tree-optimization/105726 - adjust array bound heuristic

There's heuristic to detect ptr[1].a[...] out of bound accesses
reasoning that if ptr points to an array of aggregates a trailing
incomplete array has to have size zero.  The following more
thoroughly constrains the cases this applies to avoid false
positive diagnostics.

2022-05-25  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/105726
	* gimple-ssa-warn-restrict.cc (builtin_memref::set_base_and_offset):
	Constrain array-of-flexarray case more.

	* g++.dg/warn/Warray-bounds-27.C: New testcase.

diff --git a/gcc/gimple-ssa-warn-restrict.cc b/gcc/gimple-ssa-warn-restrict.cc
index b678e80..734cdd7 100644 (file)
--- a/gcc/gimple-ssa-warn-restrict.cc
+++ b/gcc/gimple-ssa-warn-restrict.cc
@@ -525,7 +525,6 @@ builtin_memref::set_base_and_offset (tree expr)
     {
       tree memrefoff = fold_convert (ptrdiff_type_node, TREE_OPERAND (base, 1));
       extend_offset_range (memrefoff);
-      base = TREE_OPERAND (base, 0);
 
       if (refoff != HOST_WIDE_INT_MIN
          && TREE_CODE (expr) == COMPONENT_REF)
@@ -538,14 +537,19 @@ builtin_memref::set_base_and_offset (tree expr)
             REFOFF is set to s[1].b - (char*)s.  */
          offset_int off = tree_to_shwi (memrefoff);
          refoff += off;
-       }
-
-      if (!integer_zerop (memrefoff))
-       /* A non-zero offset into an array of struct with flexible array
-          members implies that the array is empty because there is no
-          way to initialize such a member when it belongs to an array.
-          This must be some sort of a bug.  */
-       refsize = 0;
+
+         if (!integer_zerop (memrefoff)
+             && !COMPLETE_TYPE_P (TREE_TYPE (expr))
+             && multiple_of_p (sizetype, memrefoff,
+                               TYPE_SIZE_UNIT (TREE_TYPE (base)), true))
+           /* A non-zero offset into an array of struct with flexible array
+              members implies that the array is empty because there is no
+              way to initialize such a member when it belongs to an array.
+              This must be some sort of a bug.  */
+           refsize = 0;
+       }
+
+      base = TREE_OPERAND (base, 0);
     }
 
   if (TREE_CODE (ref) == COMPONENT_REF)

diff --git a/gcc/testsuite/g++.dg/warn/Warray-bounds-27.C b/gcc/testsuite/g++.dg/warn/Warray-bounds-27.C
new file mode 100644 (file)
index 0000000..06ce089
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Warray-bounds-27.C
@@ -0,0 +1,16 @@
+// PR105726
+// { dg-do compile }
+// { dg-require-effective-target c++11 }
+// { dg-options "-O2 -Warray-bounds" }
+
+#include <array>
+#include <cstring>
+
+struct X {
+    char pad[4];
+    std::array<char, 1> mField;
+};
+
+void encode(char* aBuffer, const X& aMessage) {
+    strncpy(aBuffer, aMessage.mField.data(), 1); // { dg-bogus "bounds" }
+}