Re: [RFC]: Remove Mem/address type assumption in combiner

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

On Sat, May 16, 2015 at 12:36:38PM -0400, Hans-Peter Nilsson wrote:
> On Sat, 16 May 2015, Segher Boessenkool wrote:
> > On Fri, May 15, 2015 at 10:40:48PM -0400, Hans-Peter Nilsson wrote:
> > > I confess the test-case-"guarded" addi pattern should have been
> > > expressed with a shift in addition to the multiplication.
> >
> > But they wouldn't ever match so they might very well have bitrotted
> > by now :-(
> 
> It seems you're saying that the canonicalization to "ashift"
> didn't work *at all*, when starting with an expression from an
> address?  I knew it failed in part, but always thought it was
> just a partial failure.

With a plus or minus combine would always write it as a mult.
I don't think any other pass would create this combination.  I
haven't tested it though.

> > > ("In
> > > addition to" as the canonically wrong one used to be the
> > > combine-matching pattern; I'm not sure I should really drop that
> > > just yet.)
> >
> > It is harmless to leave it in.  It will rot though, eventually --
> > better take it out before then.  Add some gcc_unreachable, perhaps.
> 
> I've been trying to come up with valid reasons there'd be ever
> be canonicalization by multiplication, but failed so I guess
> I'll rip it out.

The "unreachable" thing should quickly tell you if that guess is wrong.
Not something you want to leave in a production compiler, of course.

> > Looks like quite some work for you, I'm sorry about that,
> 
> It's almost over, at least the editing part...

Great to hear that!


Segher