Re: LTO inliner -- sensitivity to increasing register pressure

[Xinliang David Li <davidxl at google dot com>]

Do you witness similar problems with LTO +FDO?

My concern is it can be tricky to get the register pressure estimate
right. The register pressure problem is created by downstream
components (code motions etc) but only exposed by the inliner.  If you
want to get it 'right' (i.e., not exposing the problems), you will
need to bake the knowledge of the downstream components (possibly
bugs) into the analysis which might not be a good thing to do longer
term.

David

On Fri, Apr 18, 2014 at 9:43 AM, Aaron Sawdey
<acsawdey@linux.vnet.ibm.com> wrote:
> Honza,
>   Seeing your recent patches relating to inliner heuristics for LTO, I
> thought I should mention some related work I'm doing.
>
> By way of introduction, I've recently joined the IBM LTC's PPC Toolchain
> team, working on gcc performance.
>
> We have not generally seen good results using LTO on IBM power processors
> and one of the problems seems to be excessive inlining that results in the
> generation of excessive spill code. So, I have set out to tackle this by
> doing some analysis at the time of the inliner pass to compute something
> analogous to register pressure, which is then used to shut down inlining of
> routines that have a lot of pressure.
>
> The analysis is basically a liveness analysis on the SSA names per basic
> block and looking for the maximum number live in any block. I've been using
> "liveness pressure" as a shorthand name for this.
>
> This can then be used in two ways.
> 1) want_inline_function_to_all_callers_p at present always says to inline
> things that have only one call site without regard to size or what this may
> do to the register allocator downstream. In particular, BZ2_decompress in
> bzip2 gets inlined and this causes the pressure reported downstream for the
> int register class to increase 10x. Looking at some combination of pressure
> in caller/callee may help avoid this kind of situation.
> 2) I also want to experiment with adding the liveness pressure in the callee
> into the badness calculation in edge_badness used by inline_small_functions.
> The idea here is to try to inline functions that are less likely to cause
> register allocator difficulty downstream first.
>
> I am just at the point of getting a prototype working, I will get a patch
> you could take a look at posted next week. In the meantime, do you have any
> comments or feedback?
>
> Thanks,
>    Aaron
>
> --
> Aaron Sawdey, Ph.D.  acsawdey@linux.vnet.ibm.com
> 050-2/C113  (507) 253-7520 home: 507/263-0782
> IBM Linux Technology Center - PPC Toolchain
>