Re: Your change of September 11, 1998

[Richard Henderson <rth at redhat dot com>]

On Mon, Jan 15, 2001 at 12:10:25PM -0500, Richard Kenner wrote:
> I am very confused by that change.  It appears to allow a PARALLEL as
> a SET_DEST.  I don't see any documentation of the form of that PARALLEL.

I don't think there is any good documentation for this.  :-(

The form was first introduced for MIPS quite some time ago as a
way to return a structure in multiple disjoint registers.  Since
then it has been used on Sparc64, PA64 and IA-64, all of which
have non-trivial structure calling conventions.

> The code in note_stores, as best as I can see, expects each element to be a
> SET or CLOBBER, since it does ...

This had previously "worked" because this parallel form only appears
as the destination of a call, and contains only call-clobbered hard
regs.  Thus for most applications, no one needed to know.

> However, in the Sparc 64 bit port, it's actually a PARALLEL where each
> entry is an EXPR_LIST, whose first expression is a register and whose
> second expression is a CONST_INT that appears to be a byte offset.
> 
> What *is* the documentation of what's supposed to be there?

That is in fact that FUNCTION_VALUE or FUNCTION_ARG is supposed
to return.  Whether that ought to make it into the call insn is
a deeper question that ought to be answered.

Bernd has previously expressed a deep-seated loathing for that,
and thinks we ought to express this as a parallel of multiple sets.

The current form has the advantage that a match_operand with no
predicate matches one of these constructs, and so we don't need
anything complicated in the .md file to handle them.



r~