From 6995a2e1bac6812ec4f48b98cfaf321f75c320d8 Mon Sep 17 00:00:00 2001
From: AbigailBuccaneer <AbigailBuccaneer@users.noreply.github.com>
Date: Mon, 24 Aug 2020 18:04:37 +0100
Subject: [PATCH] Avoid undefined behaviour when byteswapping

`a << b` is undefined when `a` is negative, and `a >> b` is
implementation-defined. The correct thing to do here is to cast to
unsigned, swap the bytes there and then swap back.

This also improves performance on some compilers: Clang is smart enough
to recognise that we're byteswapping here and reduce it to a single
`bswap` instruction on x86_64, but only for the unsigned versions.
---
 libraries/classparser/src/javaendian.h | 27 +++++---------------------
 1 file changed, 5 insertions(+), 22 deletions(-)

diff --git a/libraries/classparser/src/javaendian.h b/libraries/classparser/src/javaendian.h
index 6eb4de63..5a6e107b 100644
--- a/libraries/classparser/src/javaendian.h
+++ b/libraries/classparser/src/javaendian.h
@@ -22,21 +22,6 @@ inline uint16_t bigswap(uint16_t x)
     return x;
 }
 
-inline int64_t bigswap(int64_t x)
-{
-    return x;
-}
-
-inline int32_t bigswap(int32_t x)
-{
-    return x;
-}
-
-inline int16_t bigswap(int16_t x)
-{
-    return x;
-}
-
 #else
 inline uint64_t bigswap(uint64_t x)
 {
@@ -55,22 +40,20 @@ inline uint16_t bigswap(uint16_t x)
     return (x >> 8) | (x << 8);
 }
 
+#endif
+
 inline int64_t bigswap(int64_t x)
 {
-    return (x >> 56) | ((x << 40) & 0x00FF000000000000) | ((x << 24) & 0x0000FF0000000000) |
-           ((x << 8) & 0x000000FF00000000) | ((x >> 8) & 0x00000000FF000000) |
-           ((x >> 24) & 0x0000000000FF0000) | ((x >> 40) & 0x000000000000FF00) | (x << 56);
+    return static_cast<int64_t>(bigswap(static_cast<uint64_t>(x)));
 }
 
 inline int32_t bigswap(int32_t x)
 {
-    return (x >> 24) | ((x << 8) & 0x00FF0000) | ((x >> 8) & 0x0000FF00) | (x << 24);
+    return static_cast<int32_t>(bigswap(static_cast<uint32_t>(x)));
 }
 
 inline int16_t bigswap(int16_t x)
 {
-    return (x >> 8) | (x << 8);
+    return static_cast<int16_t>(bigswap(static_cast<uint16_t>(x)));
 }
-
-#endif
 }