Re: number_of_iterations_in_loop and a non-constant loop bound start

[Zdenek Dvorak <rakdver at atrey dot karlin dot mff dot cuni dot cz>]

Hello,

> While working on a loop pass I found that number_of_iterations_in_loop 
> reported
> that a loop with a non constant start as being undetermined which is 
> wrong for
> this case as we know that the initializer will always before the end.
> 
> Take for an example:
> 
> 
> void g(int init, int max, char *a)
> {
>   int i;
>   for (i = init; i < max; i++)
>    a[i] = 0;
> }
> 
> in the loop we know that init < max as the tree looks like:

this is a deficiency in fold; the code to determine number of
iterations takes the init < max condition into account, but
fold is unable to simplify the following expressions:

init_2 < max_4 && max_4 == -2147483648
  should be folded to 0, since there is no integer smaller than
  -2147483648

init_2 < max_4 && init_2 + 1 > max_4 - 1 + 1
  should be folded to 0, since init_2 cannot be both smaller and
  greater or equal to max_4.

In the later case a part of the reason this does not work is that
one of the "1" constants has overflow flag set, which makes fold
to behave just crazily.

Zdenek