From 7e5b390a008ccad094a39c350f385d58e8a5102a Mon Sep 17 00:00:00 2001
From: Karl Williamson <khw@cpan.org>
Date: Fri, 3 May 2019 13:57:47 -0600
Subject: [PATCH] Remove undefined behavior from IV shifting
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It is undefined behavior to shift a negative integer to the left.  This
commit avoids that by treating the value as unsigned, then casting back
to integer for return.

Petr Písař: Ported to 5.30.0 from
814735a391b874af8f00eaf89469e5ec7f38cd4aa.

Signed-off-by: Petr Písař <ppisar@redhat.com>
---
 asan_ignore |  5 -----
 pp.c        | 21 ++++++++++++++++++++-
 2 files changed, 20 insertions(+), 6 deletions(-)

diff --git a/asan_ignore b/asan_ignore
index e0f5685..f520546 100644
--- a/asan_ignore
+++ b/asan_ignore
@@ -18,11 +18,6 @@
 
 fun:Perl_pp_i_*
 
-# Perl's << is defined as using the underlying C's << operator, with the
-# same undefined behaviour for shifts greater than the word size.
-# (UVs normally, IVs with 'use integer')
-
-fun:Perl_pp_left_shift
 
 # this function numifies the field width in eg printf "%10f".
 # It has its own overflow detection, so don't warn about it
diff --git a/pp.c b/pp.c
index 7afb090..3ca04e1 100644
--- a/pp.c
+++ b/pp.c
@@ -1991,10 +1991,29 @@ static IV S_iv_shift(IV iv, int shift, bool left)
        shift = -shift;
        left = !left;
    }
+
    if (UNLIKELY(shift >= IV_BITS)) {
        return iv < 0 && !left ? -1 : 0;
    }
-   return left ? iv << shift : iv >> shift;
+   /* For left shifts, perl 5 has chosen to treat the value as unsigned for
+    * the * purposes of shifting, then cast back to signed.  This is very
+    * different from perl 6:
+    *
+    * $ perl6 -e 'say -2 +< 5'
+    * -64
+    *
+    * $ ./perl -le 'print -2 << 5'
+    * 18446744073709551552
+    * */
+   if (left) {
+       if (iv == IV_MIN) { /* Casting this to a UV is undefined behavior */
+           return 0;
+       }
+       return (IV) (((UV) iv) << shift);
+   }
+
+   /* Here is right shift */
+   return iv >> shift;
 }
 
 #define UV_LEFT_SHIFT(uv, shift) S_uv_shift(uv, shift, TRUE)
-- 
2.20.1