Re: Don't trust your optimizer :-)

ZMZ <>
Wed, 10 Nov 2010 11:27:25 CST
On Nov 10, 3:52 am, "Martin B." <> wrote:

Hi all,

I found this[1] discussion on the MS VC++Lang forum very interesting and

[1] :

It is about how you can't trust the compiler/optimizer even if you only have one
thread. (You can't trust it to do what you think it will do. What it did was
perfectly acceptable according to the std.)

The OP wanted to time some code but got his timing statements unexpectedly
reordered by the optimizer:

     start = clock();
     result = foo(ITERATIONS);
     elapsed = clock() - start;

Due to foo being known to the compiler (by whole program optimization) the
optimizer decided that since foo was a pure function:

     double foo (int iterations) {
         double acc=4.0;
         int i;
         for (i = 1 ; i <= iterations ; i++)
         acc += (i&1) ?
                ((double) -4 / (2*i+1)) : ((double) 4/ (2*i+1));
         return acc;

it could *reorder* the calls to calculate start and elapsed.
Boom! Timing gone.


Such optimization is really a very old problem inherited from C.

clock() to me is a hardware-related function.

Imagine the following pesudocode, you have to put volatile to stop the

global int event = 0;
__interrupt void timer_interrupt()
   event = 1;

int main()
     //oops, the whole if statement is optimized away
     //because compiler thinks that event is always 0
       event = 0;
   return 0;

      [ See for info about ]
      [ comp.lang.c++.moderated. First time posters: Do this! ]

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