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Re: gcc 4.3.2 vectorizes access to volatile array
H.J. Lu wrote:
> On Mon, Jun 22, 2009 at 11:14 AM, Till
> Straumann<strauman@slac.stanford.edu> wrote:
>> 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)
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>
> That may be too old. Gcc 4.3.4 revision 148680
> generates:
>
> .L5:
> leaq (%rsi,%rdx), %rax
> movzbl (%rax), %eax
> movb %al, (%rdi,%rdx)
> addq $1, %rdx
> cmpq $32, %rdx
> jne .L5
4.4.0 20090307 generates truly bizarre code, though:
gcc -m32 -c -S -O3 x.c
c:
pushl %ebp
movl %esp, %ebp
pushl %ebx
movl 12(%ebp), %edx
movl 8(%ebp), %ebx
movl %edx, %ecx
orl %ebx, %ecx
andl $3, %ecx
leal 4(%ebx), %eax
je .L10
.L2:
xorl %eax, %eax
.p2align 4,,7
.p2align 3
.L5:
leal (%edx,%eax), %ecx
movzbl (%ecx), %ecx
movb %cl, (%ebx,%eax)
addl $1, %eax
cmpl $32, %eax
jne .L5
popl %ebx
popl %ebp
ret
.p2align 4,,7
.p2align 3
.L10:
leal 4(%edx), %ecx
cmpl %ecx, %ebx
jbe .L11
.L7:
movzbl (%edx), %ecx
movl (%edx), %ecx
movl %ecx, (%ebx)
movzbl 1(%edx), %ecx
movl 4(%edx), %ecx
movl %ecx, 4(%ebx)
movzbl 2(%edx), %ecx
movl 8(%edx), %ecx
movl %ecx, 4(%eax)
movzbl 3(%edx), %ecx
movl 12(%edx), %ecx
movl %ecx, 8(%eax)
movzbl 4(%edx), %ecx
movl 16(%edx), %ecx
movl %ecx, 12(%eax)
movzbl 5(%edx), %ecx
movl 20(%edx), %ecx
movl %ecx, 16(%eax)
movzbl 6(%edx), %ebx
leal 24(%edx), %ecx
movl 24(%edx), %ebx
movl %ebx, 20(%eax)
movzbl 7(%edx), %edx
movl 4(%ecx), %edx
movl %edx, 24(%eax)
popl %ebx
popl %ebp
ret
.p2align 4,,7
.p2align 3
.L11:
cmpl %edx, %eax
jae .L2
jmp .L7