[gcc r14-9293] tree-optimization/114203 - wrong CLZ niter computation

[gcc r14-9293] tree-optimization/114203 - wrong CLZ niter computation

[Richard Biener]

https://gcc.gnu.org/g:cde50296a19b109909089b91d532d2c8455f5f10

commit r14-9293-gcde50296a19b109909089b91d532d2c8455f5f10
Author: Richard Biener <rguenther@suse.de>
Date:   Mon Mar 4 10:38:31 2024 +0100

    tree-optimization/114203 - wrong CLZ niter computation
    
    For precision less than int we apply the adjustment to make it defined
    at zero after the adjustment to make it compute CLZ rather than CTZ.
    That's wrong.
    
            PR tree-optimization/114203
            * tree-ssa-loop-niter.cc (build_cltz_expr): Apply CTZ->CLZ
            adjustment before making the result defined at zero.
    
            * gcc.dg/torture/pr114203.c: New testcase.

Diff:
---
 gcc/testsuite/gcc.dg/torture/pr114203.c | 21 +++++++++++++++++++++
 gcc/tree-ssa-loop-niter.cc              |  7 +++----
 2 files changed, 24 insertions(+), 4 deletions(-)

diff --git a/gcc/testsuite/gcc.dg/torture/pr114203.c b/gcc/testsuite/gcc.dg/torture/pr114203.c
new file mode 100644
index 00000000000..0ef6279942a
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/torture/pr114203.c
@@ -0,0 +1,21 @@
+/* { dg-do run } */
+
+int __attribute__((noipa))
+foo (unsigned char b)
+{
+  int c = 0;
+
+  while (b) {
+      b >>= 1;
+      c++;
+  }
+
+  return c;
+}
+
+int main()
+{
+  if (foo(0) != 0)
+    __builtin_abort ();
+  return 0;
+}
diff --git a/gcc/tree-ssa-loop-niter.cc b/gcc/tree-ssa-loop-niter.cc
index 038e4331661..c6d010f6d89 100644
--- a/gcc/tree-ssa-loop-niter.cc
+++ b/gcc/tree-ssa-loop-niter.cc
@@ -2288,6 +2288,9 @@ build_cltz_expr (tree src, bool leading, bool define_at_zero)
 	src = fold_convert (unsigned_type_node, src);
 
       call = build_call_expr (fn, 1, src);
+      if (leading && prec < i_prec)
+	call = fold_build2 (MINUS_EXPR, integer_type_node, call,
+			    build_int_cst (integer_type_node, i_prec - prec));
       if (define_at_zero)
 	{
 	  tree is_zero = fold_build2 (NE_EXPR, boolean_type_node, src,
@@ -2295,10 +2298,6 @@ build_cltz_expr (tree src, bool leading, bool define_at_zero)
 	  call = fold_build3 (COND_EXPR, integer_type_node, is_zero, call,
 			      build_int_cst (integer_type_node, prec));
 	}
-
-      if (leading && prec < i_prec)
-	call = fold_build2 (MINUS_EXPR, integer_type_node, call,
-			    build_int_cst (integer_type_node, i_prec - prec));
     }
 
   return call;