Re: C coding questions for [perl #58182] Unicode problem

[Nicholas Clark <nick@ccl4.org>]

On Sun, Oct 19, 2008 at 08:59:57PM -0600, karl williamson wrote:
> I only wanted to know if I have to be think about exceeding array 
> bounds.  If it's exactly 8 bits and unsigned, there's no way for it to 
> reference outside a 256 element array.

I think that the perl source code is already making that assumption in places.

But I doubt that there's a size or speed penalty in masking it with 0xFF,
as any sane optimiser would spot that it's a no-op and eliminate the code.
I tried:

$ cat index.c
#include <stdlib.h>
#include <stdio.h>

#ifndef MASK
#  define MASK
#endif

static unsigned char buffer[256];

int main (int argc, char **argv) {
  unsigned int count = sizeof(buffer);

  while (count--)
    buffer[count MASK] = ~count;

  while (*++argv) {
    const unsigned char i = (unsigned char) atoi(*argv);
    printf("%s: %d\n", *argv, buffer[i MASK]);
  }

  return 0;
}

    
$ gcc -Wall -c -O index.c 
$ ls -l index.o 
-rw-r--r-- 1 nick nick 1752 Oct 20 19:26 index.o
$ gcc -Wall -o index -O -DMASK='& 255' index.c 
$ ls -l index.o 
-rw-r--r-- 1 nick nick 1752 Oct 20 19:26 index.o


Nicholas Clark

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Follow-Ups from:

Marcus Holland-Moritz <mhx-perl@gmx.net>

References to:

karl williamson <public@khwilliamson.com>
Glenn Linderman <perl@NevCal.com>
karl williamson <public@khwilliamson.com>
"Rafael Garcia-Suarez" <rgarciasuarez@gmail.com>
karl williamson <public@khwilliamson.com>
"Rafael Garcia-Suarez" <rgarciasuarez@gmail.com>
Nicholas Clark <nick@ccl4.org>
karl williamson <public@khwilliamson.com>

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