[PATCH] vect: Fix outer loop vectorization for nested uncounted loops [PR123657]

[Victor Do Nascimento]

Given the inability of `expr_invariant_in_loop_p (loop, expr)' to
handle the `scev_not_known' node as the expression, an unknown loop
bound in the inner loop in a nested set of loops led to
`vect_analyze_loop_form' to erroneously consider the outer loop as
suitable for vectorization.  This introduces the necessary unknown
loop iteration count check to ensure correct handling of counted loops
with an embedded uncounted loop.

Bootstrapped on AArch64 and x86_64, no regressions.

gcc/ChangeLog:

        PR tree-optimization/123657
        * tree-vect-loop.cc (vect_analyze_loop_form): Add
        chrec_dont_know check.
        * tree-ssa-loop-ivopts.cc (expr_invariant_in_loop_p):
        likewise.

gcc/testsuite/ChangeLog:

        * gcc.dg/vect/vect-uncounted-run_4.c (main): New.
---
 .../gcc.dg/vect/vect-uncounted-run_4.c          | 17 +++++++++++++++++
 gcc/tree-ssa-loop-ivopts.cc                     |  4 ++++
 gcc/tree-vect-loop.cc                           |  3 ++-
 3 files changed, 23 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/gcc.dg/vect/vect-uncounted-run_4.c

diff --git a/gcc/testsuite/gcc.dg/vect/vect-uncounted-run_4.c 
b/gcc/testsuite/gcc.dg/vect/vect-uncounted-run_4.c
new file mode 100644
index 00000000000..f731daf42d9
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/vect/vect-uncounted-run_4.c
@@ -0,0 +1,17 @@
+/* Ensure we don't vectorize outer loops when the inner loop is uncounted.  */
+/* { dg-add-options vect_early_break } */
+/* { dg-require-effective-target vect_early_break_hw } */
+/* { dg-require-effective-target vect_int } */
+/* { dg-additional-options "-O3" } */
+
+int a;
+int main() {
+  for (int b = 0; b < 21; b++) {
+    int c = b;
+    while (c)
+      a = c >>= 1;
+  }
+  if (a != 0) __builtin_abort();
+}
+
+/* { dg-final { scan-tree-dump "missed:   not vectorized: inner-loop count not 
invariant." "vect" } } */
diff --git a/gcc/tree-ssa-loop-ivopts.cc b/gcc/tree-ssa-loop-ivopts.cc
index 6ecf5bef7b4..62df570003d 100644
--- a/gcc/tree-ssa-loop-ivopts.cc
+++ b/gcc/tree-ssa-loop-ivopts.cc
@@ -1825,6 +1825,10 @@ expr_invariant_in_loop_p (class loop *loop, tree expr)
 
   gcc_assert (loop_depth (loop) > 0);
 
+  /* CHREC_DONT_KNOW treated as invariant and thus, if not checked,
+     misleadingly returns TRUE.  */
+  gcc_assert (expr != chrec_dont_know);
+
   if (is_gimple_min_invariant (expr))
     return true;
 
diff --git a/gcc/tree-vect-loop.cc b/gcc/tree-vect-loop.cc
index 03339641750..9d6963a26fe 100644
--- a/gcc/tree-vect-loop.cc
+++ b/gcc/tree-vect-loop.cc
@@ -1555,7 +1555,8 @@ vect_analyze_loop_form (class loop *loop, gimple 
*loop_vectorized_call,
        return opt_result::failure_at (vect_location,
                                       "not vectorized: Bad inner loop.\n");
 
-      if (!expr_invariant_in_loop_p (loop, inner.number_of_iterations))
+      if (inner.number_of_iterations ==  chrec_dont_know
+         || !expr_invariant_in_loop_p (loop, inner.number_of_iterations))
        return opt_result::failure_at (vect_location,
                                       "not vectorized: inner-loop count not"
                                       " invariant.\n");
-- 
2.43.0