From 82b01035d1d2d4134355095f4130ba2de317690e Mon Sep 17 00:00:00 2001
From: Sam Lantinga <[EMAIL REDACTED]>
Date: Mon, 5 Dec 2022 09:42:22 -0800
Subject: [PATCH] Reduce the chance of overflow when calculating nanosecond
time
Fixes https://github.com/libsdl-org/SDL/issues/6762
---
src/timer/SDL_timer.c | 34 +++++++++++++++++++++++++++++++---
1 file changed, 31 insertions(+), 3 deletions(-)
diff --git a/src/timer/SDL_timer.c b/src/timer/SDL_timer.c
index 1fb9f2352620..d78c7d2cb2ce 100644
--- a/src/timer/SDL_timer.c
+++ b/src/timer/SDL_timer.c
@@ -471,7 +471,8 @@ SDL_bool SDL_RemoveTimer(SDL_TimerID id)
#endif /* !defined(__EMSCRIPTEN__) || !SDL_THREADS_DISABLED */
static Uint64 tick_start;
-static Uint64 tick_freq;
+static Uint32 tick_numerator;
+static Uint32 tick_denominator;
#if defined(SDL_TIMER_WINDOWS) && \
!defined(__WINRT__) && !defined(__XBOXONE__) && !defined(__XBOXSERIES__)
@@ -513,8 +514,19 @@ static void SDLCALL SDL_TimerResolutionChanged(void *userdata, const char *name,
}
}
+static Uint32 CalculateGCD(Uint32 a, Uint32 b)
+{
+ if (b == 0) {
+ return a;
+ }
+ return CalculateGCD(b, (a % b));
+}
+
void SDL_TicksInit(void)
{
+ Uint64 tick_freq;
+ Uint32 gcd;
+
if (tick_start) {
return;
}
@@ -525,6 +537,10 @@ void SDL_TicksInit(void)
SDL_TimerResolutionChanged, NULL);
tick_freq = SDL_GetPerformanceFrequency();
+ SDL_assert(tick_freq > 0 && tick_freq <= SDL_MAX_UINT32);
+ gcd = CalculateGCD(SDL_NS_PER_SECOND, (Uint32)tick_freq);
+ tick_numerator = (SDL_NS_PER_SECOND / gcd);
+ tick_denominator = (Uint32)(tick_freq / gcd);
tick_start = SDL_GetPerformanceCounter();
}
@@ -541,21 +557,33 @@ void SDL_TicksQuit(void)
Uint64
SDL_GetTickStartNS(void)
{
+ Uint64 starting_value, value;
+
if (!tick_start) {
SDL_TicksInit();
}
- return (tick_start * SDL_NS_PER_SECOND) / tick_freq;
+ starting_value = tick_start;
+ value = (starting_value * tick_numerator);
+ SDL_assert(value >= starting_value);
+ value /= tick_denominator;
+ return value;
}
Uint64
SDL_GetTicksNS(void)
{
+ Uint64 starting_value, value;
+
if (!tick_start) {
SDL_TicksInit();
}
- return ((SDL_GetPerformanceCounter() - tick_start) * SDL_NS_PER_SECOND) / tick_freq;
+ starting_value = (SDL_GetPerformanceCounter() - tick_start);
+ value = (starting_value * tick_numerator);
+ SDL_assert(value >= starting_value);
+ value /= tick_denominator;
+ return value;
}
Uint64 SDL_GetTicks(void)