Re: [PING] [PATCH] Fix asm X constraint (PR inline-asm/59155)

[Marc Glisse <marc dot glisse at inria dot fr>]

On Mon, 6 Jun 2016, Jakub Jelinek wrote:

> On Mon, Jun 06, 2016 at 12:04:04PM -0600, Jeff Law wrote:
> > On 06/06/2016 12:01 PM, Jakub Jelinek wrote:
> > > On Mon, Jun 06, 2016 at 11:54:04AM -0600, Jeff Law wrote:
> > > > > As for recog.c, I can not approve this as I am not a maintainer of it.
> > > > > I only can say that the code looks questionable to me.
> > > > I think the question on the recog part is a matter of how we choose to
> > > > interpret what the "X" constraint means.
> > > >
> > > > Does it literally mean accept anything, or accept some subset expressions?
> > > >
> > > > I tend to think the former, which means that things like
> > > > reg_overlap_mentioned_p or its callers have to be bullet-proofed.
> > >
> > > I think it is a bad idea to accept really anything, even for debug insns,
> > > which initially accepted arbitrarily large RTL expressions (and still accept
> > > stuff like subregs otherwise considered invalid etc.) we found it is highly
> > > undesirable, as it is not very good idea for the compile time complexity
> > > etc., so now we are trying to limit the complexity of the expressions there
> > > by splitting up more complex expressions into smaller ones using temporaries.
> > > So, even accept anything should always be accept anything reasonable,
> > > because most of the RTL passes don't really expect arbitrarily deep
> > > expressions, or expressions where the same reg can appear thousands of times
> > > etc.
> > The problem is how do you define this subset of expressions you're going to
> > accept and those which you are going to reject.
> >
> > I first pondered accepting RTL leaf nodes (reg, subreg, constants) and
> > rejecting everything else.  But I couldn't convince myself that some port
> > might reasonably expect (plus (x) (y)) to match the "X" constraint.
>
> It is always going to be arbitrary.
> Perhaps RTL leaf nodes (if including MEM, then perhaps with valid address
> only), and unary/binary/ternary RTL expressions with RTL leaf node operands?
> Or union of what is accepted by any other constraint?
> Or "g" plus any constants?

The last one would miss floating point registers (no 2 platforms use the 
same letter for those, hence my quest for something more generic).

The goal of the experiment is described in PR59159 (for which "+X" is 
unlikely to be the right answer, in particular because it is meaningless 
for constants). I don't know in what context people use the "X" 
constraint, or even better "=X"...

--
Marc Glisse