[PATCH] Fix PRs 79955 and 79956 (hopefully)

[PATCH] Fix PRs 79955 and 79956 (hopefully)

[Richard Biener]

The following avoids warning about completely out-of-bound accesses
as uninitialized.

For GCC8 we likely want to enhance path isolation to catch those
cases (and eventually issue a diagnostic about them).  And we
finally want to fix complete peeling to not introduce those
accesses.

Bootstrap and regtest running on x86_64-unknown-linux-gnu.

Richard.

2017-03-08  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/79955
	PR tree-optimization/79956
	* tree-ssa-uninit.c (warn_uninitialized_vars): Do not warn
	for accesses that are completely outside of the variable.

	* gcc.dg/uninit-24.c: New testcase.

Index: gcc/tree-ssa-uninit.c
===================================================================
--- gcc/tree-ssa-uninit.c	(revision 245968)
+++ gcc/tree-ssa-uninit.c	(working copy)
@@ -287,6 +287,17 @@ warn_uninitialized_vars (bool warn_possi
 		  || TREE_NO_WARNING (base))
 		continue;
 
+	      /* Do not warn if the access is fully outside of the
+	         variable.  */
+	      if (ref.size != -1
+		  && ref.max_size == ref.size
+		  && (ref.offset + ref.size <= 0
+		      || (ref.offset >= 0
+			  && TREE_CODE (DECL_SIZE (base)) == INTEGER_CST
+			  && compare_tree_int (DECL_SIZE (base),
+					       ref.offset) <= 0)))
+		continue;
+
 	      /* Limit the walking to a constant number of stmts after
 	         we overcommit quadratic behavior for small functions
 		 and O(n) behavior.  */
Index: gcc/testsuite/gcc.dg/uninit-24.c
===================================================================
--- gcc/testsuite/gcc.dg/uninit-24.c	(nonexistent)
+++ gcc/testsuite/gcc.dg/uninit-24.c	(working copy)
@@ -0,0 +1,10 @@
+/* { dg-do compile } */
+/* { dg-options "-O -Wmaybe-uninitialized" } */
+
+int foo (int x)
+{
+  int y;
+  if (x)
+    return *(&y + 1); /* { dg-bogus "may be used uninitialized" } */
+  return 0;
+}

Re: [PATCH] Fix PRs 79955 and 79956 (hopefully)

[Jakub Jelinek]

On Wed, Mar 08, 2017 at 11:12:47AM +0100, Richard Biener wrote:
> 
> The following avoids warning about completely out-of-bound accesses
> as uninitialized.
> 
> For GCC8 we likely want to enhance path isolation to catch those
> cases (and eventually issue a diagnostic about them).  And we
> finally want to fix complete peeling to not introduce those
> accesses.
> 
> Bootstrap and regtest running on x86_64-unknown-linux-gnu.

LGTM, thanks.

> 2017-03-08  Richard Biener  <rguenther@suse.de>
> 
> 	PR tree-optimization/79955
> 	PR tree-optimization/79956
> 	* tree-ssa-uninit.c (warn_uninitialized_vars): Do not warn
> 	for accesses that are completely outside of the variable.
> 
> 	* gcc.dg/uninit-24.c: New testcase.

	Jakub

Re: [PATCH] Fix PRs 79955 and 79956 (hopefully)

[Jeff Law]

On 03/08/2017 03:12 AM, Richard Biener wrote:
>
> The following avoids warning about completely out-of-bound accesses
> as uninitialized.
>
> For GCC8 we likely want to enhance path isolation to catch those
> cases (and eventually issue a diagnostic about them).  And we
> finally want to fix complete peeling to not introduce those
> accesses.
FWIW, I want to do isolation of o-o-b array accesses and this would 
likely fit into that reasonably well.

jeff