[PING^2][PATCH][omp, simt] Handle alternative IV
Tom de Vries
tdevries@suse.de
Thu Dec 17 16:46:28 GMT 2020
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^2.
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;
>> +}
>>
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