From 51c06fb2bfe238bf76b0ec0da3175c04f4bea2f2 Mon Sep 17 00:00:00 2001
From: TAKAI Kousuke <62541129+t-a-k@users.noreply.github.com>
Date: Fri, 8 Oct 2021 22:58:54 +0900
Subject: Simplify IV abs operation in pp_abs.

Transforming -iv into "(UV)-(iv + 1) + 1" can avoid signed integer
overflow even if iv was IV_MIN in 2's compelement representation
(as long as iv < 0), so that it makes special treatment for IV_MIN
unnecessary and saves one conditional jump.
---
 pp.c | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

(limited to 'pp.c')

diff --git a/pp.c b/pp.c
index 25d3a319cd..643a8758b7 100644
--- a/pp.c
+++ b/pp.c
@@ -3047,13 +3047,12 @@ PP(pp_abs)
           if (iv >= 0) {
             uv = (UV)iv;
           } else {
-            if (iv != IV_MIN) {
-              uv = (UV)-iv;
-            } else {
-              /* 2s complement assumption. Also, not really needed as
-                 IV_MIN and -IV_MIN should both be %100...00 and NV-able  */
-              uv = (UV)IV_MIN;
-            }
+              /* "(UV)-(iv + 1) + 1" below is mathematically "-iv", but
+                 transformed so that every subexpression will never trigger
+                 overflows even on 2's complement representation (note that
+                 iv is always < 0 here), and modern compilers could optimize
+                 this to a single negation.  */
+              uv = (UV)-(iv + 1) + 1;
           }
         }
       set_uv:
-- 
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