This is the mail archive of the mailing list for the GCC project.

Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

Fix loop_only_exit_p

while looking into loop code I noticed that loop_only_exit_p seems overly
simplistic. The purpose of this predicate is to decide whether the loop
must terminate by the exit given (so for example it is known that the number
of iteration test in that particular exit won't overflow because undefined
effect will happen then).

The code checks for calls with side effects, but it misses the fact that loop
can exit by an external EH (with -fnon-call-exceptions) or by an asm statement.
Exactly the same problem is being solved by the gcov code where it is necessary
which tries to verify that given the first statement of bb is executed, the
control flow will continue by one of the outgoing edges (which is necesssary
to make Kirhoff law applicable).

This patch thus unifies the logic and also fixes probably ages long bug about
ignoring external EH in profiling code.

Bootstrapped/regtested x86_64-linux, seems sane?


	* gimple.h (stmt_can_terminate_bb_p): New function.
	* tree-cfg.c (need_fake_edge_p): Rename to ...
	(stmt_can_terminate_bb_p): ... this; return true if stmt can
	throw external; handle const and pure calls.
	* tree-ssa-loop-niter.c (loop_only_exit_p): Use it.
Index: gimple.h
--- gimple.h	(revision 238191)
+++ gimple.h	(working copy)
@@ -1526,6 +1526,7 @@ extern void gimple_seq_set_location (gim
 extern void gimple_seq_discard (gimple_seq);
 extern void maybe_remove_unused_call_args (struct function *, gimple *);
 extern bool gimple_inexpensive_call_p (gcall *);
+extern bool stmt_can_terminate_bb_p (gimple *);
 /* Formal (expression) temporary table handling: multiple occurrences of
    the same scalar expression are evaluated into the same temporary.  */
Index: tree-cfg.c
--- tree-cfg.c	(revision 238191)
+++ tree-cfg.c	(working copy)
@@ -7919,15 +7919,20 @@ gimple_block_ends_with_condjump_p (const
-/* Return true if we need to add fake edge to exit at statement T.
-   Helper function for gimple_flow_call_edges_add.  */
+/* Return true if statement T may terminate execution of BB in ways not
+   explicitly represtented in the CFG.  */
-static bool
-need_fake_edge_p (gimple *t)
+stmt_can_terminate_bb_p (gimple *t)
   tree fndecl = NULL_TREE;
   int call_flags = 0;
+  /* Eh exception not handled internally terminates execution of the whole
+     function.  */
+  if (stmt_can_throw_external (t))
+    return true;
   /* NORETURN and LONGJMP calls already have an edge to exit.
      CONST and PURE calls do not need one.
      We don't currently check for CONST and PURE here, although
@@ -7960,6 +7965,13 @@ need_fake_edge_p (gimple *t)
       edge e;
       basic_block bb;
+      if (call_flags & (ECF_PURE | ECF_CONST)
+	  && !(call_flags & ECF_LOOPING_CONST_OR_PURE))
+	return false;
+      /* Function call may do longjmp, terminate program or do other things.
+	 Special case noreturn that have non-abnormal edges out as in this case
+	 the fact is sufficiently represented by lack of edges out of T.  */
       if (!(call_flags & ECF_NORETURN))
 	return true;
@@ -8024,7 +8036,7 @@ gimple_flow_call_edges_add (sbitmap bloc
       if (!gsi_end_p (gsi))
 	t = gsi_stmt (gsi);
-      if (t && need_fake_edge_p (t))
+      if (t && stmt_can_terminate_bb_p (t))
 	  edge e;
@@ -8059,7 +8071,7 @@ gimple_flow_call_edges_add (sbitmap bloc
 	      stmt = gsi_stmt (gsi);
-	      if (need_fake_edge_p (stmt))
+	      if (stmt_can_terminate_bb_p (stmt))
 		  edge e;
Index: tree-ssa-loop-niter.c
--- tree-ssa-loop-niter.c	(revision 238191)
+++ tree-ssa-loop-niter.c	(working copy)
@@ -2159,7 +2159,6 @@ loop_only_exit_p (const struct loop *loo
   basic_block *body;
   gimple_stmt_iterator bsi;
   unsigned i;
-  gimple *call;
   if (exit != single_exit (loop))
     return false;
@@ -2168,17 +2167,8 @@ loop_only_exit_p (const struct loop *loo
   for (i = 0; i < loop->num_nodes; i++)
       for (bsi = gsi_start_bb (body[i]); !gsi_end_p (bsi); gsi_next (&bsi))
-	{
-	  call = gsi_stmt (bsi);
-	  if (gimple_code (call) != GIMPLE_CALL)
-	    continue;
-	  if (gimple_has_side_effects (call))
-	    {
-	      free (body);
-	      return false;
-	    }
-	}
+	if (stmt_can_terminate_bb_p (gsi_stmt (bsi)))
+	  return true;
   free (body);
       if (EDGE_COUNT (bb->succs) > 1)

Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]