Warning when compiling dwarf2out.c with -ftree-parallelize-loops=4

[Zdenek Dvorak]

Hi,

> As far as I get it, there is no real failure here.
> Parloop, unaware of the array's upper bound, inserts the 'enough 
> iterations' condition (i>400-1), and thereby 
> makes the last iteration range from 400 upwards. 
> VRP now has a constant it can compare to the array's upper bound.
> Correct programs that do not exceed the array bound will now fail because 
> VRP is not aware of the 
> fact that the parallel code (and last iteration it is looking at) is 
> actually not executed in these cases.
> 
> I'm not sure how to proceed, in order to avoid this warning.
> Any ideas?

I would propose changing the test

expected_loop_iterations (loop) <= n_threads

in tree-parloops.c to

((nit = estimated_loop_iterations_int (loop, false)) >= 0
 && nit < MIN_PER_THREAD * n_threads)

Which should prevent the parallelization for this loop, as we should be
able to derive that the loop iterates at most 4 times due to the array
access.

Zdenek