Re: gcc 4.3.2 vectorizes access to volatile array

[Till Straumann <strauman at slac dot stanford dot edu>]

Andrew Haley wrote:
> Till Straumann wrote:
>   
> > gcc-4.3.2 seems to produce bad code when
> > accessing an array of small 'volatile'
> > objects -- it may try to access multiple
> > such objects in a 'parallel' fashion.
> > E.g., instead of reading two consecutive
> > 'volatile short's sequentially it reads
> > a single 32-bit longword. This may crash
> > e.g., when accessing a memory-mapped device
> > which allows only 16-bit accesses.
> >
> > If I compile this code fragment
> >
> > void volarrcpy(short *d, volatile short *s, int n)
> > {
> > int i;
> >  for (i=0; i<n; i++)
> >    d[i] = s[i];
> > }
> >
> >
> > with '-O3' (the critical option seems to be '-ftree-vectorize')
> > then gcc-4.3.2 produces quite complicated code
> > but the essential section is (powerpc)
> >
> > .L7:
> >    lhz 0,0(11)
> >    addi 11,11,2
> >    lwzx 0,4,9
> >    stwx 0,3,9
> >    addi 9,9,4
> >    bdnz .L7
> >
> > or i386
> >
> > .L7:
> >    movw    (%ecx), %ax
> >    movl    (%esi,%edx,4), %eax
> >    movl    %eax, (%ebx,%edx,4)
> >    incl    %edx
> >    addl    $2, %ecx
> >    cmpl    %edx, -20(%ebp)
> >    ja  .L7
> >
> >
> > Disassembled back into C-code, this reads
> >
> > uint32_t *dst_l = (uint32_t*)d;
> > uint32_t *src_l = (uint32_t*)s;
> >
> > for (i=0; i<n/2; i++) {
> >    d[i]     = s[i];
> >    dst_l[i] = src_l[i];
> > }
> >
> > This code seems neither optimal nor correct.
> > Besides reading half of the locations twice
> > which violates the semantics of volatile
> > objects accessing such objects in a 'vectorized'
> > way (in this case: instead of reading
> > two adjacent short addresses gcc emits
> > a single 32-bit read) seems illegal to me.
> >
> > Similar behavior seems to be present in 4.3.3.
> >
> > Does anybody have some insight? Should I file
> > a bug report?
> >     
>
> I can't reproduce this with "GCC: (GNU) 4.3.3 20081110 (prerelease)"
>
> .L8:
> 	movzwl	(%ecx), %eax
> 	addl	$1, %ebx
> 	addl	$2, %ecx
> 	movw	%ax, (%edx)
> 	addl	$2, %edx
> 	cmpl	%ebx, 16(%ebp)
> 	jg	.L8
>
> I think you should upgrade.
>
> Andrew.
>   

OK, try this then:

void
c(char *d, volatile char *s)
{
int i;
   for ( i=0; i<32; i++ )
       d[i]=s[i];
}


(gcc --version: gcc (Ubuntu 4.3.3-5ubuntu4) 4.3.3)

gcc -m32 -c -S -O3
produces an unrolled sequence:

   movzbl  (%ecx), %eax
   leal    20(%ebx), %edx
   movl    (%ecx), %eax
   movl    %eax, (%edi)
   movzbl  1(%ecx), %eax
   movl    4(%ecx), %eax
   movl    %eax, 4(%edi)
   movzbl  2(%ecx), %eax
   movl    4(%ebx), %eax
   movl    %eax, 4(%esi)
   movzbl  3(%ecx), %eax
   movl    8(%ebx), %eax
   movl    %eax, 8(%esi)
... < snip >...

The 64-bit version even uses SSE registers to
load the volatile data:

(gcc -c -S -O3)

.L7:
   movzbl  (%rsi), %eax
   movdqu  (%rsi), %xmm0
   movdqa  %xmm0, (%rdi)
   movzbl  1(%rsi), %eax
   movdqu  (%rdx), %xmm0
   movdqa  %xmm0, 16(%rdi)

Not sure an upgrade helps ;-)

-- Till