Backport: Bug [#2115]. Fix undefined behaviour of shifting negative values.

Backport of changeset 7612:d70ccc4f172a.

1 files changed, 3 insertions, 2 deletions

diff --git a/src/PositionCache.cxx b/src/PositionCache.cxx
index 8910a2955..03393536d 100644
--- a/src/PositionCache.cxx
+++ b/src/PositionCache.cxx
@@ -615,10 +615,11 @@ bool PositionCacheEntry::Retrieve(unsigned int styleNumber_, const char *s_,
 }
 
 unsigned int PositionCacheEntry::Hash(unsigned int styleNumber_, const char *s, unsigned int len_) noexcept {
-	unsigned int ret = s[0] << 7;
+	const unsigned char *us = reinterpret_cast<const unsigned char *>(s);
+	unsigned int ret = us[0] << 7;
 	for (unsigned int i=0; i<len_; i++) {
 		ret *= 1000003;
-		ret ^= s[i];
+		ret ^= us[i];
 	}
 	ret *= 1000003;
 	ret ^= len_;