Re: RFC: Using mode and code macros in *.md files

[Richard Sandiford <rsandifo at redhat dot com>]

Thanks for the feedback.

James E Wilson <wilson@specifixinc.com> writes:
> My first question would be how this affects debugging gcc.  Being able
> to match rtl insns to the md pattern for them is important.  These
> macros obscure the connection.  If I have a insn that has been
> recognized, and claims to match the slesi pattern, then I will be
> confused if I can't find such a pattern.  How am I supposed to know to
> look for the s<floatcond><mode> pattern instead?  The port maintainer
> would know this, but someone else wouldn't.  Maybe it would help to
> print md file line numbers instead of or in addition to pattern names
> when dumping rtl.

Sound like a good idea.

> If I have an insn that hasn't been recognized, then I have a similar but
> slightly different problem.  If I try grepping for the operator (le:SI I
> am not going to find it.  How I am supposed to know to search for
> (<floatcond>:<mode> instead?

I can see that could be a problem.  I guess it depends on the habits
of the coder.

FWIW, I tend to use '\ble\b' (within emacs) when searching for an
operator name, and the first hit for that would be the macro definition.
(I mostly use '\b' because I can never remember which re syntaxes use
'(' as a grouping operator and which use '\(').

>> For example, code macros allow us to combine 7 of the c.cond.fmt patterns:
>
> You can already do this via match_operator.  Just define a predicate
> that accepts the 7 comparison codes you care about, and you can write a
> single pattern to patch all 7.  This gives a somewhat different affect
> though, as your macro approach gives 7 patterns whereas we only have one
> pattern if we use match_operator.

For matching, yes, but the point is that these are named patterns.
OK, so at the moment, only gen_slt_sf() is actually used (by one
of the reload patterns), but match_operator gives a less friendly
gen_*() interface.  If you have:

    (set (match_operand:SF 0 "register_operand" "=f")
         (match_operator:SF 1 "float_cmp_operator"
            [(match_operand:SF 2 "register_operand" "f")
             (match_operand:SF 2 "register_operand" "f")]))

then the gen_*() function will have four arguments.  From memory,
arguments 2 and 3 are ignored, and you need to pass gen_rtx_LT (...)
for argument 1.

In practice, we'd probably end up adding a new expander specifically for
"slt_sf", or perhaps just synthesising it directly using gen_rtx_*()
functions.

> We should consider whether we need or want two different mechanisms that
> do the same thing.

Well, match_operator is a bit more general, in that it can (if necessary)
match codes with different formats.  I don't know how important that is
in practice.

Richard