From 4f40aee87d1598407aafd9bb35a491d46c766c19 Mon Sep 17 00:00:00 2001
From: Aldy Hernandez <aldyh@redhat.com>
Date: Wed, 11 Mar 2020 11:05:46 +0100
Subject: Document narrowing cast discrepancy.

---
 gcc/tree-vrp.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

(limited to 'gcc')

diff --git a/gcc/tree-vrp.c b/gcc/tree-vrp.c
index 1270d19..a9d85ed 100644
--- a/gcc/tree-vrp.c
+++ b/gcc/tree-vrp.c
@@ -2134,6 +2134,18 @@ register_edge_assert_for_2 (tree name, edge e,
          simply register the same assert for it.  */
       if (CONVERT_EXPR_CODE_P (rhs_code))
 	{
+	  // This disables narrowing casts:
+	  //
+	  // long unsigned int _2;
+	  // char _3;
+	  // _3 = (char) _2;
+	  // if (_3 == 0)
+	  //   goto <bb 6>;
+	  //
+	  // evrp gets [0,0]
+	  // gori gets [0,0][256, 18446744073709551360]
+	  //
+	  // See: See tree-ssa/ivopts-lower_base.c
 	  gori_computable set_gori_computable (false);
 	  wide_int rmin, rmax;
 	  tree rhs1 = gimple_assign_rhs1 (def_stmt);
-- 
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