[RFC] type promotion pass

[Segher Boessenkool]

On Fri, Sep 15, 2017 at 08:40:41PM +0200, Jakub Jelinek wrote:
> > > I'm greatly oversimplifying here.  Type promotion/demotion is fairly
> > > complex to get right.
> > 
> > Yeah :-(
> > 
> > Maybe the best thing is to promote really early, but to keep track of which
> > bits matter.  And then adjust some passes to take that into account.  Not a
> > trivial amount of work.
> 
> Is type promotion actually what we want to do early?  I'd think type
> demotion is what better canonicalizes the IL and removes redundant
> operations (e.g. those affecting only high bits if we only care about low
> bits).

On gimple we already have smallest type possible, I think?  When expanding
to RTL that then needs to only use instructions that exist for the target.
And then problems happen -- we only have instructions that work on full
registers on many targets (or also on 32-bit items), but we do not care
about the higher bits in some cases, or we only need it to be sign/zero
extended and we do not need a separate extend insn in many cases (but the
RTL passes do not realise that).

Or do you see problems during gimple as well?


Segher