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thread-safe timer monotonicity check. fixes #653

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(agree that DBL_EPSILON is an unnecessary hack, since division by
(t+eps)-(t) will yield a number not much more useful than infinity)

This was SVN commit r8516.
This commit is contained in:
janwas 2010-11-01 10:19:22 +00:00
parent 657ec9aa45
commit b27d6d38e3
1 changed files with 25 additions and 15 deletions
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40
source/lib/timer.cpp
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@ -82,22 +82,32 @@ void timer_LatchStartTime()
}
static void EnsureMonotonic(double& t)
// NB: does not guarantee strict monotonicity - callers must avoid
// dividing by the difference of two equal times.
// (this avoids having to update maxRepresentation when
// updating newTime to the (more recent) oldTime)
static void EnsureMonotonic(double& newTime)
{
//retry:
// static i64 t_lastBits; // initially 0.0
// memcpy(&t_lastBits, &m_seconds, sizeof(t_lastBits));
//
// i64 tBits;
// memcpy(&tBits, &seconds, sizeof(tBits));
//
// if(!cpu_CAS64((volatile i64*)&m_seconds, t_lastBits, tBits))
// goto retry;
//
// static double t_last = 0.0;
// if(t < t_last)
// t = t_last;
// t_last = t;
i64 newRepresentation;
memcpy(&newRepresentation, &newTime, sizeof(newRepresentation));
// representation of the latest time reported by any thread
static volatile i64 maxRepresentation; // initially 0.0
retry:
const i64 oldRepresentation = maxRepresentation; // latch
double oldTime;
memcpy(&oldTime, &oldRepresentation, sizeof(oldTime));
// a previous or concurrent call got a more recent time -
// return that and avoid updating maxRepresentation.
if(newTime < oldTime)
{
newTime = oldTime;
return;
}
if(!cpu_CAS64(&maxRepresentation, oldRepresentation, newRepresentation))
goto retry;
}