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 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.
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.
fc14ff6111