On 07/04/2013 02:43 AM, dw wrote:
> I think I may have answered my own question. While talking about
> clobbers, the docs say:
>
> "You may not write a clobber description in a way that overlaps with an
> input or output operand."
> "There is no way for you to specify that an input operand is modified
> without also specifying it as an output operand."
>
> So (by implication) if you modify an input, you are expected to specify
> that fact, and you can't do that with a clobber. So, reading between
> the lines, the assumption here is that modifying input-only parameters
> is bad.
>
> Which brings us to the next question: If input-only parameters are
> assumed to be unchanged, why are some being re-loaded? These
> unnecessary instructions affect both program size and execution performance.
>
> The code below shows 2 instances of unnecessary assembly code being
> generated:
>
> 1) rdi being loaded (unnecessarily) at 407818.
> 2) eax being zeroed twice.
>
> And I can provide code that shows more.
>
> I'm trying to decide how important this is. If this is only happening
> because I'm using an asm block, that's one thing. While not good, I'm
> not optimistic about what's going to happen if I open a bug that only
> affects performance around inline asm.
>
> On the other hand, it seems possible that this is a more general problem
> where the optimizer is simply missing a category of optimization
> opportunities. Ok, not likely, but possible.
>
> To file or not to file. That is the question.
>
> If anyone has any thoughts or insight to share, I'd love to hear it.
>
> dw
>
> On 7/2/2013 6:41 PM, dw wrote:
>> I'm trying to understand how input parameters are used by gcc's
>> extended asm. Each time I think I've got a handle on it, it does
>> something else unexpected.
>>
>> For example, this c++ code:
>>
>> inline void moo(unsigned char *Dest, unsigned char Data, int Count) {
>> asm volatile (
>> "rep stosb"
>> : /* no outputs */
>> : "D" (Dest), "a" (Data), "c" (Count)
>> : "memory");
>> }
>>
>> int main()
>> {
>> unsigned char buff1[32];
>>
>> moo(buff1, 0, sizeof(buff1));
>> moo(buff1, 0, sizeof(buff1));
>>
>> return 0;
>> }
>>
>> Compiling for 64bit on i386 using -Os, I get:
I get:
main:
.LFB1:
leaq -32(%rsp), %rdi
movl $32, %ecx
xorl %eax, %eax
#APP
# 2 "z.c" 1
rep stosb
# 0 "" 2
# 2 "z.c" 1
rep stosb
# 0 "" 2
#NO_APP
xorl %eax, %eax
ret
By the way, your asm is wrong: stosb modifies its registers, and you
must tell GCC that.
Andrew.
