From 001e4e11ca1dfd0ded1bbc29e26f71be50c86aac Mon Sep 17 00:00:00 2001 From: Tasos Sahanidis Date: Thu, 17 May 2018 17:55:38 +0300 Subject: [PATCH] Correct C FixedMul() off-by-one errors The FixedMul() C implementation would produce off by one results, causing constant desyncs on 64 bit builds and builds without an ASM implementation of the function. This is fixed by shifting instead of dividing, possibly avoiding rounding errors. --- src/m_fixed.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) diff --git a/src/m_fixed.c b/src/m_fixed.c index ce7471a2..01445738 100644 --- a/src/m_fixed.c +++ b/src/m_fixed.c @@ -33,7 +33,9 @@ */ fixed_t FixedMul(fixed_t a, fixed_t b) { - return (fixed_t)((((INT64)a * b) ) / FRACUNIT); + // Need to cast to unsigned before shifting to avoid undefined behaviour + // for negative integers + return (fixed_t)(((UINT64)((INT64)a * b)) >> FRACBITS); } #endif //__USE_C_FIXEDMUL__