[patch] RFC: Hook for insn costs?

[Segher Boessenkool]

On Mon, Jul 17, 2017 at 12:20:16PM +0200, Richard Biener wrote:
> > So I argue that we want to have a hook for insn cost.
> 
> And I think at previous GNU Cauldrons we agreed to that.

Excellent!  So now it just needs to happen ;-)

> > Now what should it take as input?  An rtx_insn, or just the pattern
> > (as insn_rtx_cost does)?
> 
> Is there any useful info on the other operands of an rtx_insn?  If not
> then passing in the pattern (a rtx) might be somewhat more flexible.
> Of course it's then way easier to confuse rtx_cost and insn_cost ...
> 
> > And if an rtx_insn, should that than be an
> > rtx_insn that is already linked into the insn chain (so we can see what
> > other insns generate its inputs, and which of its outputs are unused,
> > etc.)?

I think you need an insn for the dataflow things.  But, not all callers
can use that I suppose (combine can -- although combine currently does
not keep useful DF info).  I guess we should start with just pattern_cost.


Segher