re PR tree-optimization/19828 (LIM is pulling out a pure function even though there is something which can modify global memory)

	PR tree-optimization/19828
	* tree-ssa-loop-im.c: Add a TODO comment.
	(movement_possibility): Return MOVE_PRESERVE_EXECUTION for calls
	without side-effects.

	* gcc.dg/tree-ssa/loop-7.c: New test.
	* gcc.c-torture/execute/20050218-1.c: New test.

Co-Authored-By: Jakub Jelinek <jakub@redhat.com>

From-SVN: r95275

1 files changed, 44 insertions, 0 deletions

diff --git a/gcc/tree-ssa-loop-im.c b/gcc/tree-ssa-loop-im.c
index 65a2a5f..770b71b 100644
--- a/gcc/tree-ssa-loop-im.c
+++ b/gcc/tree-ssa-loop-im.c
@@ -38,6 +38,28 @@ Software Foundation, 59 Temple Place - Suite 330, Boston, MA
 #include "tree-pass.h"
 #include "flags.h"
 
+/* TODO:  Support for predicated code motion.  I.e.
+
+   while (1)
+     {
+       if (cond)
+	 {
+	   a = inv;
+	   something;
+	 }
+     }
+
+   Where COND and INV are is invariants, but evaluating INV may trap or be
+   invalid from some other reason if !COND.  This may be transformed to
+
+   if (cond)
+     a = inv;
+   while (1)
+     {
+       if (cond)
+	 something;
+     }  */
+
 /* A type for the list of statements that have to be moved in order to be able
    to hoist an invariant computation.  */
 
@@ -227,6 +249,28 @@ movement_possibility (tree stmt)
       || tree_could_trap_p (rhs))
     return MOVE_PRESERVE_EXECUTION;
 
+  if (get_call_expr_in (stmt))
+    {
+      /* While pure or const call is guaranteed to have no side effects, we
+	 cannot move it arbitrarily.  Consider code like
+
+	 char *s = something ();
+
+	 while (1)
+	   {
+	     if (s)
+	       t = strlen (s);
+	     else
+	       t = 0;
+	   }
+
+	 Here the strlen call cannot be moved out of the loop, even though
+	 s is invariant.  In addition to possibly creating a call with
+	 invalid arguments, moving out a function call that is not executed
+	 may cause performance regressions in case the call is costly and
+	 not executed at all.  */
+      return MOVE_PRESERVE_EXECUTION;
+    }
   return MOVE_POSSIBLE;
 }