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.
Thanks,
- Tom
> [omp, simt] Handle alternative IV
>
> gcc/ChangeLog:
>
> 2020-10-02 Tom de Vries <tdevr...@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 <tdevr...@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;
> +}
>
