On Thu, Dec 22, 2011 at 9:25 AM, Jiangning Liu <jiangning....@arm.com> wrote:
>> Yes, the number of iterations of the i loop simply is too difficult fo
>> our loop iteration calculator to comprehend:
>>
>> for (i=k; i<500; i+=k)
>>
>> iterates for roundup((500-k)/k) time. In particular if the step is
>> non-constant our nr-of-iteration calculator gives up.
>>
>
> I'm trying to give an even smaller case,
>
> int a[512] ;
> int *a_p ;
>
> int f(int k)
> {
> int i ;
>
> for(i=0; i<k; i++)
> {
> a_p = &a[i] ;
> *a_p = 7 ;
> }
> }
>
> For this case, we have a very simple loop step "i++", then we would have the
> GIMPLE before expand like below,
>
> <bb 5>:
> # i_13 = PHI <i_6(5), 0(4)>
> # ivtmp.10_9 = PHI <ivtmp.10_1(5), ivtmp.10_15(4)>
> a_p_lsm.6_4 = &a[i_13];
> ivtmp.10_1 = ivtmp.10_9 + 4;
> D.4085_16 = (void *) ivtmp.10_1;
> MEM[base: D.4085_16, offset: 0B] = 7;
> i_6 = i_13 + 1;
> if (i_6 != k_3(D))
> goto <bb 5>;
> else
> goto <bb 6>;
>
> <bb 6>:
> # a_p_lsm.6_11 = PHI <a_p_lsm.6_4(5)>
> a_p = a_p_lsm.6_11;
> goto <bb 3>;
>
> Why can't we still sunk &a[i_13] out of loop? For example, I expect to
> generate the code like below,
>
> <bb 5>:
> # i_13 = PHI <i_6(5), 0(4)>
> # ivtmp.10_9 = PHI <ivtmp.10_1(5), ivtmp.10_15(4)>
> i_14 = i_13;
> ivtmp.10_1 = ivtmp.10_9 + 4;
> D.4085_16 = (void *) ivtmp.10_1;
> MEM[base: D.4085_16, offset: 0B] = 7;
> i_6 = i_13 + 1;
> if (i_6 != k_3(D))
> goto <bb 5>;
> else
> goto <bb 6>;
>
> <bb 6>:
> # a_p_lsm.6_11 = PHI <a_p_lsm.6_4(5)>
> a_p_lsm.6_4 = &a[i_14];
> a_p = a_p_lsm.6_11;
> goto <bb 3>;
>
> This way the computation of &a[i] would be saved within the loop.
>
> Any idea?
The job to do this is final value replacement, not sinking (we do not
sink non-invariant expressions - you'd have to translate them through
the loop-closed SSA exit PHI node, certainly doable, patches welcome ;)).
Richard.
> Thanks,
> -Jiangning
>
>
>
