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[omp, ftracer] Don't duplicate blocks in SIMT region

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When running the libgomp testsuite on x86_64-linux with nvptx accelerator on
the test-case included in this patch, we run into:
...
FAIL: libgomp.fortran/pr95654.f90 -O3 -fomit-frame-pointer -funroll-loops \
  -fpeel-loops -ftracer -finline-functions  execution test
...

The test-case is a minimal version of this FAIL:
...
FAIL: libgomp.fortran/pr66199-5.f90 -O3 -fomit-frame-pointer -funroll-loops \
  -fpeel-loops -ftracer -finline-functions  execution test
...
but that one has stopped failing at commit c2ebf4f10d "openmp: Add support
for non-rect simd and improve collapsed simd support".

The problem is that ftracer duplicates a block containing GOMP_SIMT_VOTE_ANY.

That is, before ftracer we have (dropping the GOMP_SIMT_ prefix):
...
bb4(ENTER_ALLOC)
*----------+
|           \
|            \
|             v
|             *
v             bb8
*<------------*
bb5(VOTE_ANY)
*-------------+
|             |
|             |
|             |
|             |
|             v
|             *
v             bb7(XCHG_IDX)
*<------------*
bb6(EXIT)
...

The XCHG_IDX internal-fn does inter-SIMT-lane communication, which for nvptx
maps onto shfl, an operator which has the requirement that the warp executing
the operator is convergent.  The warp diverges at bb4, and
reconverges at bb5, and does not diverge by going to bb7, so the shfl is
indeed executed by a convergent warp.

After ftracer, we have:
...
bb4(ENTER_ALLOC)
*----------+
|           \
|            \
|             \
|              \
v               v
*               *
bb5(VOTE_ANY)   bb8(VOTE_ANY)
*               *
|\             /|
| \  +--------+ |
|  \/           |
|  /\           |
| /  +----------v
|/              *
v               bb7(XCHG_IDX)
*<--------------*
bb6(EXIT)
...

The warp diverges again at bb5, but does not reconverge again before bb6, so
the shfl is executed by a divergent warp, which causes the FAIL.

Fix this by making ftracer ignore blocks containing ENTER_ALLOC, VOTE_ANY and
EXIT, effectively treating the SIMT region conservatively.

An argument can be made that the test needs to be added in a more
generic place, like gimple_can_duplicate_bb_p or some such, and that ftracer
then needs to use the generic test.  But that's a discussion with a much
broader scope, so I'm leaving that for another patch.

Bootstrapped and reg-tested on x86_64-linux.

Build on x86_64-linux with nvptx accelerator, tested with libgomp.

gcc/ChangeLog:

	PR fortran/95654
	* tracer.c (ignore_bb_p): Ignore GOMP_SIMT_ENTER_ALLOC,
	GOMP_SIMT_VOTE_ANY and GOMP_SIMT_EXIT.

libgomp/ChangeLog:

2020-10-05  Tom de Vries  <tdevries@suse.de>

	PR fortran/95654
	* testsuite/libgomp.fortran/pr95654.f90: New test.
This commit is contained in:
Tom de Vries 2020-09-22 16:38:07 +02:00
parent 4347d36f93
commit ab3f4b27ab
2 changed files with 29 additions and 0 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

18
gcc/tracer.c
View File

@ -108,6 +108,24 @@ ignore_bb_p (const_basic_block bb)
return true;
}
for (gimple_stmt_iterator gsi = gsi_start_bb (CONST_CAST_BB (bb));
!gsi_end_p (gsi); gsi_next (&gsi))
{
gimple *g = gsi_stmt (gsi);
/* An IFN_GOMP_SIMT_ENTER_ALLOC/IFN_GOMP_SIMT_EXIT call must be
duplicated as part of its group, or not at all.
The IFN_GOMP_SIMT_VOTE_ANY is currently part of such a group,
so the same holds there, but it could be argued that the
IFN_GOMP_SIMT_VOTE_ANY could be generated after that group,
in which case it could be duplicated. */
if (is_gimple_call (g)
&& (gimple_call_internal_p (g, IFN_GOMP_SIMT_ENTER_ALLOC)
|| gimple_call_internal_p (g, IFN_GOMP_SIMT_EXIT)
|| gimple_call_internal_p (g, IFN_GOMP_SIMT_VOTE_ANY)))
return true;
}
return false;
}

11
libgomp/testsuite/libgomp.fortran/pr95654.f90 Normal file
View File

@ -0,0 +1,11 @@
! { dg-do run }
program main
implicit none
integer :: d1
!$omp target map(from: d1)
!$omp teams distribute parallel do simd default(none) lastprivate(d1) num_teams (2) num_threads (1)
do d1 = 0, 31
end do
!$omp end target
if (d1 /= 32) stop 3
end program main