> On 22 Jun 2017, at 09:52, Richard Biener <rguent...@suse.de> wrote:
>
> On Thu, 22 Jun 2017, Richard Biener wrote:
>
>> On Wed, 21 Jun 2017, Richard Biener wrote:
>>
>>>
>>> During my attempt to refactor reduction vectorization I ran across
>>> the special casing of inital values for INTEGER_INDUC_COND_REDUCTION
>>> and tried to see what it is about. So I ended up implementing
>>> cond reduction support for targets w/o REDUC_MAX_EXPR by simply
>>> doing the reduction in scalar code -- while that results in an
>>> expensive epilogue the vector loop should be reasonably fast.
>>>
>>> I still didn't run into any exec FAILs in vect.exp with removing
>>> the INTEGER_INDUC_COND_REDUCTION special case thus the following
>>> patch.
>>>
>>> Alan -- is there a testcase (maybe full bootstrap & regtest will
>>> unconver one) that shows how this is necessary?
>>
>> Testing has some fallout and I discovered what this special-case is
>> about. I'll commit the below testcase to exercise the case.
>
> After fixing the typo (sorry) the testcase also shows a latent
> wrong-code bug. We use an induction vector starting at index 1
> for COND_REDUCTION to allow for a special value of zero but it
> seems the INTEGER_INDUC_COND_REDUCTION special-casing of the
> default value relies on the value of zero being special but
> in the testcase it is meaningful…
>
Yes, looks like you’re right. Apologies.
> In fact, given that for INTEGER_INDUC_COND_REDUCTION the
> values, while based on an induction variable, do not have to
> match 1:1, we can modify the testcase to use last = 2*i; or
> last = i - 1; Which means we have to add a second vector
> to specify whether a lane was set or not, basically use
> the COND_REDUCTION code?
>
We could only allow loops that start from 1, but that would exclude the
most common uses.
I was wondering if we could special case “-1” as the default value index,
but that would break the max operation in the epilogue.
How about, in the loop the vectcond, instead of storing index, could store
index+offset where offset is the value required to get the loop initial
value to 1.
Then in the epilogue, the result is:
“If max == 0 ? default : max - offset"
Using offset would also allow loops that start negative -
for (int i = -2; i < N; i++) would have offset set to 3.
is_nonwrapping_integer_induction() would have to be updated too.
Alan.
> Richard.
>
>> Richard.
>>
>> 2017-06-22 Richard Biener <rguent...@suse.de>
>>
>> * gcc.dg/vect/pr65947-14.c: New testcase.
>>
>> Index: gcc/testsuite/gcc.dg/vect/pr65947-14.c
>> ===================================================================
>> --- gcc/testsuite/gcc.dg/vect/pr65947-14.c (nonexistent)
>> +++ gcc/testsuite/gcc.dg/vect/pr65947-14.c (working copy)
>> @@ -0,0 +1,44 @@
>> +/* { dg-require-effective-target vect_condition } */
>> +
>> +#include "tree-vect.h"
>> +
>> +extern void abort (void) __attribute__ ((noreturn));
>> +
>> +#define N 27
>> +
>> +/* Condition reduction with matches only in even lanes at runtime. */
>> +
>> +int
>> +condition_reduction (int *a, int min_v)
>> +{
>> + int last = N + 96;
>> +
>> + for (int i = 0; i < N; i++)
>> + if (a[i] > min_v)
>> + last = i;
>> +
>> + return last;
>> +}
>> +
>> +int
>> +main (void)
>> +{
>> + int a[N] = {
>> + 47, 12, 13, 14, 15, 16, 17, 18, 19, 20,
>> + 1, 2, 3, 4, 5, 6, 7, 8, 9, 10,
>> + 21, 22, 23, 24, 25, 26, 27
>> + };
>> +
>> + check_vect ();
>> +
>> + int ret = condition_reduction (a, 46);
>> +
>> + /* loop should have found a value of 0, not default N + 96. */
>> + if (ret != 0)
>> + abort ();
>> +
>> + return 0;
>> +}
>> +
>> +/* { dg-final { scan-tree-dump-times "LOOP VECTORIZED" 2 "vect" { xfail { !
>> vect_max_reduc } } } */
>> +/* { dg-final { scan-tree-dump-times "condition expression based on integer
>> induction." 4 "vect" } } */
>>
>
> --
> Richard Biener <rguent...@suse.de>
> SUSE LINUX GmbH, GF: Felix Imendoerffer, Jane Smithard, Graham Norton, HRB
> 21284 (AG Nuernberg)
