peephole2: dead regs not marked as dead

[Paolo Bonzini]

On 11/10/2010 11:58 AM, Georg Lay wrote:
> In the old 3.4.x (private port) I introduced a target hook in combine,
> just prior to where recog_for_combine gets called. The hook did some
> canonicalization of rtx and thereby considerably reduced the number of
> patterns that would have been necessary without that hook. It
> transformed some unspecs.

How is that related to delegitimize_address?

> A second use case of such a hook could look as follows: Imagine a
> combined pattern that does not match but would be legal if the backend
> knew that some register contains some specific value like, e.g.,
> non-negative, zero-or-storeflagvalue, combiner has proved that some
> bits will always be set or always be cleared;

You can use nonzero_bits or num_signbit_copies in define_splits.  In 
_this_ case, a define_insn_or_split doesn't really help, I agree with 
Joern on this. :)

> II.
> Suppose insns A, B and C with costs f(A) = f(B) = f(C) = 2.
>
> Combine combines A+B and sees costs f(A+B) = 5. This makes combine
> reject the pattern, not looking deeper.

Does it really do that?  I see nothing in combine_instructions which 
would prevent going deeper.

Paolo