Re: question about rtl loop-iv analysis

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

Hello,

> And finally at the stage of rtl unrolling it looks like this:
> [6]   r186 = r2 + C;
>       r318 = r186 + 160;
>       loop:
>       r186 = r186 + 16
>       if (r186 != r318) then goto loop else exit
> 
> Then, in loop-unroll.c we call iv_number_of_iterations, which eventually
> calls iv_analyze_biv (with r258/r186), which in turn calls
> latch_dominating_def.
> There, when processing the vectorized case, it encounters the def in the
> loop ('r186+16'), and then the def outside the loop ('r2+C'), at which
> point it fails cause it found two defs (and so we get that this is not a
> simple iv, and not a simple loop, and unrolling fails: "Unable to prove
> that the loop iterates constant times").
> When processing the unvectorized case, we also first encounter the def in
> the loop ('r258+16'), and then the def outside the loop ('0'), but this def
> succeeds the test "if (!bitmapset (bb_info->out,....))", and so we don't
> fail when we encounter the second def, and all is well.
> 
> So one question I have is what is that bitmap exactly, and why does loop
> [6] fail rtl iv-analysis?

the bitmap bb_info->out should contain reaching definitions at the end
of the latch block; so the definition of r186 outside of the loop
should not be contained in it.  This seems to be a dataflow analysis
problem.

Zdenek