[Bug tree-optimization/64277] [4.9/5 Regression] Incorrect warning "array subscript is above array bounds"

[enkovich.gnu at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64277

--- Comment #13 from Ilya Enkovich <enkovich.gnu at gmail dot com> ---
Ranges have to be used for maxiter computations to have consistent analysis in
complete unroll and vrp.  Following patch allows to refine maxiter estimation
using ranges and avoid warnings.

diff --git a/gcc/tree-ssa-loop-niter.c b/gcc/tree-ssa-loop-niter.c
index 919f5c0..14cce2a 100644
--- a/gcc/tree-ssa-loop-niter.c
+++ b/gcc/tree-ssa-loop-niter.c
@@ -2754,6 +2754,7 @@ record_nonwrapping_iv (struct loop *loop, tree base, tree
step, gimple stmt,
 {
   tree niter_bound, extreme, delta;
   tree type = TREE_TYPE (base), unsigned_type;
+  tree orig_base = base;

   if (TREE_CODE (step) != INTEGER_CST || integer_zerop (step))
     return;
@@ -2777,7 +2778,14 @@ record_nonwrapping_iv (struct loop *loop, tree base,
tree step, gimple stmt,

   if (tree_int_cst_sign_bit (step))
     {
+      wide_int min, max, highwi = high;
       extreme = fold_convert (unsigned_type, low);
+      if (TREE_CODE (orig_base) == SSA_NAME
+         && !POINTER_TYPE_P (TREE_TYPE (orig_base))
+         && SSA_NAME_RANGE_INFO (orig_base)
+         && get_range_info (orig_base, &min, &max) == VR_RANGE
+         && wi::gts_p (highwi, max))
+       base = wide_int_to_tree (unsigned_type, max);
       if (TREE_CODE (base) != INTEGER_CST)
        base = fold_convert (unsigned_type, high);
       delta = fold_build2 (MINUS_EXPR, unsigned_type, base, extreme);
@@ -2785,8 +2793,15 @@ record_nonwrapping_iv (struct loop *loop, tree base,
tree step, gimple stmt,
     }
   else
     {
+      wide_int min, max, lowwi = low;
       extreme = fold_convert (unsigned_type, high);
-      if (TREE_CODE (base) != INTEGER_CST)
+      if (TREE_CODE (orig_base) == SSA_NAME
+         && !POINTER_TYPE_P (TREE_TYPE (orig_base))
+         && SSA_NAME_RANGE_INFO (orig_base)
+         && get_range_info (orig_base, &min, &max) == VR_RANGE
+         && wi::gts_p (min, lowwi))
+       base = wide_int_to_tree (unsigned_type, min);
+      else if (TREE_CODE (base) != INTEGER_CST)
        base = fold_convert (unsigned_type, low);
       delta = fold_build2 (MINUS_EXPR, unsigned_type, extreme, base);
     }
diff --git a/gcc/testsuite/gcc.dg/pr64277.c b/gcc/testsuite/gcc.dg/pr64277.c
new file mode 100644
index 0000000..0d5ef11
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr64277.c
@@ -0,0 +1,21 @@
+/* PR tree-optimization/64277 */
+/* { dg-do compile } */
+/* { dg-options "-O3 -Wall -Werror" } */
+
+
+int f1[10];
+void test1 (short a[], short m, unsigned short l)
+{
+  int i = l;
+  for (i = i + 5; i < m; i++)
+    f1[i] = a[i]++;
+}
+
+void test2 (short a[], short m, short l)
+{
+  int i;
+  if (m > 5)
+    m = 5;
+  for (i = m; i > l; i--)
+    f1[i] = a[i]++;
+}