[gcc r13-4357] tree-optimization/107493 - SCEV analysis with conversions

[Richard Biener <rguenth@gcc.gnu.org>]

https://gcc.gnu.org/g:2b293a949c0fbe21e47c4bc99f807dc941c02bb6

commit r13-4357-g2b293a949c0fbe21e47c4bc99f807dc941c02bb6
Author: Richard Biener <rguenther@suse.de>
Date:   Mon Nov 28 10:25:44 2022 +0100

    tree-optimization/107493 - SCEV analysis with conversions
    
    This shows another case where trying to validate conversions during
    the CHREC SCC analysis fails because said analysis assumes we are
    converting a complete SCC.  Like the constraint on the initial
    conversion seen restrict all conversions handled to sign-changes.
    
            PR tree-optimization/107493
            * tree-scalar-evolution.cc (scev_dfs::follow_ssa_edge_expr):
            Only handle no-op and sign-changing conversions.
    
            * gcc.dg/torture/pr107493.c: New testcase.

Diff:
---
 gcc/testsuite/gcc.dg/torture/pr107493.c | 13 +++++++++++++
 gcc/tree-scalar-evolution.cc            | 10 ++++++++--
 2 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/gcc/testsuite/gcc.dg/torture/pr107493.c b/gcc/testsuite/gcc.dg/torture/pr107493.c
new file mode 100644
index 00000000000..b36692133d8
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/torture/pr107493.c
@@ -0,0 +1,13 @@
+/* { dg-do run { target int32plus } } */
+
+int a;
+int b(int c) { return c; }
+int main()
+{
+  a = -21;
+  for (; a <= 0; a = (unsigned short)(b(a + 2) + 8))
+    ;
+  if (a != 65525)
+    __builtin_abort();
+  return 0;
+}
diff --git a/gcc/tree-scalar-evolution.cc b/gcc/tree-scalar-evolution.cc
index 60060337804..f75398afb7c 100644
--- a/gcc/tree-scalar-evolution.cc
+++ b/gcc/tree-scalar-evolution.cc
@@ -1227,10 +1227,16 @@ scev_dfs::follow_ssa_edge_expr (gimple *at_stmt, tree expr,
     {
     CASE_CONVERT:
       {
-	/* This assignment is under the form "a_1 = (cast) rhs.  */
+	/* This assignment is under the form "a_1 = (cast) rhs.  We cannot
+	   validate any precision altering conversion during the SCC
+	   analysis, so don't even try.  */
+	if (!tree_nop_conversion_p (type, TREE_TYPE (rhs0)))
+	  return t_false;
 	t_bool res = follow_ssa_edge_expr (at_stmt, rhs0,
 					   evolution_of_loop, limit);
-	*evolution_of_loop = chrec_convert (type, *evolution_of_loop, at_stmt);
+	if (res == t_true)
+	  *evolution_of_loop = chrec_convert (type, *evolution_of_loop,
+					      at_stmt);
 	return res;
       }