't got an example handy, but as far as i understood, i can't access to
the low 8-bit data of a 32-bit variable directly, right?

Tolga Dalman.

Re: type aliasing (was: gcc 3.3 -03 produces bad code)

[Daniel Jacobowitz]

On Fri, Apr 18, 2003 at 12:32:24AM +0200, Tolga Dalman wrote:
> hello, 
> 
> i missed to reply to the group, only Daniel got the message. sorry about
> that.
> 
> On Thu, 17 Apr 2003 18:25:24 -0400 Daniel Jacobowitz <drow@mvista.com> wrote:
> 
> > On Fri, Apr 18, 2003 at 12:21:54AM +0200, Tolga Dalman wrote:
> > > hello, 
> > > 
> > > On Thu, 17 Apr 2003 16:04:24 -0400 Daniel Jacobowitz <drow@mvista.com>
> > > wrote:
> > > 
> > > > On Thu, Apr 17, 2003 at 09:32:23PM +0200, Ronald Kukuck wrote:
> > > > > Hello,
> > > > > 
> > > > > I'm using:
> > > > > 
> > > > > gcc version 3.3 20030226 (prerelease) (SuSE Linux)
> > > > > 
> > > > > A large application (~25MB), which worked fine with the gcc included
> > > > > in Suse 8.1, is now crashing in some modules if I compile it with "-O3".
> > > > > 
> > > > > I have tried to make the bug reproducible in a small file (test1.C).
> > > > > To see it working OK use:
> > > > > 
> > > > > g++ -O -o test test1.C
> > > > > 
> > > > > to see it fail use:
> > > > > 
> > > > > g++ -O3 -funroll-loops -mcpu=pentiumpro -o test test1.C
> > > > > 
> > > > > or use simply:
> > > > > 
> > > > > g++ -O3 -o test test1.C
> > > > > 
> > > > > which is producing different results then above one.
> > > > > 
> > > > > 
> > > > > Is there a workaround for this problem or is there a place where I can 
> > > > > download a new binary?
> > > > 
> > > > > inline int ReadDbRecHead(int *pred,int *succ,int *next, short *infopa,
> > > > > const int rec_no, const int dbunit) {
> > > > >       int *adress, *iptr;
> > > > >       
> > > > >      // DbsFindReadAdr(dbunit, rec_no);
> > > > >       adress = (int*)&s_in;
> > > > >       iptr=(int*)infopa;
> > > > >       *pred=*adress++;
> > > > >       *succ=*adress++;
> > > > >       *next=*adress++;
> > > > >       *iptr++=*adress++;
> > > > >       *iptr=*adress;
> > > > >       return(*adress);
> > > > >    }
> > > > 
> > > > You can't cast a short * to an int * that way.  I recommend searching
> > > > for information on type aliasing, or seeing the effect of
> > > > -fno-strict-aliasing on your code.
> > > 
> > > well, i was wondering about type aliasing and the effect of
> > > -fno-strict-aliasing for a long time. could you give a brief explaination
> > > about aliasing? the example in the man page was not really enlightening to
> > > me :( when "should" i use -fno-strict-aliasing and when not?
> > 
> > You should never use it - fix the code.  I really can't explain it any
> > better than that example.
> 
> can't there be a case, where it can't be avoided to violate aliasing rules?
> i haven't got an example handy, but as far as i understood, i can't access to
> the low 8-bit data of a 32-bit variable directly, right?

If you use "char *", that's not an aliasing violation.  It's often not
the best way to do it, but it's not an aliasing violation.

-- 
Daniel Jacobowitz
MontaVista Software                         Debian GNU/Linux Developer

Re: type aliasing (was: gcc 3.3 -03 produces bad code)

[Tolga Dalman]

On Thu, 17 Apr 2003 18:41:49 -0400 Daniel Jacobowitz <drow@mvista.com> wrote:

> On Fri, Apr 18, 2003 at 12:32:24AM +0200, Tolga Dalman wrote:
> > hello, 
> > 
> > i missed to reply to the group, only Daniel got the message. sorry about
> > that.
> > 
> > On Thu, 17 Apr 2003 18:25:24 -0400 Daniel Jacobowitz <drow@mvista.com>
> > wrote:
> > 
> > > On Fri, Apr 18, 2003 at 12:21:54AM +0200, Tolga Dalman wrote:
> > > > hello, 
> > > > 
> > > > On Thu, 17 Apr 2003 16:04:24 -0400 Daniel Jacobowitz <drow@mvista.com>
> > > > wrote:
> > > > 
> > > > > On Thu, Apr 17, 2003 at 09:32:23PM +0200, Ronald Kukuck wrote:
> > > > > > Hello,
> > > > > > 
> > > > > > I'm using:
> > > > > > 
> > > > > > gcc version 3.3 20030226 (prerelease) (SuSE Linux)
> > > > > > 
> > > > > > A large application (~25MB), which worked fine with the gcc included
> > > > > > in Suse 8.1, is now crashing in some modules if I compile it with
> > > > > > "-O3".
> > > > > > 
> > > > > > I have tried to make the bug reproducible in a small file (test1.C).
> > > > > > To see it working OK use:
> > > > > > 
> > > > > > g++ -O -o test test1.C
> > > > > > 
> > > > > > to see it fail use:
> > > > > > 
> > > > > > g++ -O3 -funroll-loops -mcpu=pentiumpro -o test test1.C
> > > > > > 
> > > > > > or use simply:
> > > > > > 
> > > > > > g++ -O3 -o test test1.C
> > > > > > 
> > > > > > which is producing different results then above one.
> > > > > > 
> > > > > > 
> > > > > > Is there a workaround for this problem or is there a place where I
> > > > > > can download a new binary?
> > > > > 
> > > > > > inline int ReadDbRecHead(int *pred,int *succ,int *next, short
> > > > > > *infopa, const int rec_no, const int dbunit) {
> > > > > >       int *adress, *iptr;
> > > > > >       
> > > > > >      // DbsFindReadAdr(dbunit, rec_no);
> > > > > >       adress = (int*)&s_in;
> > > > > >       iptr=(int*)infopa;
> > > > > >       *pred=*adress++;
> > > > > >       *succ=*adress++;
> > > > > >       *next=*adress++;
> > > > > >       *iptr++=*adress++;
> > > > > >       *iptr=*adress;
> > > > > >       return(*adress);
> > > > > >    }
> > > > > 
> > > > > You can't cast a short * to an int * that way.  I recommend searching
> > > > > for information on type aliasing, or seeing the effect of
> > > > > -fno-strict-aliasing on your code.
> > > > 
> > > > well, i was wondering about type aliasing and the effect of
> > > > -fno-strict-aliasing for a long time. could you give a brief
> > > > explaination about aliasing? the example in the man page was not really
> > > > enlightening to me :( when "should" i use -fno-strict-aliasing and when
> > > > not?
> > > 
> > > You should never use it - fix the code.  I really can't explain it any
> > > better than that example.
> > 
> > can't there be a case, where it can't be avoided to violate aliasing rules?
> > i haven't got an example handy, but as far as i understood, i can't access
> > to the low 8-bit data of a 32-bit variable directly, right?
> 
> If you use "char *", that's not an aliasing violation.  It's often not
> the best way to do it, but it's not an aliasing violation.

sorry for insisting, but what is it then? my example was not meant to be a
"char*", but how about:

uint32_t i = /* a value */;
uint8_t  l; 

l = ((uint8_t*) &i)[1];

would that be a violation? or this one:

char* c = "teststring";
int* p;

for (p = (int*) c; p; p++) {
     /* do something */
}


thanks, 
Tolga Dalman.

Re: type aliasing (was: gcc 3.3 -03 produces bad code)

[Daniel Jacobowitz]

On Fri, Apr 18, 2003 at 12:59:45AM +0200, Tolga Dalman wrote:
> On Thu, 17 Apr 2003 18:41:49 -0400 Daniel Jacobowitz <drow@mvista.com> wrote:
> 
> > On Fri, Apr 18, 2003 at 12:32:24AM +0200, Tolga Dalman wrote:
> > > hello, 
> > > 
> > > i missed to reply to the group, only Daniel got the message. sorry about
> > > that.
> > > 
> > > On Thu, 17 Apr 2003 18:25:24 -0400 Daniel Jacobowitz <drow@mvista.com>
> > > wrote:
> > > 
> > > > On Fri, Apr 18, 2003 at 12:21:54AM +0200, Tolga Dalman wrote:
> > > > > hello, 
> > > > > 
> > > > > On Thu, 17 Apr 2003 16:04:24 -0400 Daniel Jacobowitz <drow@mvista.com>
> > > > > wrote:
> > > > > 
> > > > > > On Thu, Apr 17, 2003 at 09:32:23PM +0200, Ronald Kukuck wrote:
> > > > > > > Hello,
> > > > > > > 
> > > > > > > I'm using:
> > > > > > > 
> > > > > > > gcc version 3.3 20030226 (prerelease) (SuSE Linux)
> > > > > > > 
> > > > > > > A large application (~25MB), which worked fine with the gcc included
> > > > > > > in Suse 8.1, is now crashing in some modules if I compile it with
> > > > > > > "-O3".
> > > > > > > 
> > > > > > > I have tried to make the bug reproducible in a small file (test1.C).
> > > > > > > To see it working OK use:
> > > > > > > 
> > > > > > > g++ -O -o test test1.C
> > > > > > > 
> > > > > > > to see it fail use:
> > > > > > > 
> > > > > > > g++ -O3 -funroll-loops -mcpu=pentiumpro -o test test1.C
> > > > > > > 
> > > > > > > or use simply:
> > > > > > > 
> > > > > > > g++ -O3 -o test test1.C
> > > > > > > 
> > > > > > > which is producing different results then above one.
> > > > > > > 
> > > > > > > 
> > > > > > > Is there a workaround for this problem or is there a place where I
> > > > > > > can download a new binary?
> > > > > > 
> > > > > > > inline int ReadDbRecHead(int *pred,int *succ,int *next, short
> > > > > > > *infopa, const int rec_no, const int dbunit) {
> > > > > > >       int *adress, *iptr;
> > > > > > >       
> > > > > > >      // DbsFindReadAdr(dbunit, rec_no);
> > > > > > >       adress = (int*)&s_in;
> > > > > > >       iptr=(int*)infopa;
> > > > > > >       *pred=*adress++;
> > > > > > >       *succ=*adress++;
> > > > > > >       *next=*adress++;
> > > > > > >       *iptr++=*adress++;
> > > > > > >       *iptr=*adress;
> > > > > > >       return(*adress);
> > > > > > >    }
> > > > > > 
> > > > > > You can't cast a short * to an int * that way.  I recommend searching
> > > > > > for information on type aliasing, or seeing the effect of
> > > > > > -fno-strict-aliasing on your code.
> > > > > 
> > > > > well, i was wondering about type aliasing and the effect of
> > > > > -fno-strict-aliasing for a long time. could you give a brief
> > > > > explaination about aliasing? the example in the man page was not really
> > > > > enlightening to me :( when "should" i use -fno-strict-aliasing and when
> > > > > not?
> > > > 
> > > > You should never use it - fix the code.  I really can't explain it any
> > > > better than that example.
> > > 
> > > can't there be a case, where it can't be avoided to violate aliasing rules?
> > > i haven't got an example handy, but as far as i understood, i can't access
> > > to the low 8-bit data of a 32-bit variable directly, right?
> > 
> > If you use "char *", that's not an aliasing violation.  It's often not
> > the best way to do it, but it's not an aliasing violation.
> 
> sorry for insisting, but what is it then? my example was not meant to be a
> "char*", but how about:
> 
> uint32_t i = /* a value */;
> uint8_t  l; 
> 
> l = ((uint8_t*) &i)[1];
> 
> would that be a violation? or this one:
> 
> char* c = "teststring";
> int* p;
> 
> for (p = (int*) c; p; p++) {
>      /* do something */
> }

If it's a char *, then it's OK.  If it's a uint8_t, then it's a
violation.  I'm not sure if the second one would be a problem but I
think it might be, since you're accessing char data through an int*. 
You have to access an object through a pointer of the appropriate type,
OR a pointer to char.

I seriously recommend picking up a copy of the language standard, which
explains this quite well.

-- 
Daniel Jacobowitz
MontaVista Software                         Debian GNU/Linux Developer

Re: type aliasing (was: gcc 3.3 -03 produces bad code)

[Tolga Dalman]

On Thu, 17 Apr 2003 19:22:54 -0400 Daniel Jacobowitz <drow@mvista.com> wrote:

> On Fri, Apr 18, 2003 at 12:59:45AM +0200, Tolga Dalman wrote:
> > On Thu, 17 Apr 2003 18:41:49 -0400 Daniel Jacobowitz <drow@mvista.com>
> > wrote:
> > 
> > > On Fri, Apr 18, 2003 at 12:32:24AM +0200, Tolga Dalman wrote:
> > > > hello, 
> > > > 
> > > > i missed to reply to the group, only Daniel got the message. sorry about
> > > > that.
> > > > 
> > > > On Thu, 17 Apr 2003 18:25:24 -0400 Daniel Jacobowitz <drow@mvista.com>
> > > > wrote:
> > > > 
> > > > > On Fri, Apr 18, 2003 at 12:21:54AM +0200, Tolga Dalman wrote:
> > > > > > hello, 
> > > > > > 
> > > > > > On Thu, 17 Apr 2003 16:04:24 -0400 Daniel Jacobowitz
> > > > > > <drow@mvista.com> wrote:
> > > > > > 
> > > > > > > On Thu, Apr 17, 2003 at 09:32:23PM +0200, Ronald Kukuck wrote:
> > > > > > > > Hello,
> > > > > > > > 
> > > > > > > > I'm using:
> > > > > > > > 
> > > > > > > > gcc version 3.3 20030226 (prerelease) (SuSE Linux)
> > > > > > > > 
> > > > > > > > A large application (~25MB), which worked fine with the gcc
> > > > > > > > included in Suse 8.1, is now crashing in some modules if I
> > > > > > > > compile it with"-O3".
> > > > > > > > 
> > > > > > > > I have tried to make the bug reproducible in a small file
> > > > > > > > (test1.C). To see it working OK use:
> > > > > > > > 
> > > > > > > > g++ -O -o test test1.C
> > > > > > > > 
> > > > > > > > to see it fail use:
> > > > > > > > 
> > > > > > > > g++ -O3 -funroll-loops -mcpu=pentiumpro -o test test1.C
> > > > > > > > 
> > > > > > > > or use simply:
> > > > > > > > 
> > > > > > > > g++ -O3 -o test test1.C
> > > > > > > > 
> > > > > > > > which is producing different results then above one.
> > > > > > > > 
> > > > > > > > 
> > > > > > > > Is there a workaround for this problem or is there a place where
> > > > > > > > I can download a new binary?
> > > > > > > 
> > > > > > > > inline int ReadDbRecHead(int *pred,int *succ,int *next, short
> > > > > > > > *infopa, const int rec_no, const int dbunit) {
> > > > > > > >       int *adress, *iptr;
> > > > > > > >       
> > > > > > > >      // DbsFindReadAdr(dbunit, rec_no);
> > > > > > > >       adress = (int*)&s_in;
> > > > > > > >       iptr=(int*)infopa;
> > > > > > > >       *pred=*adress++;
> > > > > > > >       *succ=*adress++;
> > > > > > > >       *next=*adress++;
> > > > > > > >       *iptr++=*adress++;
> > > > > > > >       *iptr=*adress;
> > > > > > > >       return(*adress);
> > > > > > > >    }
> > > > > > > 
> > > > > > > You can't cast a short * to an int * that way.  I recommend
> > > > > > > searching for information on type aliasing, or seeing the effect
> > > > > > > of-fno-strict-aliasing on your code.
> > > > > > 
> > > > > > well, i was wondering about type aliasing and the effect of
> > > > > > -fno-strict-aliasing for a long time. could you give a brief
> > > > > > explaination about aliasing? the example in the man page was not
> > > > > > really enlightening to me :( when "should" i use
> > > > > > -fno-strict-aliasing and when not?
> > > > > 
> > > > > You should never use it - fix the code.  I really can't explain it any
> > > > > better than that example.
> > > > 
> > > > can't there be a case, where it can't be avoided to violate aliasing
> > > > rules? i haven't got an example handy, but as far as i understood, i
> > > > can't access to the low 8-bit data of a 32-bit variable directly, right?
> > > 
> > > If you use "char *", that's not an aliasing violation.  It's often not
> > > the best way to do it, but it's not an aliasing violation.
> > 
> > sorry for insisting, but what is it then? my example was not meant to be a
> > "char*", but how about:
> > 
> > uint32_t i = /* a value */;
> > uint8_t  l; 
> > 
> > l = ((uint8_t*) &i)[1];
> > 
> > would that be a violation? or this one:
> > 
> > char* c = "teststring";
> > int* p;
> > 
> > for (p = (int*) c; p; p++) {
> >      /* do something */
> > }
> 
> If it's a char *, then it's OK.  If it's a uint8_t, then it's a
> violation.  I'm not sure if the second one would be a problem but I
> think it might be, since you're accessing char data through an int*. 
> You have to access an object through a pointer of the appropriate type,
> OR a pointer to char.
> 
> I seriously recommend picking up a copy of the language standard, which
> explains this quite well.
> 

after some digging in the iso c99 standard, most of my questions resolved. 
so, whenever i access to a variable with it's type or any sub-type, no violation
will be performed. 

thanks alot...

Re: type aliasing (was: gcc 3.3 -03 produces bad code)

[Andrew Haley]

Tolga Dalman writes:
 > On Thu, 17 Apr 2003 18:41:49 -0400 Daniel Jacobowitz <drow@mvista.com> wrote:
 > 
 > > If you use "char *", that's not an aliasing violation.  It's often not
 > > the best way to do it, but it's not an aliasing violation.
 > 
 > sorry for insisting, but what is it then? my example was not meant to be a
 > "char*", but how about:
 > 
 > uint32_t i = /* a value */;
 > uint8_t  l; 
 > 
 > l = ((uint8_t*) &i)[1];
 > 
 > would that be a violation?

Yes.

 > or this one:
 > 
 > char* c = "teststring";
 > int* p;
 > 
 > for (p = (int*) c; p; p++) {
 >      /* do something */
 > }

Yes.

This isn't the place to have this discussion.  For information about
type based optimization, please read
http://www.ddj.com/documents/s=880/ddj0010d/0010d.htm.  Please also
note that if you want to access data via an lvalue of a different
type, use a union.

Redirected to gcc-help@gcc.gnu.org.

Andrew.