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Re: [RFC] type promotion pass
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
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?