[PATCH] libgomp, nvptx, v3: Honor OpenMP 5.1 num_teams lower bound

Jakub Jelinek jakub@redhat.com
Fri Nov 12 17:58:04 GMT 2021


On Fri, Nov 12, 2021 at 02:27:16PM +0100, Jakub Jelinek via Gcc-patches wrote:
> On Fri, Nov 12, 2021 at 02:20:23PM +0100, Jakub Jelinek via Gcc-patches wrote:
> > This patch assumes that .shared variables are initialized to 0,
> > https://docs.nvidia.com/cuda/parallel-thread-execution/index.html lists
> > in Table 7. .shared as non-initializable.  If that isn't the case,
> > we need to initialize it somewhere for the case of #pragma omp target
> > without #pragma omp teams in it, maybe in libgcc/config/nvptx/crt0.c ?
> 
> A quick look at libgcc/config/nvptx/crt0.c shows the target supports
> __attribute__((shared)), so perhaps either following instead, or, if
> .shared isn't preinitialized to zero, defining the variable in
> libgcc/config/nvptx/crt0.c , adding there __gomp_team_num = 0;
> and adding extern keyword before int __gomp_team_num __attribute__((shared));
> in libgomp/config/nvptx/target.c.

And finally here is a third version, which fixes a typo in the previous
patch (in instead of int) and actually initializes the shared var because
PTX documentation doesn't say anything about how the shared vars are
initialized.

Tested on x86_64-linux with nvptx-none offloading, ok for trunk?

2021-11-12  Jakub Jelinek  <jakub@redhat.com>

	* config/nvptx/team.c (__gomp_team_num): Define as
	__attribute__((shared)) var.
	(gomp_nvptx_main): Initialize __gomp_team_num to 0.
	* config/nvptx/target.c (__gomp_team_num): Declare as
	extern __attribute__((shared)) var.
	(GOMP_teams4): Use __gomp_team_num as the team number instead of
	%ctaid.x.  If first, initialize it to %ctaid.x.  If num_teams_lower
	is bigger than num_blocks, use num_teams_lower teams and arrange for
	bumping of __gomp_team_num if !first and returning false once we run
	out of teams.
	* config/nvptx/teams.c (__gomp_team_num): Declare as
	extern __attribute__((shared)) var.
	(omp_get_team_num): Return __gomp_team_num value instead of %ctaid.x.

--- libgomp/config/nvptx/team.c.jj	2021-05-25 13:43:02.793121350 +0200
+++ libgomp/config/nvptx/team.c	2021-11-12 17:49:02.847341650 +0100
@@ -32,6 +32,7 @@
 #include <string.h>
 
 struct gomp_thread *nvptx_thrs __attribute__((shared,nocommon));
+int __gomp_team_num __attribute__((shared));
 
 static void gomp_thread_start (struct gomp_thread_pool *);
 
@@ -57,6 +58,7 @@ gomp_nvptx_main (void (*fn) (void *), vo
       /* Starting additional threads is not supported.  */
       gomp_global_icv.dyn_var = true;
 
+      __gomp_team_num = 0;
       nvptx_thrs = alloca (ntids * sizeof (*nvptx_thrs));
       memset (nvptx_thrs, 0, ntids * sizeof (*nvptx_thrs));
 
--- libgomp/config/nvptx/target.c.jj	2021-11-12 15:57:29.400632875 +0100
+++ libgomp/config/nvptx/target.c	2021-11-12 17:47:39.499533296 +0100
@@ -26,28 +26,41 @@
 #include "libgomp.h"
 #include <limits.h>
 
+extern int __gomp_team_num __attribute__((shared));
+
 bool
 GOMP_teams4 (unsigned int num_teams_lower, unsigned int num_teams_upper,
 	     unsigned int thread_limit, bool first)
 {
+  unsigned int num_blocks, block_id;
+  asm ("mov.u32 %0, %%nctaid.x;" : "=r" (num_blocks));
   if (!first)
-    return false;
+    {
+      unsigned int team_num;
+      if (num_blocks > gomp_num_teams_var)
+	return false;
+      team_num = __gomp_team_num;
+      if (team_num > gomp_num_teams_var - num_blocks)
+	return false;
+      __gomp_team_num = team_num + num_blocks;
+      return true;
+    }
   if (thread_limit)
     {
       struct gomp_task_icv *icv = gomp_icv (true);
       icv->thread_limit_var
 	= thread_limit > INT_MAX ? UINT_MAX : thread_limit;
     }
-  unsigned int num_blocks, block_id;
-  asm ("mov.u32 %0, %%nctaid.x;" : "=r" (num_blocks));
-  asm ("mov.u32 %0, %%ctaid.x;" : "=r" (block_id));
-  /* FIXME: If num_teams_lower > num_blocks, we want to loop multiple
-     times for some CTAs.  */
-  (void) num_teams_lower;
-  if (!num_teams_upper || num_teams_upper >= num_blocks)
+  if (!num_teams_upper)
     num_teams_upper = num_blocks;
-  else if (block_id >= num_teams_upper)
+  else if (num_blocks < num_teams_lower)
+    num_teams_upper = num_teams_lower;
+  else if (num_blocks < num_teams_upper)
+    num_teams_upper = num_blocks;
+  asm ("mov.u32 %0, %%ctaid.x;" : "=r" (block_id));
+  if (block_id >= num_teams_upper)
     return false;
+  __gomp_team_num = block_id;
   gomp_num_teams_var = num_teams_upper - 1;
   return true;
 }
--- libgomp/config/nvptx/teams.c.jj	2021-05-25 13:43:02.793121350 +0200
+++ libgomp/config/nvptx/teams.c	2021-11-12 17:37:18.933361024 +0100
@@ -28,6 +28,8 @@
 
 #include "libgomp.h"
 
+extern int __gomp_team_num __attribute__((shared));
+
 void
 GOMP_teams_reg (void (*fn) (void *), void *data, unsigned int num_teams,
 		unsigned int thread_limit, unsigned int flags)
@@ -48,9 +50,7 @@ omp_get_num_teams (void)
 int
 omp_get_team_num (void)
 {
-  int ctaid;
-  asm ("mov.u32 %0, %%ctaid.x;" : "=r" (ctaid));
-  return ctaid;
+  return __gomp_team_num;
 }
 
 ialias (omp_get_num_teams)


	Jakub



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