On Tue, Sep 20, 2016 at 08:52:46AM -0600, Jeff Law wrote:
> >>>>>- JUMP_LABEL can be a return which is not an insn.  That also seems
> >>>>>  like something that should be improved at some point.
> >>>>The JUMP_LABEL field within a JUMP_INSN?  That sounds like a bug.
> >>>
> >>>yes, see the comment before the declaration of
> >>>rtx_jump_insn::jump_label ()
> >>>it can be either a possibly deleted label, or a RETURN or SIMPLE_RETURN
> >>>expression.  Memory usage concerns aside its a tempting design to
> >>>change, but at the moment itts definitely intended to work this way,
> >>>there's plenty of code that checks ANY_RETURN_P (JUMP_LABEL (x)).
> >>
> >>Yes.  But rtl.texi still says:
> >>
> >>  Return insns count as jumps, but since they do not refer to any
> >>  labels, their @code{JUMP_LABEL} is @code{NULL_RTX}.
> >
> >And the comment at the top of jump.c:
> >
> >   Each CODE_LABEL has a count of the times it is used
> >   stored in the LABEL_NUSES internal field, and each JUMP_INSN
> >   has one label that it refers to stored in the
> >   JUMP_LABEL internal field.  With this we can detect labels that
> >   become unused because of the deletion of all the jumps that
> >   formerly used them.  The JUMP_LABEL info is sometimes looked
> >   at by later passes.  For return insns, it contains either a
> >   RETURN or a SIMPLE_RETURN rtx.
> >
> >It's intentional, and really there's nothing wrong with it, or with
> >operands[] containing CODE_LABELs. The problem is trying to force a

I'm just pointing out the documentation is out-of-date here.  I'll do
a patch.

> >static type system onto a dynamically typed data structure.
> But what this doesn't answer is why we stuff RETURN/SIMPLE_RETURN into 
> the JUMP_LABEL field of a return.  I simply can't remember any rationale 
> behind that.

It is very inconvenient to parse the whole rtx to see if this is a
RETURN vs. a SIMPLE_RETURN (it can be inside a conditional or inside
a PARALLEL, etc.)

Ideally we would not have RETURN at all anymore, just SIMPLE_RETURN,
but that isn't going to happen any time soon.

> I suspect that if we dig further, we'll find that we can accomplish 
> whatever the goal was behind that decision in a way that easily works in 
> a world where we have separated rtx objects from objects that are part 
> of the insn chain.

We cannot easily extract the JUMP_LABEL of a normal jump from its rtx
pattern either (if at all).  So it seems the extra field of a JUMP_INSN
is here to stay.  We'll have to figure out how to nicely do INSN vs.
JUMP_INSN in an rtx_insn world.

> Just to be clear, I don't see us going to a world where everything we 
> have as an rtx today is a statically typed object. But there are things 
> that I think make sense to carve out, the most obvious being objects 
> which are part of the insn chain.

Agreed on both.  I have nightmares about the first, so please don't even
talk about going there :-)


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