[PATCH] tree-optimization/26854 - compile-time hog in SSA forwprop

[Richard Biener <rguenther@suse.de>]

The following addresses

 tree forward propagate             :  12.41 (  9%)

seen with the compile.i testcase of this PR which points at
the has_use_on_stmt function which, for SSA names with many
uses is slow.  The solution is to instead of immediate uses,
look at stmt operands to identify whether a name has a use
on a stmt.  That improves SSA forwprop to

 tree forward propagate             :   1.30 (  0%)

for this testcase.

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

	PR tree-optimization/26854
	* gimple-fold.cc (has_use_on_stmt): Look at stmt operands
	instead of immediate uses.
---
 gcc/gimple-fold.cc | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/gcc/gimple-fold.cc b/gcc/gimple-fold.cc
index 379d2e930ea..935e8006413 100644
--- a/gcc/gimple-fold.cc
+++ b/gcc/gimple-fold.cc
@@ -5767,15 +5767,17 @@ gimple_fold_call (gimple_stmt_iterator *gsi, bool inplace)
 }
 
 
-/* Return true whether NAME has a use on STMT.  */
+/* Return true whether NAME has a use on STMT.  Note this can return
+   false even though there's a use on STMT if SSA operands are not
+   up-to-date.  */
 
 static bool
 has_use_on_stmt (tree name, gimple *stmt)
 {
-  imm_use_iterator iter;
-  use_operand_p use_p;
-  FOR_EACH_IMM_USE_FAST (use_p, iter, name)
-    if (USE_STMT (use_p) == stmt)
+  ssa_op_iter iter;
+  tree op;
+  FOR_EACH_SSA_TREE_OPERAND (op, stmt, iter, SSA_OP_USE)
+    if (op == name)
       return true;
   return false;
 }
-- 
2.35.3