[PATCH] tree-optimization/105726 - adjust array bound heuristic

[Richard Biener <rguenther@suse.de>]

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.

Bootstrapped and tested on x86_64-unknown-linux-gnu.

OK?

Thanks,
Richard.

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.
---
 gcc/gimple-ssa-warn-restrict.cc              | 22 ++++++++++++--------
 gcc/testsuite/g++.dg/warn/Warray-bounds-27.C | 16 ++++++++++++++
 2 files changed, 29 insertions(+), 9 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/warn/Warray-bounds-27.C

diff --git a/gcc/gimple-ssa-warn-restrict.cc b/gcc/gimple-ssa-warn-restrict.cc
index b678e806da3..734cdd7f5b4 100644
--- 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
index 00000000000..06ce089c4b0
--- /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" }
+}
-- 
2.35.3