Re: [PATCH] doc: Document order of define_peephole2 scanning

[Hans-Peter Nilsson <hp@axis.com>]

> From: Paul Koning <paulkoning@comcast.net>

> Date: Tue, 18 Apr 2023 14:32:07 -0400
> 
> I'm not sure about the meaning of part of this.
> "...resumes at the last generated insn."  Does that mean:

(Neither...)
 
> 1. If a match is found at some insn, the replacement
> defined by the matching define_peephole2 is performed, and
> then the scan resumes at the first of the insns produced
> by the replacement.

This was what I expected.  If it had been this, I wouldn't
have suggested the doc update.  But it isn't: no, it's the
*last produced one*.  If you look at the referenced example
(unfortunately outside of the diff context) it should all be
clear.

> or
> 
> 2. If a match is found at some insn, the replacement
> defined by the matching define_peephole2 is performed, and
> then the scan resumes at the insn immediately following
> the ones just matched.

No, from the last of the replacement insns.

> "Last generated" seems to fit option 1,

Sorry, your confusement confuses me.  I just don't see how
to confuse last with first or matched with generated. :)

> but I'm not sure
> if that's what you meant.  Maybe you could add some words
> to say more explicitly which it is.

I'm referring to an example on the same pdf page.

But perhaps s/resumes at the last generated insn/resumes at
the last insn in the replacement sequence/ would help?

brgds, H-P


> 
> 	paul
> 
> > On Apr 18, 2023, at 1:55 PM, Hans-Peter Nilsson via Gcc-patches <gcc-patches@gcc.gnu.org> wrote:
> > 
> > Generated pdf inspected.  Ok to commit?
> > 
> > Thoughts on fixing the IMHO wart to also expose all
> > replacements to all define_peephole2?  Looks feasible
> > (famous last words), but then again I haven't checked the
> > history yet.
> > 
> > -- >8 --
> > I was a bit surprised when my define_peephole2 didn't match,
> > but it was because it was expected to partially match the
> > generated output of a previous define_peephole2.  I had
> > assumed that the algorithm exposed newly created opportunities
> > to all define_peephole2's.  While things can change in that
> > direction, let's start with documenting the current state.
> > 
> > 	* doc/md.texi (define_peephole2): Document order of scanning.
> > ---
> > gcc/doc/md.texi | 8 ++++++++
> > 1 file changed, 8 insertions(+)
> > 
> > diff --git a/gcc/doc/md.texi b/gcc/doc/md.texi
> > index 07bf8bdebffb..0f9e32d2c648 100644
> > --- a/gcc/doc/md.texi
> > +++ b/gcc/doc/md.texi
> > @@ -9362,6 +9362,14 @@ If the preparation falls through (invokes neither @code{DONE} nor
> > @code{FAIL}), then the @code{define_peephole2} uses the replacement
> > template.
> > 
> > +Insns are scanned in forward order from beginning to end for each basic
> > +block, but the basic blocks are scanned in reverse order of appearance
> > +in a function.  After a successful replacement, scanning for further
> > +opportunities for @code{define_peephole2} matches, resumes at the last
> > +generated insn.  I.e. for the example above, the first insn that can be
> > +matched by another @code{define_peephole2}, is @code{(set (match_dup 3)
> > +(match_dup 4))}.
> > +
> > @end ifset
> > @ifset INTERNALS
> > @node Insn Attributes
> > -- 
> > 2.30.2
> > 
>