Re: [patch] for PR 34244

["Richard Guenther" <richard dot guenther at gmail dot com>]

On Nov 29, 2007 5:28 AM, Zdenek Dvorak <rakdver@kam.mff.cuni.cz> wrote:
> Hi,
>
> we have a code like
>
> if (something)
>   offset = 100;
> else
>   offset = -100;
>
> while (offset > 0)
>   offset--;
>
> if (offset == 0)
>   launch_nuclear_rockets ();
>
> VRP starts simulating the code, first executing the true branch of the if
> (something) condition, getting offset = 100.  It then proceeds with the loop,
> determining that number of iterations is equal to offset (since we just now
> believe that offset==100, this is correct, without any assumptions), thus the
> final value of offset is 0 and launch_nuclear_rockets is considered to be
> always executed.
>
> Later, VRP evaluates the false branch of the if (something) condition, setting
> the value range of offset to [-100,100], and proceeds to re-evaluate the
> effects of the loop.  However, scev caches the number of iterations of the
> loop, so it is not re-evaluated, and we keep believing that the final value of
> offset is always zero.
>
> Forcing scev to recompute the number of iterations every time some value range
> changes would probably lead to compile time problems (computing number of iterations
> is fairly expensive, and we used to have compile time problems with that in other
> passes).  Instead, the patch forces scev to compute and cache the number of iterations
> of each loop before vrp analysis starts, and cache these values (that do not use
> the partial results of VRP).
>
> Bootstrapped & regtested on amd64-linux.

This is ok.

Thanks,
Richard.

> Zdenek
>
>         PR tree-optimization/34244
>         * tree-vrp.c (adjust_range_with_scev): Clear scev cache.
>         (record_numbers_of_iterations): New function.
>         (execute_vrp): Cache the numbers of iterations of loops.
>         * tree-scalar-evolution.c (scev_reset_except_niters):
>         New function.
>         (scev_reset): Use scev_reset_except_niters.
>         * tree-scalar-evolution.h (scev_reset_except_niters): Declare.
>
>         * gcc.dg/tree-ssa/pr34244.c: New test.