[gcc r12-2583] Correct uninitialized object offset and size computation [PR101494].

[gcc r12-2583] Correct uninitialized object offset and size computation [PR101494].

[Martin Sebor]

https://gcc.gnu.org/g:1121e495b70105deeb82295f8210e30f2080bc37

commit r12-2583-g1121e495b70105deeb82295f8210e30f2080bc37
Author: Martin Sebor <msebor@redhat.com>
Date:   Wed Jul 28 16:25:40 2021 -0600

    Correct uninitialized object offset and size computation [PR101494].
    
    Resolves:
    PR middle-end/101494 - -Wuninitialized false alarm with memrchr of size 0
    
    gcc/ChangeLog:
    
            PR middle-end/101494
            * tree-ssa-uninit.c (maybe_warn_operand): Correct object offset
            and size computation.
    
    gcc/testsuite/ChangeLog:
    
            PR middle-end/101494
            * gcc.dg/uninit-pr101494.c: New test.

Diff:
---
 gcc/testsuite/gcc.dg/uninit-pr101494.c | 60 ++++++++++++++++++++++++++++++++++
 gcc/tree-ssa-uninit.c                  | 20 +++++++-----
 2 files changed, 72 insertions(+), 8 deletions(-)

diff --git a/gcc/testsuite/gcc.dg/uninit-pr101494.c b/gcc/testsuite/gcc.dg/uninit-pr101494.c
new file mode 100644
index 00000000000..d3267b6e43a
--- /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_pi (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 + 1, 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" }
+}
+
diff --git a/gcc/tree-ssa-uninit.c b/gcc/tree-ssa-uninit.c
index ab64a680568..5d7bc800419 100644
--- a/gcc/tree-ssa-uninit.c
+++ b/gcc/tree-ssa-uninit.c
@@ -371,16 +371,20 @@ 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