[PING^3][PATCH][omp, simt] Handle alternative IV

[Tom de Vries <tdevries@suse.de>]

On 12/17/20 5:46 PM, Tom de Vries wrote:
> On 10/15/20 5:05 PM, Tom de Vries wrote:
>> On 10/2/20 3:21 PM, Tom de Vries wrote:
>>> Hi,
>>>
>>> Consider the test-case libgomp.c/pr81778.c added in this commit, with
>>> this core loop (note: CANARY_SIZE set to 0 for simplicity):
>>> ...
>>>   int s = 1;
>>>   #pragma omp target simd
>>>   for (int i = N - 1; i > -1; i -= s)
>>>     a[i] = 1;
>>> ...
>>> which, given that N is 32, sets a[0..31] to 1.
>>>
>>> After omp-expand, this looks like:
>>> ...
>>>   <bb 5> :
>>>   simduid.7 = .GOMP_SIMT_ENTER (simduid.7);
>>>   .omp_simt.8 = .GOMP_SIMT_ENTER_ALLOC (simduid.7);
>>>   D.3193 = -s;
>>>   s.9 = s;
>>>   D.3204 = .GOMP_SIMT_LANE ();
>>>   D.3205 = -s.9;
>>>   D.3206 = (int) D.3204;
>>>   D.3207 = D.3205 * D.3206;
>>>   i = D.3207 + 31;
>>>   D.3209 = 0;
>>>   D.3210 = -s.9;
>>>   D.3211 = D.3210 - i;
>>>   D.3210 = -s.9;
>>>   D.3212 = D.3211 / D.3210;
>>>   D.3213 = (unsigned int) D.3212;
>>>   D.3213 = i >= 0 ? D.3213 : 0;
>>>
>>>   <bb 19> :
>>>   if (D.3209 < D.3213)
>>>     goto <bb 6>; [87.50%]
>>>   else
>>>     goto <bb 7>; [12.50%]
>>>
>>>   <bb 6> :
>>>   a[i] = 1;
>>>   D.3215 = -s.9;
>>>   D.3219 = .GOMP_SIMT_VF ();
>>>   D.3216 = (int) D.3219;
>>>   D.3220 = D.3215 * D.3216;
>>>   i = D.3220 + i;
>>>   D.3209 = D.3209 + 1;
>>>   goto <bb 19>; [100.00%]
>>> ...
>>>
>>> On nvptx, the first time bb6 is executed, i is in the 0..31 range (depending
>>> on the lane that is executing) at bb entry.
>>>
>>> So we have the following sequence:
>>> - a[0..31] is set to 1
>>> - i is updated to -32..-1
>>> - D.3209 is updated to 1 (being 0 initially)
>>> - bb19 is executed, and if condition (D.3209 < D.3213) == (1 < 32) evaluates
>>>   to true
>>> - bb6 is once more executed, which should not happen because all the elements
>>>   that needed to be handled were already handled.
>>> - consequently, elements that should not be written are written
>>> - with CANARY_SIZE == 0, we may run into a libgomp error:
>>>   ...
>>>   libgomp: cuCtxSynchronize error: an illegal memory access was encountered
>>>   ...
>>>   and with CANARY_SIZE unmodified, we run into:
>>>   ...
>>>   Expected 0, got 1 at base[-961]
>>>   Aborted (core dumped)
>>>   ...
>>>
>>> The cause of this is as follows:
>>> - because the step s is a variable rather than a constant, an alternative
>>>   IV (D.3209 in our example) is generated in expand_omp_simd, and the
>>>   loop condition is tested in terms of the alternative IV rather than
>>>   the original IV (i in our example).
>>> - the SIMT code in expand_omp_simd works by modifying step and initial value.
>>> - The initial value fd->loop.n1 is loaded into a variable n1, which is
>>>   modified by the SIMT code and then used there-after.
>>> - The step fd->loop.step is loaded into a variable step, which is is modified
>>>   by the SIMT code, but afterwards there are uses of both step and
>>>   fd->loop.step.
>>> - There are uses of fd->loop.step in the alternative IV handling code,
>>>   which should use step instead.
>>>
>>> Fix this by introducing an additional variable orig_step, which is not
>>> modified by the SIMT code and replacing all remaining uses of fd->loop.step
>>> by either step or orig_step.
>>>
>>> Build on x86_64-linux with nvptx accelerator, tested libgomp.
>>>
>>> This fixes for-5.c and for-6.c FAILs I'm currently seeing on a quadro m1200
>>> with driver 450.66.
>>>
>>> OK for trunk?
>>>
>>

Ping^3.

Thanks,
- Tom

>>> [omp, simt] Handle alternative IV
>>>
>>> gcc/ChangeLog:
>>>
>>> 2020-10-02  Tom de Vries  <tdevries@suse.de>
>>>
>>> 	* omp-expand.c (expand_omp_simd): Add step_orig, and replace uses of
>>> 	fd->loop.step by either step or orig_step.
>>>
>>> libgomp/ChangeLog:
>>>
>>> 2020-10-02  Tom de Vries  <tdevries@suse.de>
>>>
>>> 	* testsuite/libgomp.c/pr81778.c: New test.
>>>
>>> ---
>>>  gcc/omp-expand.c                      | 11 ++++----
>>>  libgomp/testsuite/libgomp.c/pr81778.c | 48 +++++++++++++++++++++++++++++++++++
>>>  2 files changed, 54 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/gcc/omp-expand.c b/gcc/omp-expand.c
>>> index 99cb4f9dda4..80e35ac0294 100644
>>> --- a/gcc/omp-expand.c
>>> +++ b/gcc/omp-expand.c
>>> @@ -6307,6 +6307,7 @@ expand_omp_simd (struct omp_region *region, struct omp_for_data *fd)
>>>        n2 = OMP_CLAUSE_DECL (innerc);
>>>      }
>>>    tree step = fd->loop.step;
>>> +  tree orig_step = step; /* May be different from step if is_simt.  */
>>>  
>>>    bool is_simt = omp_find_clause (gimple_omp_for_clauses (fd->for_stmt),
>>>  				  OMP_CLAUSE__SIMT_);
>>> @@ -6455,7 +6456,7 @@ expand_omp_simd (struct omp_region *region, struct omp_for_data *fd)
>>>    tree altv = NULL_TREE, altn2 = NULL_TREE;
>>>    if (fd->collapse == 1
>>>        && !broken_loop
>>> -      && TREE_CODE (fd->loops[0].step) != INTEGER_CST)
>>> +      && TREE_CODE (orig_step) != INTEGER_CST)
>>>      {
>>>        /* The vectorizer currently punts on loops with non-constant steps
>>>  	 for the main IV (can't compute number of iterations and gives up
>>> @@ -6471,7 +6472,7 @@ expand_omp_simd (struct omp_region *region, struct omp_for_data *fd)
>>>  	itype = signed_type_for (itype);
>>>        t = build_int_cst (itype, (fd->loop.cond_code == LT_EXPR ? -1 : 1));
>>>        t = fold_build2 (PLUS_EXPR, itype,
>>> -		       fold_convert (itype, fd->loop.step), t);
>>> +		       fold_convert (itype, step), t);
>>>        t = fold_build2 (PLUS_EXPR, itype, t, fold_convert (itype, n2));
>>>        t = fold_build2 (MINUS_EXPR, itype, t,
>>>  		       fold_convert (itype, fd->loop.v));
>>> @@ -6479,10 +6480,10 @@ expand_omp_simd (struct omp_region *region, struct omp_for_data *fd)
>>>  	t = fold_build2 (TRUNC_DIV_EXPR, itype,
>>>  			 fold_build1 (NEGATE_EXPR, itype, t),
>>>  			 fold_build1 (NEGATE_EXPR, itype,
>>> -				      fold_convert (itype, fd->loop.step)));
>>> +				      fold_convert (itype, step)));
>>>        else
>>>  	t = fold_build2 (TRUNC_DIV_EXPR, itype, t,
>>> -			 fold_convert (itype, fd->loop.step));
>>> +			 fold_convert (itype, step));
>>>        t = fold_convert (TREE_TYPE (altv), t);
>>>        altn2 = create_tmp_var (TREE_TYPE (altv));
>>>        expand_omp_build_assign (&gsi, altn2, t);
>>> @@ -6630,7 +6631,7 @@ expand_omp_simd (struct omp_region *region, struct omp_for_data *fd)
>>>    if (is_simt)
>>>      {
>>>        gsi = gsi_start_bb (l2_bb);
>>> -      step = fold_build2 (MINUS_EXPR, TREE_TYPE (step), fd->loop.step, step);
>>> +      step = fold_build2 (MINUS_EXPR, TREE_TYPE (step), orig_step, step);
>>>        if (POINTER_TYPE_P (type))
>>>  	t = fold_build_pointer_plus (fd->loop.v, step);
>>>        else
>>> diff --git a/libgomp/testsuite/libgomp.c/pr81778.c b/libgomp/testsuite/libgomp.c/pr81778.c
>>> new file mode 100644
>>> index 00000000000..571668eb36a
>>> --- /dev/null
>>> +++ b/libgomp/testsuite/libgomp.c/pr81778.c
>>> @@ -0,0 +1,48 @@
>>> +/* Minimized from for-5.c.  */
>>> +
>>> +#include <stdio.h>
>>> +#include <stdlib.h>
>>> +
>>> +/* Size of array we want to write.  */
>>> +#define N 32
>>> +
>>> +/* Size of extra space before and after.  */
>>> +#define CANARY_SIZE (N * 32)
>>> +
>>> +/* Start of array we want to write.  */
>>> +#define BASE (CANARY_SIZE)
>>> +
>>> +// Total size to be allocated.
>>> +#define ALLOC_SIZE (CANARY_SIZE + N + CANARY_SIZE)
>>> +
>>> +#pragma omp declare target
>>> +int a[ALLOC_SIZE];
>>> +#pragma omp end declare target
>>> +
>>> +int
>>> +main (void)
>>> +{
>>> +  /* Use variable step in for loop.  */
>>> +  int s = 1;
>>> +
>>> +#pragma omp target update to(a)
>>> +
>>> +  /* Write a[BASE] .. a[BASE + N - 1].  */
>>> +#pragma omp target simd
>>> +  for (int i = N - 1; i > -1; i -= s)
>>> +    a[BASE + i] = 1;
>>> +
>>> +#pragma omp target update from(a)
>>> +
>>> +  for (int i = 0; i < ALLOC_SIZE; i++)
>>> +    {
>>> +      int expected = (BASE <= i && i < BASE + N) ? 1 : 0;
>>> +      if (a[i] == expected)
>>> +	continue;
>>> +
>>> +      printf ("Expected %d, got %d at base[%d]\n", expected, a[i], i - BASE);
>>> +      abort ();
>>> +    }
>>> +
>>> +  return 0;
>>> +}
>>>