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 <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; >>> +} >>>