Re: Fix PHI handling in ipa-split

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Fri, Jun 25, 2010 at 9:57 AM, Jan Hubicka <hubicka@ucw.cz> wrote:
> Hi,
> fixing the PHIs it turned out I got PHI handling completely wrong in visit_bb.
> Using FOR_EACH_SSA_TREE_OPERAND on PHI is bad idea (that code got copied from
> normal statement handling).
>
> So this patch fixes this problem and allows splitting blocks with PHI in entry_bb
> when either PHI is virtual, or all incomming edges from header have same values
> (this is to allow split blocks starting with a loop).
>
> tree-inline needs updating to handle PHIs in entry_bb. ?This is quite easy,
> the edge needs to be created first and then regular PHI copying code sees
> an edge that has no direct equivalent in original function body. ?Instead
> it needs to look for edge from basic block that was not copied.
>
> Alternative would be to split BB in ipa-split and avoid PHIs in entry BB,
> but I think it is easier this way also for future other users of partial
> clonning (that would be, for example, autopar)
>
> Finally to make splitting effective on regions starting by loop, one needs
> to be cureful about entry frequency. ?It is not entry_bb frequency, but rather
> sum of frequencies of edges incomming to entry_bb from the header.
>
> Doing similar analysis on reasons for not splitting we now get:
> ? ?110 ?split part has non-ssa uses
> ? ?833 ?need to pass non-param values
> ? ?982 ?entry BB has PHI with multiple variants
>
> ? 5042 ?split size is smaller than call overhead
> ? 6813 ?incomming frequency is too large.
> ?25300 ?header size is too large for inline candidate
>
> There are about 800 splits, so teaching the code to pass non-param values still
> has potential to triple count of splits, but we definitly got a lot better:
> most of time we invalidate split because we do now want to split there.
>
> Bootstrapped/regtested x86_64-linux, OK?
>
> Honza
>
> ? ? ? ?* gcc.dg/tree-ssa/ipa-split-2.c: New testcase.
> ? ? ? ?* ipa-split.c (consider_split): PHI in entry block is OK as long as all
> ? ? ? ?edges comming from header are equivalent.
> ? ? ? ?(visit_bb): Handle PHIs correctly.
> ? ? ? ?* tree-inline.c (copy_phis_for_bb): Be able to copy
> ? ? ? ?PHI from entry edge.
> ? ? ? ?(copy_cfg_body): Produce edge from entry BB before copying
> ? ? ? ?PHIs.

This caused:

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47141


-- 
H.J.