[PATCH] openmp: Change to using a hashtab to lookup offload target addresses for indirect function calls

[Kwok Cheung Yeung <kcy@codesourcery.com>]

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Hi

There was a bug in the declare-target-indirect-2.c libgomp testcase 
(testing indirect calls in offloaded target regions, spread over 
multiple teams/threads) that due to an errant fallthrough in a switch 
statement resulted in only one indirect function ever getting called:

switch (i % 3)
   {
     case 0: fn_ptr[i] = &foo;  // Missing break
     case 1: fn_ptr[i] = &bar;  // Missing break
     case 2: fn_ptr[i] = &baz;
   }

However, when the missing break statements are added, the testcase fails 
with an invalid memory access. Upon investigation, this is due to the 
use of a splay-tree as the lookup structure for indirect addresses, as 
the splay-tree moves frequently accessed elements closer to the root 
node and so needs locking when used from multiple threads. However, this 
would end up partially serialising all the threads and kill performance. 
I have switched the lookup structure from a splay tree to a hashtab 
instead to avoid locking during lookup.

I have also tidied up the initialisation of the lookup table by calling 
it only from the first thread of the first team, instead of redundantly 
calling it from every thread and only having the first one reached do 
the initialisation. This removes the need for locking during initialisation.

Tested with offloading to NVPTX and GCN with a x86_64 host. Okay for master?

Thanks

Kwok

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From 721ec33bec2fddc7ee37e227358e36fec923f8da Mon Sep 17 00:00:00 2001
From: Kwok Cheung Yeung <kcy@codesourcery.com>
Date: Wed, 17 Jan 2024 16:53:40 +0000
Subject: [PATCH 1/2] openmp: Change to using a hashtab to lookup offload
 target addresses for indirect function calls

A splay-tree was previously used to lookup equivalent target addresses
for a given host address on offload targets. However, as splay-trees can
modify their structure on lookup, they are not suitable for concurrent
access from separate teams/threads without some form of locking.  This
patch changes the lookup data structure to a hashtab instead, which does
not have these issues.

The call to build_indirect_map to initialize the data structure is now
called from just the first thread of the first team to avoid redundant
calls to this function.

2024-01-19  Kwok Cheung Yeung  <kcy@codesourcery.com>

	libgomp/
	* config/accel/target-indirect.c: Include string.h and hashtab.h.
	Remove include of splay-tree.h.
	(splay_tree_prefix, splay_tree_c): Delete.
	(struct indirect_map_t): New.
	(hash_entry_type, htab_alloc, htab_free, htab_hash, htab_eq): New.
	(GOMP_INDIRECT_ADD_MAP): Remove volatile qualifier.
	(USE_SPLAY_TREE_LOOKUP): Rename to...
	(USE_HASHTAB_LOOKUP): ..this.
	(indirect_map, indirect_array): Delete.
	(indirect_htab): New.
	(build_indirect_map): Remove locking.  Build indirect map using
	hashtab.
	(GOMP_target_map_indirect_ptr): Use indirect_htab to lookup target
	address.
	* config/gcn/team.c (gomp_gcn_enter_kernel): Call build_indirect_map
	from first thread of first team only.
	* config/nvptx/team.c (gomp_nvptx_main): Likewise.
	* testsuite/libgomp.c-c++-common/declare-target-indirect-2.c (main):
	Add missing break statements.
---
 libgomp/config/accel/target-indirect.c        | 75 +++++++++++--------
 libgomp/config/gcn/team.c                     |  7 +-
 libgomp/config/nvptx/team.c                   |  9 ++-
 .../declare-target-indirect-2.c               | 14 ++--
 4 files changed, 59 insertions(+), 46 deletions(-)

diff --git a/libgomp/config/accel/target-indirect.c b/libgomp/config/accel/target-indirect.c
index c60fd547cb6..6dad85076d6 100644
--- a/libgomp/config/accel/target-indirect.c
+++ b/libgomp/config/accel/target-indirect.c
@@ -25,22 +25,43 @@
    <http://www.gnu.org/licenses/>.  */
 
 #include <assert.h>
+#include <string.h>
 #include "libgomp.h"
 
-#define splay_tree_prefix indirect
-#define splay_tree_c
-#include "splay-tree.h"
+struct indirect_map_t
+{
+  void *host_addr;
+  void *target_addr;
+};
+
+typedef struct indirect_map_t *hash_entry_type;
+
+static inline void * htab_alloc (size_t size) { return gomp_malloc (size); }
+static inline void htab_free (void *ptr) { free (ptr); }
+
+#include "hashtab.h"
+
+static inline hashval_t
+htab_hash (hash_entry_type element)
+{
+  return hash_pointer (element->host_addr);
+}
 
-volatile void **GOMP_INDIRECT_ADDR_MAP = NULL;
+static inline bool
+htab_eq (hash_entry_type x, hash_entry_type y)
+{
+  return x->host_addr == y->host_addr;
+}
+
+void **GOMP_INDIRECT_ADDR_MAP = NULL;
 
 /* Use a splay tree to lookup the target address instead of using a
    linear search.  */
-#define USE_SPLAY_TREE_LOOKUP
+#define USE_HASHTAB_LOOKUP
 
-#ifdef USE_SPLAY_TREE_LOOKUP
+#ifdef USE_HASHTAB_LOOKUP
 
-static struct indirect_splay_tree_s indirect_map;
-static indirect_splay_tree_node indirect_array = NULL;
+static htab_t indirect_htab = NULL;
 
 /* Build the splay tree used for host->target address lookups.  */
 
@@ -48,37 +69,29 @@ void
 build_indirect_map (void)
 {
   size_t num_ind_funcs = 0;
-  volatile void **map_entry;
-  static int lock = 0; /* == gomp_mutex_t lock; gomp_mutex_init (&lock); */
+  void **map_entry;
 
   if (!GOMP_INDIRECT_ADDR_MAP)
     return;
 
-  gomp_mutex_lock (&lock);
-
-  if (!indirect_array)
+  if (!indirect_htab)
     {
       /* Count the number of entries in the NULL-terminated address map.  */
       for (map_entry = GOMP_INDIRECT_ADDR_MAP; *map_entry;
 	   map_entry += 2, num_ind_funcs++);
 
-      /* Build splay tree for address lookup.  */
-      indirect_array = gomp_malloc (num_ind_funcs * sizeof (*indirect_array));
-      indirect_splay_tree_node array = indirect_array;
+      /* Build hashtab for address lookup.  */
+      indirect_htab = htab_create (num_ind_funcs);
       map_entry = GOMP_INDIRECT_ADDR_MAP;
 
-      for (int i = 0; i < num_ind_funcs; i++, array++)
+      for (int i = 0; i < num_ind_funcs; i++, map_entry += 2)
 	{
-	  indirect_splay_tree_key k = &array->key;
-	  k->host_addr = (uint64_t) *map_entry++;
-	  k->target_addr = (uint64_t) *map_entry++;
-	  array->left = NULL;
-	  array->right = NULL;
-	  indirect_splay_tree_insert (&indirect_map, array);
+	  struct indirect_map_t element = { *map_entry, NULL };
+	  hash_entry_type *slot = htab_find_slot (&indirect_htab, &element,
+						  INSERT);
+	  *slot = (hash_entry_type) map_entry;
 	}
     }
-
-  gomp_mutex_unlock (&lock);
 }
 
 void *
@@ -88,15 +101,11 @@ GOMP_target_map_indirect_ptr (void *ptr)
   if (!ptr)
     return ptr;
 
-  assert (indirect_array);
-
-  struct indirect_splay_tree_key_s k;
-  indirect_splay_tree_key node = NULL;
-
-  k.host_addr = (uint64_t) ptr;
-  node = indirect_splay_tree_lookup (&indirect_map, &k);
+  assert (indirect_htab);
 
-  return node ? (void *) node->target_addr : ptr;
+  struct indirect_map_t element = { ptr, NULL };
+  hash_entry_type entry = htab_find (indirect_htab, &element);
+  return entry ? entry->target_addr : ptr;
 }
 
 #else
diff --git a/libgomp/config/gcn/team.c b/libgomp/config/gcn/team.c
index 61e9c616b67..bd3df448b52 100644
--- a/libgomp/config/gcn/team.c
+++ b/libgomp/config/gcn/team.c
@@ -52,14 +52,15 @@ gomp_gcn_enter_kernel (void)
 {
   int threadid = __builtin_gcn_dim_pos (1);
 
-  /* Initialize indirect function support.  */
-  build_indirect_map ();
-
   if (threadid == 0)
     {
       int numthreads = __builtin_gcn_dim_size (1);
       int teamid = __builtin_gcn_dim_pos(0);
 
+      /* Initialize indirect function support.  */
+      if (teamid == 0)
+	build_indirect_map ();
+
       /* Set up the global state.
 	 Every team will do this, but that should be harmless.  */
       gomp_global_icv.nthreads_var = 16;
diff --git a/libgomp/config/nvptx/team.c b/libgomp/config/nvptx/team.c
index 0cf5dad39ca..d5361917a24 100644
--- a/libgomp/config/nvptx/team.c
+++ b/libgomp/config/nvptx/team.c
@@ -60,9 +60,6 @@ gomp_nvptx_main (void (*fn) (void *), void *fn_data)
   asm ("mov.u32 %0, %%tid.y;" : "=r" (tid));
   asm ("mov.u32 %0, %%ntid.y;" : "=r" (ntids));
 
-  /* Initialize indirect function support.  */
-  build_indirect_map ();
-
   if (tid == 0)
     {
       gomp_global_icv.nthreads_var = ntids;
@@ -74,6 +71,12 @@ gomp_nvptx_main (void (*fn) (void *), void *fn_data)
       nvptx_thrs = alloca (ntids * sizeof (*nvptx_thrs));
       memset (nvptx_thrs, 0, ntids * sizeof (*nvptx_thrs));
 
+      /* Initialize indirect function support.  */
+      unsigned int block_id;
+      asm ("mov.u32 %0, %%ctaid.x;" : "=r" (block_id));
+      if (block_id == 0)
+	build_indirect_map ();
+
       /* Find the low-latency heap details ....  */
       uint32_t *shared_pool;
       uint32_t shared_pool_size = 0;
diff --git a/libgomp/testsuite/libgomp.c-c++-common/declare-target-indirect-2.c b/libgomp/testsuite/libgomp.c-c++-common/declare-target-indirect-2.c
index 9fe190efce8..545f1a9fcbf 100644
--- a/libgomp/testsuite/libgomp.c-c++-common/declare-target-indirect-2.c
+++ b/libgomp/testsuite/libgomp.c-c++-common/declare-target-indirect-2.c
@@ -17,17 +17,17 @@ int main (void)
     {
       switch (i % 3)
 	{
-	case 0: fn_ptr[i] = &foo;
-	case 1: fn_ptr[i] = &bar;
-	case 2: fn_ptr[i] = &baz;
+	case 0: fn_ptr[i] = &foo; break;
+	case 1: fn_ptr[i] = &bar; break;
+	case 2: fn_ptr[i] = &baz; break;
 	}
       expected += (*fn_ptr[i]) ();
     }
 
-#pragma omp target teams distribute parallel for reduction(+: x) \
-		map (to: fn_ptr) map (tofrom: x)
-  for (int i = 0; i < N; i++)
-    x += (*fn_ptr[i]) ();
+  #pragma omp target teams distribute parallel for \
+	reduction (+: x) map (to: fn_ptr) map (tofrom: x)
+    for (int i = 0; i < N; i++)
+      x += (*fn_ptr[i]) ();
 
   return x - expected;
 }
-- 
2.34.1