On Fri, Mar 2, 2018 at 3:12 PM, Richard Sandiford
<richard.sandif...@linaro.org> wrote:
> Richard Biener <richard.guent...@gmail.com> writes:
>> On Thu, Mar 1, 2018 at 12:38 PM, Richard Sandiford
>> <richard.sandif...@linaro.org> wrote:
>>> Richard Biener <richard.guent...@gmail.com> writes:
>>>> On Wed, Feb 28, 2018 at 3:20 PM, Richard Sandiford
>>>> <richard.sandif...@linaro.org> wrote:
>>>>> GCC 6 and 7 would vectorise:
>>>>>
>>>>> void
>>>>> f (unsigned long incx, unsigned long incy,
>>>>> float *restrict dx, float *restrict dy)
>>>>> {
>>>>> unsigned long ix = 0, iy = 0;
>>>>> for (unsigned long i = 0; i < 512; ++i)
>>>>> {
>>>>> dy[iy] += dx[ix];
>>>>> ix += incx;
>>>>> iy += incy;
>>>>> }
>>>>> }
>>>>>
>>>>> without first proving that incy is nonzero. This is a regression from
>>>>> GCC 5. It was fixed on trunk in r223486, which versioned the loop based
>>>>> on whether incy is zero, but that's obviously too invasive to backport.
>>>>> This patch instead bails out for non-constant steps in the place that
>>>>> trunk would try a check for zeroness.
>>>>>
>>>>> I did wonder about trying to use range information to prove nonzeroness
>>>>> for SSA_NAMEs, but that would be entirely new code and didn't seem
>>>>> suitable for a release branch.
>>>>>
>>>>> Tested on aarch64-linux-gnu. OK for GCC 7 and 6? I'll add the testcase
>>>>> to trunk too.
>>>>
>>>> Given dist == 0 isn't it enough to test either DR_STEP (dra) or
>>>> DR_STEP (drb)?
>>>> That seems what trunk is doing (just look at dr_zero_step_indicator of
>>>> dra).
>>>> Even when not using range-info I think that using !
>>>> tree_expr_nonzero_p (DR_STEP (dra))
>>>> is more to the point of the issue we're fixing -- that also would catch
>>>> integer_zerop (DR_STEP (dra)) which trunk handles by failing as well but
>>>> your
>>>> patch wouldn't so it looks like a more "complete" fix.
>>>
>>> OK.
>>>
>>>> Last but not least trunk and your patch guards all this by
>>>> !loop->force_vectorize.
>>>> But force_vectorize doesn't give any such guarantee that step isn't
>>>> zero so I wonder
>>>> why we deliberately choose to possibly miscompile stuff here?
>>>
>>> This was based on the pre-existing:
>>>
>>> if (loop_vinfo && integer_zerop (step))
>>> {
>>> GROUP_FIRST_ELEMENT (vinfo_for_stmt (stmt)) = NULL;
>>> if (!nested_in_vect_loop_p (loop, stmt))
>>> return DR_IS_READ (dr);
>>> /* Allow references with zero step for outer loops marked
>>> with pragma omp simd only - it guarantees absence of
>>> loop-carried dependencies between inner loop iterations. */
>>> if (!loop->force_vectorize)
>>> {
>>> if (dump_enabled_p ())
>>> dump_printf_loc (MSG_NOTE, vect_location,
>>> "zero step in inner loop of nest\n");
>>> return false;
>>> }
>>> }
>>>
>>> AIUI #pragma omp simd really does guarantee that iterations of
>>> the loop can be executed concurrently (up to the limit given by
>>> safelen if present). So something like:
>>>
>>> #pragma omp simd
>>> for (int i = 0; i < n; ++i)
>>> a[i * step] += 1;
>>>
>>> would be incorrect for step==0. (#pragma ordered simd forces
>>> things to be executed in order where necessary.)
>>
>> But then we should check safelen, not force_vectorize...
>
> OK. Following on from irc, this version uses expr_not_equal_to
> instead of tree_expr_nonzero_p. It also adds safelen to the existing
> use of force_vectorize (which seemed safer than replacing it).
>
> Tested on aarch64-linux-gnu. OK to install?
Ok.
Richard.
> Richard
>
>
> 2018-02-28 Richard Sandiford <richard.sandif...@linaro.org>
>
> gcc/
> PR tree-optimization/84485
> * tree-vect-data-refs.c (vect_analyze_data_ref_dependence): Return
> true for zero dependence distances if the step might be zero,
> and if there is no metadata that guarantees correctness.
> (vect_analyze_data_ref_access): Check safelen as well as
> force_vectorize.
>
> gcc/testsuite/
> PR tree-optimization/84485
> * gcc.dg/vect/pr84485.c: New test.
>
> Index: gcc/tree-vect-data-refs.c
> ===================================================================
> --- gcc/tree-vect-data-refs.c 2018-02-28 14:19:22.326678337 +0000
> +++ gcc/tree-vect-data-refs.c 2018-03-02 13:28:06.339321095 +0000
> @@ -394,6 +394,16 @@ vect_analyze_data_ref_dependence (struct
> }
> }
>
> + unsigned int step_prec = TYPE_PRECISION (TREE_TYPE (DR_STEP (dra)));
> + if (loop->safelen < 2
> + && !expr_not_equal_to (DR_STEP (dra), wi::zero (step_prec)))
> + {
> + if (dump_enabled_p ())
> + dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
> + "step could be zero.\n");
> + return true;
> + }
> +
> continue;
> }
>
> @@ -2515,7 +2525,7 @@ vect_analyze_data_ref_access (struct dat
> /* Allow references with zero step for outer loops marked
> with pragma omp simd only - it guarantees absence of
> loop-carried dependencies between inner loop iterations. */
> - if (!loop->force_vectorize)
> + if (!loop->force_vectorize || loop->safelen < 2)
> {
> if (dump_enabled_p ())
> dump_printf_loc (MSG_NOTE, vect_location,
> Index: gcc/testsuite/gcc.dg/vect/pr84485.c
> ===================================================================
> --- /dev/null 2018-03-01 08:17:49.562264353 +0000
> +++ gcc/testsuite/gcc.dg/vect/pr84485.c 2018-03-02 13:28:06.338321095 +0000
> @@ -0,0 +1,34 @@
> +/* { dg-do run } */
> +
> +#include "tree-vect.h"
> +
> +#define N 256
> +
> +void __attribute__ ((noinline, noclone))
> +f (unsigned long incx, unsigned long incy,
> + float *restrict dx, float *restrict dy)
> +{
> + unsigned long ix = 0, iy = 0;
> + for (unsigned long i = 0; i < N; ++i)
> + {
> + dy[iy] += dx[ix];
> + ix += incx;
> + iy += incy;
> + }
> +}
> +
> +float a = 0.0;
> +float b[N];
> +
> +int
> +main (void)
> +{
> + check_vect ();
> +
> + for (int i = 0; i < N; ++i)
> + b[i] = i;
> + f (1, 0, b, &a);
> + if (a != N * (N - 1) / 2)
> + __builtin_abort ();
> + return 0;
> +}
