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From: Martin Steuer <martin AT x DOT invalid>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: new gcc 4.10 behaviour
Date: Wed, 29 Mar 2006 12:08:03 +0200
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Hans-Bernhard Broeker wrote:
> I.e. there's nothing in the language definition forbidding the compiler
> to change 
> 
> 
>>  clock_t t0=clock();
>>  int ackret=Ack(3, n);
>>  clock_t t1=clock();
> 
> 
>>  printf("Ack(3,%d): %d\n", n, ackret);
>>  printf("Time: %g secs\n", 1.0*(t1-t0)/CLOCKS_PER_SEC);
> 
> 
> into
> 
> 
>>  clock_t t0=clock();
>>  clock_t t1=clock();
>>  int ackret=Ack(3, n);
> 
> 
>>  printf("Ack(3,%d): %d\n", n, ackret);
>>  printf("Time: %g secs\n", 1.0*(t1-t0)/CLOCKS_PER_SEC);
> 
> 
> or even
> 
> 
>>  int ackret=Ack(3, n);
>>  printf("Ack(3,%d): %d\n", n, ackret);
>>  clock_t t0=clock();
>>  clock_t t1=clock();
>>  printf("Time: %g secs\n", 1.0*(t1-t0)/CLOCKS_PER_SEC);
> 
> 

I'm curios, would it also be possible, that the optimizer reorders the 
calls to clock to:

clock_t t1 = clock();
clock_t t0 = clock();

And if not, why? Would the optimizer have to assume that the first call 
to clock has changed the internal state of that function?

And in general: how can one guarantee a certain order of execution for 
code like that from the OP? He suggested that using static or global 
variables seems to be a solution, but is this really the way to do it?

Martin