[PATCH] tree-optimization/103168 - Improve VN of pure function calls

Richard Biener rguenther@suse.de
Wed Nov 24 11:40:07 GMT 2021


This improves value-numbering of calls that read memory, calls
to const functions with aggregate arguments and calls to
pure functions where the latter include const functions we
demoted to pure for the fear of interposing with a less
optimized version.  Note that for pure functions we do not
handle functions that access global memory.

Bootstrapped and tested on x86_64-unknown-linux-gnu, pushed.

Richard.

2021-11-24  Richard Biener  <rguenther@suse.de>
	Jan Hubicka  <jh@suse.cz>

	PR tree-optimization/103168
	* ipa-modref.h (struct modref_summary): Add load_accesses.
	* ipa-modref.c (modref_summary::finalize): Initialize load_accesses.
	* tree-ssa-sccvn.c (visit_reference_op_call): Use modref
	info to walk the virtual use->def chain to CSE const/pure
	function calls possibly reading from memory.

	* g++.dg/tree-ssa/pr103168.C: New testcase.
---
 gcc/ipa-modref.c                         |  17 +++
 gcc/ipa-modref.h                         |   2 +
 gcc/testsuite/g++.dg/tree-ssa/pr103168.C |  24 +++++
 gcc/tree-ssa-sccvn.c                     | 126 +++++++++++++++++++++++
 4 files changed, 169 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/tree-ssa/pr103168.C

diff --git a/gcc/ipa-modref.c b/gcc/ipa-modref.c
index 79d7d774715..923ae6c1dd3 100644
--- a/gcc/ipa-modref.c
+++ b/gcc/ipa-modref.c
@@ -721,6 +721,23 @@ modref_summary::finalize (tree fun)
 	    break;
 	}
     }
+  if (loads->every_base)
+    load_accesses = 1;
+  else
+    {
+      load_accesses = 0;
+      for (auto base_node : loads->bases)
+	{
+	  if (base_node->every_ref)
+	    load_accesses++;
+	  else
+	    for (auto ref_node : base_node->refs)
+	      if (ref_node->every_access)
+		load_accesses++;
+	      else
+		load_accesses += ref_node->accesses->length ();
+	}
+    }
 }
 
 /* Get function summary for FUNC if it exists, return NULL otherwise.  */
diff --git a/gcc/ipa-modref.h b/gcc/ipa-modref.h
index f868eb6de07..a0247f5449f 100644
--- a/gcc/ipa-modref.h
+++ b/gcc/ipa-modref.h
@@ -53,6 +53,8 @@ struct GTY(()) modref_summary
 
   /* Flags coputed by finalize method.  */
 
+  /* Total number of accesses in loads tree.  */
+  unsigned int load_accesses;
   /* global_memory_read is not set for functions calling functions
      with !binds_to_current_def which, after interposition, may read global
      memory but do nothing useful with it (except for crashing if some
diff --git a/gcc/testsuite/g++.dg/tree-ssa/pr103168.C b/gcc/testsuite/g++.dg/tree-ssa/pr103168.C
new file mode 100644
index 00000000000..82924a3e3ce
--- /dev/null
+++ b/gcc/testsuite/g++.dg/tree-ssa/pr103168.C
@@ -0,0 +1,24 @@
+// { dg-do compile }
+// { dg-options "-O2 -fdump-tree-fre1-details" }
+
+struct a
+{
+  int a;
+  static __attribute__ ((noinline))
+      int ret (int v) {return v;}
+
+  __attribute__ ((noinline))
+      int inca () {return a++;}
+};
+
+int
+test()
+{
+  struct a av;
+  av.a=1;
+  int val = av.ret (0) + av.inca();
+  av.a=2;
+  return val + av.ret(0) + av.inca();
+}
+
+/* { dg-final { scan-tree-dump-times "Replaced a::ret" 1 "fre1" } } */
diff --git a/gcc/tree-ssa-sccvn.c b/gcc/tree-ssa-sccvn.c
index 149674e6a16..d31bf329d2e 100644
--- a/gcc/tree-ssa-sccvn.c
+++ b/gcc/tree-ssa-sccvn.c
@@ -71,6 +71,8 @@ along with GCC; see the file COPYING3.  If not see
 #include "tree-ssa-loop-niter.h"
 #include "builtins.h"
 #include "fold-const-call.h"
+#include "ipa-modref-tree.h"
+#include "ipa-modref.h"
 #include "tree-ssa-sccvn.h"
 
 /* This algorithm is based on the SCC algorithm presented by Keith
@@ -5084,12 +5086,136 @@ visit_reference_op_call (tree lhs, gcall *stmt)
   struct vn_reference_s vr1;
   vn_reference_t vnresult = NULL;
   tree vdef = gimple_vdef (stmt);
+  modref_summary *summary;
 
   /* Non-ssa lhs is handled in copy_reference_ops_from_call.  */
   if (lhs && TREE_CODE (lhs) != SSA_NAME)
     lhs = NULL_TREE;
 
   vn_reference_lookup_call (stmt, &vnresult, &vr1);
+
+  /* If the lookup did not succeed for pure functions try to use
+     modref info to find a candidate to CSE to.  */
+  const unsigned accesses_limit = 8;
+  if (!vnresult
+      && !vdef
+      && lhs
+      && gimple_vuse (stmt)
+      && (((summary = get_modref_function_summary (stmt, NULL))
+	   && !summary->global_memory_read
+	   && summary->load_accesses < accesses_limit)
+	  || gimple_call_flags (stmt) & ECF_CONST))
+    {
+      /* First search if we can do someting useful and build a
+	 vector of all loads we have to check.  */
+      bool unknown_memory_access = false;
+      auto_vec<ao_ref, accesses_limit> accesses;
+      unsigned load_accesses = summary ? summary->load_accesses : 0;
+      if (!unknown_memory_access)
+	/* Add loads done as part of setting up the call arguments.
+	   That's also necessary for CONST functions which will
+	   not have a modref summary.  */
+	for (unsigned i = 0; i < gimple_call_num_args (stmt); ++i)
+	  {
+	    tree arg = gimple_call_arg (stmt, i);
+	    if (TREE_CODE (arg) != SSA_NAME
+		&& !is_gimple_min_invariant (arg))
+	      {
+		if (accesses.length () >= accesses_limit - load_accesses)
+		  {
+		    unknown_memory_access = true;
+		    break;
+		  }
+		accesses.quick_grow (accesses.length () + 1);
+		ao_ref_init (&accesses.last (), arg);
+	      }
+	  }
+      if (summary && !unknown_memory_access)
+	{
+	  /* Add loads as analyzed by IPA modref.  */
+	  for (auto base_node : summary->loads->bases)
+	    if (unknown_memory_access)
+	      break;
+	    else for (auto ref_node : base_node->refs)
+	      if (unknown_memory_access)
+		break;
+	      else for (auto access_node : ref_node->accesses)
+		{
+		  accesses.quick_grow (accesses.length () + 1);
+		  ao_ref *r = &accesses.last ();
+		  if (!access_node.get_ao_ref (stmt, r))
+		    {
+		      /* Initialize a ref based on the argument and
+			 unknown offset if possible.  */
+		      tree arg = access_node.get_call_arg (stmt);
+		      if (arg && TREE_CODE (arg) == SSA_NAME)
+			arg = SSA_VAL (arg);
+		      if (arg
+			  && TREE_CODE (arg) == ADDR_EXPR
+			  && (arg = get_base_address (arg))
+			  && DECL_P (arg))
+			{
+			  ao_ref_init (r, arg);
+			  r->ref = NULL_TREE;
+			  r->base = arg;
+			}
+		      else
+			{
+			  unknown_memory_access = true;
+			  break;
+			}
+		    }
+		  r->base_alias_set = base_node->base;
+		  r->ref_alias_set = ref_node->ref;
+		}
+	}
+
+      /* Walk the VUSE->VDEF chain optimistically trying to find an entry
+	 for the call in the hashtable.  */
+      unsigned limit = (unknown_memory_access
+			? 0
+			: (param_sccvn_max_alias_queries_per_access
+			   / (accesses.length () + 1)));
+      tree saved_vuse = vr1.vuse;
+      hashval_t saved_hashcode = vr1.hashcode;
+      while (limit > 0 && !vnresult && !SSA_NAME_IS_DEFAULT_DEF (vr1.vuse))
+	{
+	  vr1.hashcode = vr1.hashcode - SSA_NAME_VERSION (vr1.vuse);
+	  gimple *def = SSA_NAME_DEF_STMT (vr1.vuse);
+	  /* ???  We could use fancy stuff like in walk_non_aliased_vuses, but
+	     do not bother for now.  */
+	  if (is_a <gphi *> (def))
+	    break;
+	  vr1.vuse = vuse_ssa_val (gimple_vuse (def));
+	  vr1.hashcode = vr1.hashcode + SSA_NAME_VERSION (vr1.vuse);
+	  vn_reference_lookup_1 (&vr1, &vnresult);
+	  limit--;
+	}
+
+      /* If we found a candidate to CSE to verify it is valid.  */
+      if (vnresult && !accesses.is_empty ())
+	{
+	  tree vuse = vuse_ssa_val (gimple_vuse (stmt));
+	  while (vnresult && vuse != vr1.vuse)
+	    {
+	      gimple *def = SSA_NAME_DEF_STMT (vuse);
+	      for (auto &ref : accesses)
+		{
+		  /* ???  stmt_may_clobber_ref_p_1 does per stmt constant
+		     analysis overhead that we might be able to cache.  */
+		  if (stmt_may_clobber_ref_p_1 (def, &ref, true))
+		    {
+		      vnresult = NULL;
+		      break;
+		    }
+		}
+	      vuse = vuse_ssa_val (gimple_vuse (def));
+	    }
+	}
+      vr1.vuse = saved_vuse;
+      vr1.hashcode = saved_hashcode;
+    }
+
   if (vnresult)
     {
       if (vnresult->result_vdef && vdef)
-- 
2.31.1


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