Fix PR41454 (dom miscompiles stuff)

[Michael Matz <matz at suse dot de>]

Hi,

the basic issue is that dom doesn't call update_stmt early enough, but 
instead queues this to when leaving the block.  That's not going to work 
as leave_block can thread over backedges.  There are counter-measures to 
not generate endless loops by that but they depends on updated statement 
for the target block (a dom parent in our case, hence not already 
processed by leave_block, hence with modified but not updated statements).  
In particular FOR_EACH_SSA_USE_OPERAND doesn't work with unupdated 
statements.

The fix is obviously to call update_stmt early enough.  With that we can 
also do away with the stmts_to_rescan set, that since some time wasn't 
doing interesting things anymore except defering update_stmt calls.  It 
once was used to defer changing virtual ops that are part of the hash 
code, that has to stay the same until avail expressions are removed.  But 
that can't happen anymore, and even more so even the removal code doesn't 
really seem to rely on this awkward condition.

Fixes the testcase, regstrapping on x86_64-linux in progress (all 
langs+Ada).  Okay if that passes?


Ciao,
Michael.
-- 
	PR tree-optimization/41454
	* tree-ssa-dom (stmts_to_rescan): Remove variable.
	(tree_ssa_dominator_optimize): Don't allocate and free it.
	(dom_opt_leave_block): Don't iterate over it.
	(optimize_stmt): Don't defer updating stmts.

testsuite/
	* gcc.dg/pr41454.c: New test.

Index: tree-ssa-dom.c
===================================================================
--- tree-ssa-dom.c	(revision 152118)
+++ tree-ssa-dom.c	(working copy)
@@ -127,15 +127,6 @@ DEF_VEC_ALLOC_P(expr_hash_elt_t,heap);
 
 static VEC(expr_hash_elt_t,heap) *avail_exprs_stack;
 
-/* Stack of statements we need to rescan during finalization for newly
-   exposed variables.
-
-   Statement rescanning must occur after the current block's available
-   expressions are removed from AVAIL_EXPRS.  Else we may change the
-   hash code for an expression and be unable to find/remove it from
-   AVAIL_EXPRS.  */
-static VEC(gimple,heap) *stmts_to_rescan;
-
 /* Structure for entries in the expression hash table.  */
 
 struct expr_hash_elt
@@ -623,7 +614,6 @@ tree_ssa_dominator_optimize (void)
   avail_exprs = htab_create (1024, real_avail_expr_hash, avail_expr_eq, free_expr_hash_elt);
   avail_exprs_stack = VEC_alloc (expr_hash_elt_t, heap, 20);
   const_and_copies_stack = VEC_alloc (tree, heap, 20);
-  stmts_to_rescan = VEC_alloc (gimple, heap, 20);
   need_eh_cleanup = BITMAP_ALLOC (NULL);
 
   /* Setup callbacks for the generic dominator tree walker.  */
@@ -733,7 +723,6 @@ tree_ssa_dominator_optimize (void)
   
   VEC_free (expr_hash_elt_t, heap, avail_exprs_stack);
   VEC_free (tree, heap, const_and_copies_stack);
-  VEC_free (gimple, heap, stmts_to_rescan);
   
   /* Free the value-handle array.  */
   threadedge_finalize_values ();
@@ -1773,20 +1762,6 @@ dom_opt_leave_block (struct dom_walk_dat
 
   remove_local_expressions_from_table ();
   restore_vars_to_original_value ();
-
-  /* If we queued any statements to rescan in this block, then
-     go ahead and rescan them now.  */
-  while (VEC_length (gimple, stmts_to_rescan) > 0)
-    {
-      gimple stmt = VEC_last (gimple, stmts_to_rescan);
-      basic_block stmt_bb = gimple_bb (stmt);
-
-      if (stmt_bb != bb)
-	break;
-
-      VEC_pop (gimple, stmts_to_rescan);
-      update_stmt (stmt);
-    }
 }
 
 /* Search for redundant computations in STMT.  If any are found, then
@@ -2218,6 +2193,8 @@ optimize_stmt (basic_block bb, gimple_st
   if (gimple_modified_p (stmt) || modified_p)
     {
       tree val = NULL;
+      
+      update_stmt_if_modified (stmt);
 
       if (gimple_code (stmt) == GIMPLE_COND)
         val = fold_binary_loc (gimple_location (stmt),
@@ -2238,13 +2215,6 @@ optimize_stmt (basic_block bb, gimple_st
 	    fprintf (dump_file, "  Flagged to clear EH edges.\n");
 	}
     }
-
-  /* Queue the statement to be re-scanned after all the
-     AVAIL_EXPRS have been processed.  The change buffer stack for
-     all the pushed statements will be processed when this queue
-     is emptied.  */
-  if (may_have_exposed_new_symbols)
-    VEC_safe_push (gimple, heap, stmts_to_rescan, gsi_stmt (si));
 }
 
 /* Search for an existing instance of STMT in the AVAIL_EXPRS table.
Index: testsuite/gcc.dg/pr41454.c
===================================================================
--- testsuite/gcc.dg/pr41454.c	(revision 0)
+++ testsuite/gcc.dg/pr41454.c	(revision 0)
@@ -0,0 +1,22 @@
+/* { dg-do run } */
+/* { dg-options "-O2 -fno-tree-vrp" } */
+
+extern void abort (void);
+
+int main ()
+{
+  int BM_tab2[0400];
+  int *BM_tab = BM_tab2;
+  int *BM_tab_base;
+
+  BM_tab_base = BM_tab;
+  BM_tab += 0400;
+  while (BM_tab_base != BM_tab)
+    {
+      *--BM_tab = 6;
+      *--BM_tab = 6;
+    }
+  if (BM_tab2[0] != 6)
+    abort ();
+  return 0;
+}