Re: Bug in gcc-20011231

[Peter Barada <pbarada at mail dot wm dot sps dot mot dot com>]

>I modified the code to look like:


#include <stdio.h>

typedef unsigned char ufix8;
typedef unsigned short ufix16;

ufix8 c_val = 1;
ufix8 c_expected = 1;
ufix16 s_val = 1;
ufix16 s_expected = 1;

#define CHECK(val,i,result, expected) \
  result = val % i; \
  if (result != expected) { \
   fprintf(stderr,"Huh? %s %d %% %d = %d\n", (sizeof(val) == 1 ? \
                                              "char" : \
           "short"), val, i, result); \
  }

main(int argc, char *argv[])
{
  ufix8 c_result;
  ufix16 s_result;
  ufix8 i;

  CHECK(c_val, 3, c_result, c_expected);
  CHECK(s_val, 3, s_result, s_expected);
}


And the result is:

[pbarada: /tmp] > /tmp/crap8/bin/i686-linux-gcc -v
Reading specs from /tmp/crap8/bin/../lib/gcc-lib/i686-linux/3.1/specs
Configured with: /home/pbarada/work/cvs-wavemark/cross-linux-tools/gcc-20011231/configure --target=i686-linux --prefix=/tmp/crap8/ --enable-languages=c --with-local-prefix=/tmp/crap/i686-linux
Thread model: single
gcc version 3.1 20011231 (experimental)
[pbarada: /tmp] > /tmp/crap8/bin/i686-linux-gcc -O test2.c
[pbarada: /tmp] > ./a.out
Huh? char 1 % 3 = 132

The assembler for the char modulus is:

.LM2:
	movzbl	c_val, %edx
	movzbw	%dl, %ax
	imull	$-85, %eax, %eax       <----------  0xffffffab
	shrl	$8, %eax
	movb	%al, %cl
	shrb	%cl
	movb	%cl, %al
	addb	%al, %al
	addb	%cl, %al
	movb	%dl, %cl
	subb	%al, %cl
	cmpb	c_expected, %cl
	je	.L2

And for the short is:

.LM3:
	movzwl	s_val, %ecx
	movl	$-1431655765, %edx       <----------  0xaaaaaaab
	movl	%ecx, %eax
	mull	%edx
	shrl	%edx
	leal	(%edx,%edx,2), %edx
	movl	%ecx, %eax
	subl	%edx, %eax
	movzwl	%ax, %edx
	cmpw	%dx, s_expected
	je	.L3


notice that the value calculated for the multiplier is 8 bit accurate
in the char case, and 32 bit accurate in the short case.  This is
because compute_mode in expand_dvmod is QImode in the char case, and
SImode in the short case.

Any ideas???

-- 
Peter Barada                                   Peter.Barada@motorola.com
Wizard                                         781-852-2768 (direct)
WaveMark Solutions(wholly owned by Motorola)   781-270-0193 (fax)