Re: [PATCH] PR 62173, re-shuffle insns for RTL loop invariant hoisting

[Segher Boessenkool <segher@kernel.crashing.org>]

On Thu, Dec 18, 2014 at 05:00:01PM +0000, Jiong Wang wrote:
> On 17/12/14 15:54, Richard Biener wrote:
> >ick.  I realize we don't have SSA form on RTL but doesn't DF provide
> >at least some help in looking up definition statements for pseudos?
> >In fact we want to restrict the transform to single-use pseudos, thus
> >hopefully it can at least tell us that... (maybe not and this is what
> >LOG_LINKS are for in combine...?)  At least loop-invariant alreadly
> >computes df_chain with DF_UD_CHAIN which seems exactly what
> >is needed (apart from maybe getting at single-use info).
> 
> thanks very much for these inspiring questions.
> 
> yes, we want to restrict the transformation on single-use pseudo only,
> and it's better the transformation could re-use existed info and helper
> function to avoid increase compile time. but I haven't found anything I
> can reuse at the stage the transformation happen.
> 
> the info similar as LOG_LINKS is what I want, but maybe simpler. I'd study
> the code about build LOG_LINKS, and try to see if we can do some factor out.

LOG_LINKs in combine are just historical.  combine should be converted
to use DF fully.

LOG_LINKs have nothing to do with single use; they point from the _first_
use to its corresponding def.

You might want to look at what fwprop does instead.


Segher