Mail Archives: djgpp/2006/04/05/10:15:10
Bob W <dontsend AT operamail DOT com> wrote:
> As mentioned already in a previous post, gcc 4.10
> tends to optimize code in a way so that it gets out
> of sequence.
Well, sort of. The tricky part is that if the 2 of the three things
you're doing hadn't been the clock() function, of all things, this
would have been unnoticeable to the program, which would tend to make
it an allowed optimization.
On top of that, the function you call between the two is local to the
translation unit, eligible for (partial) inlining, and doesn't have
any side effects. On a lot of modern processors, intermixing the
calls to clock() and the evaluation of the Ack() function would allow
the CPU to parallelize them (--> Hyperthreading, ...) and finish the
overall job faster. In other words, for *any* other function but
clock() and some close relatives of it, this would be a desirable
optimization.
> - gcc calls clock() to start timing
> - Ack() is called the 1st time
> - gcc calls clock() again to end the timing
> - now Ack() is called again and this obsoletes timing
More to the point, these two calls of Ack() are part of an unrolled,
inline rendition of this highly recursive function.
[Workarounds...]:
> - assign the return value of Ack() to a static or
> global variable instead of using a local var
This works because as far as GCC knows, clock() might be reading that
global variable, so the sequence point after calling Ack() requires
finishing it before calling clock().
> - 'hide' the second clock() call after the first
> printf() statement
That's not hiding, that's just that printf() can't be called before
Ack() is finished, and again, as far as GCC knows, printf() might
change data clock() uses, so clock() can't be called until printf()
returns.
> If gcc's out-of-sequence optimisation is prevented
> by using one of the methods mentioned previously,
> the program actually executes a touch faster.
... on the CPU you tried it on. Which may not be the one it was
optimizing for.
--
Hans-Bernhard Broeker (broeker AT physik DOT rwth-aachen DOT de)
Even if all the snow were burnt, ashes would remain.
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