Correct uninitialized object offset and size computation [PR101494].

Resolves:
PR middle-end/101494 - -uninitialized false alarm with memrchr of size 0

gcc/ChangeLog:

	PR middle-end/101494
	* tree-ssa-uninit.c (builtin_call_nomodifying_p):
	(check_defs):
	(maybe_warn_operand):

gcc/testsuite/ChangeLog:

	PR middle-end/101494
	* gcc.dg/uninit-38.c:
	* gcc.dg/uninit-41.c: New test.
	* gcc.dg/uninit-pr101494.c: New test.

@@ -304,16 +344,20 @@ maybe_warn_operand (ao_ref &ref, gimple *stmt, tree lhs, tree rhs,
       || get_no_uninit_warning (base))
     return NULL_TREE;
 
-  /* Do not warn if the access is fully outside of the variable.  */
+  /* Do not warn if the access is zero size or if it's fully outside
+     the object.  */
   poly_int64 decl_size;
+  if (known_size_p (ref.size)
+      && known_eq (ref.max_size, ref.size)
+      && (known_eq (ref.size, 0)
+	  || known_le (ref.offset + ref.size, 0)))
+    return NULL_TREE;
+
   if (DECL_P (base)
-      && ((known_size_p (ref.size)
-	   && known_eq (ref.max_size, ref.size)
-	   && known_le (ref.offset + ref.size, 0))
-	  || (known_ge (ref.offset, 0)
-	      && DECL_SIZE (base)
-	      && poly_int_tree_p (DECL_SIZE (base), &decl_size)
-	      && known_le (decl_size, ref.offset))))
+      && known_ge (ref.offset, 0)
+      && DECL_SIZE (base)
+      && poly_int_tree_p (DECL_SIZE (base), &decl_size)
+      && known_le (decl_size, ref.offset))
     return NULL_TREE;
 
   /* Do not warn if the result of the access is then used for
diff --git a/gcc/testsuite/gcc.dg/uninit-pr101494.c b/gcc/testsuite/gcc.dg/uninit-pr101494.c
new file mode 100644
index 00000000000..4fcb5f2dc79
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/uninit-pr101494.c
@@ -0,0 +1,60 @@
+/* PR middle-end/101494 - bogus -Wmaybe-uninitialized on memrchr of size 0
+   { dg-do compile }
+   { dg-options "-O2 -Wall" } */
+
+typedef __SIZE_TYPE__ size_t;
+
+void* alloca (size_t);
+
+__attribute__ ((malloc, alloc_size (1))) void* alloc (size_t);
+
+__attribute__ ((access (read_only, 1, 2))) void* sink (void*, size_t);
+
+void test_alloca_zero (size_t i)
+{
+  char *p = alloca (0);
+  sink (p, 0);      // { dg-bogus "\\\[-Wuninitialized" }
+}
+
+void test_alloca_zero_p1 (size_t i)
+{
+  char *p = alloca (0);
+  sink (p + i, 0);
+}
+
+void test_alloca_cst (void)
+{
+  char *p = alloca (7);
+  sink (p, 0);      // { dg-bogus "\\\[-Wuninitialized" }
+}
+
+void test_alloca_cst_p1 (void)
+{
+  char *p = alloca (7);
+  sink (p, 0);      // { dg-bogus "\\\[-Wuninitialized" }
+}
+
+void test_alloca_cst_p7 (void)
+{
+  char *p = alloca (7);
+  sink (p + 7, 0);  // { dg-bogus "\\\[-Wuninitialized" }
+}
+
+void test_alloca_var (size_t n)
+{
+  char *p = alloca (n);
+  sink (p, 0);      // { dg-bogus "\\\[-Wuninitialized" }
+}
+
+void test_alloca_var_p1 (size_t n)
+{
+  char *p = alloca (n);
+  sink (p + 1, 0);  // { dg-bogus "\\\[-Wuninitialized" }
+}
+
+void test_alloca_var_pn (size_t n)
+{
+  char *p = alloca (n);
+  sink (p + n, 0);  // { dg-bogus "\\\[-Wuninitialized" }
+}
+