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From:	"Bob W" <dontsend AT operamail DOT com>
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Subject:	Re: new gcc 4.10 behaviour
Date:	4 Apr 2006 12:42:10 -0700
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Brian Inglis wrote:

.............

>
> Then it's a bug, perhaps over zealous optimization,

..............

> It's also a suspicious coincidence that the poster reports a speed
> increase of 5 times, and that ticks are 18.2/sec but multiplied by 5
> so that CLK_TCK and CLOCKS_PER_SEC can be kept as an integral 91;
> perhaps the ticks are being multiplied by 5 elsewhere, as well as in
> clock.c?

As already mentioned elsewhere - the following happens:

- From the 1st optimisation level onwards gcc chooses to
modify Ack() which might get called more than once - unlike
the source code would suggest. Whether Ack() is called a
second time or not depends on the parameter supplied.

- If that happens, the 2nd clock() function call, which gcc
has placed between the Ack() calls, gets executed too early.
This seems to be the reason for the reported timing distortion
(aka "five-fold speed increase").

As a conclusion I'd rule out that clock ticks are multiplied
by 5 elsewhere. It is just a strange gcc optimisation attempt
which is obviously breaking the intended code sequence.

- Raw text -