Re: [3.1.1] Re: GCC performance regression - its memset!

[Jack Lloyd <lloyd at acm dot jhu dot edu>]

This looks to be an endian issue.

Some code:
------------- CUT ----------------
#include <stdio.h>

void foo(void *x, int y)
   {
   __builtin_memset(x, 0, y);
   }

int main()
   {
   int x[2] = { 0x5a5a5a5a, 0x5a5a5a5a };
   printf("%08X %08X\n", x[0], x[1]);
   foo(x, 5);
   printf("%08X %08X\n", x[0], x[1]);
   return 0;
   }
------------- CUT ----------------

Let's try this on x86:

5A5A5A5A 5A5A5A5A
00000000 5A5A5A00

And again on SPARC:

5A5A5A5A 5A5A5A5A
00000000 005A5A5A

where it comes out what you expect, at least if you expect what I expect
(or something...)

And, honestly, I'm not sure if this is how it's supposed to be or not.
What are the rules for doing a memset across part of an object? Certainly,
in both cases it sets 5 bytes to zero, and those bytes are contiguous in
memory. The thing is, I can't imagine how you could actually change this on
x86 to work like it does on a big-endian machine, because you would have to
know the size of the type involved.

HTH,
  -Jack

On Mon, 20 May 2002, Glen Nakamura wrote:

> > Hi Jakub,
> >
> > As I've mentioned in a previous e-mail, I believe that this test
> > case is poorly formed; quite clearly the x argument to foo is
> > incorrectly aligned for the type.  On many platforms, the above
> > code (and even with foo defined as "*x = 0;") will fail at runtime.
> >
> > The benefit of Jan's patch is that on machines without alignment
> > restrictions, we'll still produce the "expected behaviour" for
> > incorrectly written programs.  Mozilla's source code is incorrect
> > and needs to be fixed (use void* rather than void**).  This isn't
> > really a correctness PR, more a quality of implementation for
> > undefined behaviour PR.
> >
> > Roger
>
> Aloha,
>
> FWIW, I think this really is a regression...  The problem is really
> that memset doesn't properly set bytes at the tail of the buffer.
> How about this testcase?
>
> - Glen Nakamura
>
>
> /* PR optimization/6703 */
> extern void abort (void);
> extern void exit (int);
>
> void foo (int *x, int y)
> {
>   __builtin_memset (x, 0, y);
> }
>
> int main ()
> {
>   int x[2] = { 0x5a5a5a5a, 0x5a5a5a5a };
>
>   if (x[1] != 0x5a5a5a5a)
>       abort ();
>   foo (x, 5);
>   if (x[1] == 0x5a5a5a5a)
>       abort ();
>   exit (0);
> }
>