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Reduce the chance of overflow when calculating nanosecond time

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Fixes https://github.com/libsdl-org/SDL/issues/6762
This commit is contained in:
Sam Lantinga 2022-12-05 09:42:22 -08:00
parent ed6d401616
commit 82b01035d1
1 changed files with 31 additions and 3 deletions
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34
src/timer/SDL_timer.c
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@ -471,7 +471,8 @@ SDL_bool SDL_RemoveTimer(SDL_TimerID id)
#endif /* !defined(__EMSCRIPTEN__) || !SDL_THREADS_DISABLED */ #endif /* !defined(__EMSCRIPTEN__) || !SDL_THREADS_DISABLED */
static Uint64 tick_start; static Uint64 tick_start;
static Uint64 tick_freq; static Uint32 tick_numerator;
static Uint32 tick_denominator;
#if defined(SDL_TIMER_WINDOWS) && \ #if defined(SDL_TIMER_WINDOWS) && \
!defined(__WINRT__) && !defined(__XBOXONE__) && !defined(__XBOXSERIES__) !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) void SDL_TicksInit(void)
{ {
Uint64 tick_freq;
Uint32 gcd;
if (tick_start) { if (tick_start) {
return; return;
} }
@ -525,6 +537,10 @@ void SDL_TicksInit(void)
SDL_TimerResolutionChanged, NULL); SDL_TimerResolutionChanged, NULL);
tick_freq = SDL_GetPerformanceFrequency(); 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(); tick_start = SDL_GetPerformanceCounter();
} }
@ -541,21 +557,33 @@ void SDL_TicksQuit(void)
Uint64 Uint64
SDL_GetTickStartNS(void) SDL_GetTickStartNS(void)
{ {
Uint64 starting_value, value;
if (!tick_start) { if (!tick_start) {
SDL_TicksInit(); 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 Uint64
SDL_GetTicksNS(void) SDL_GetTicksNS(void)
{ {
Uint64 starting_value, value;
if (!tick_start) { if (!tick_start) {
SDL_TicksInit(); 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) Uint64 SDL_GetTicks(void)