[llvm-dev] [RFC] Tablegen-erated GlobalISel Combine Rules

[David Greene via llvm-dev]

Daniel Sanders via llvm-dev <llvm-dev at lists.llvm.org> writes:

> I've been working on the GlobalISel combiner recently and I'd like to
> share the plan for how Combine Rules will be defined in GlobalISel and
> solicit feedback on it.

This is really great stuff!  I agree with pretty much everything Nicolai
said, particularly the use of DAGs.  That's seems much more natually
TableGen-y to me.  Specific comments are below.

But before that, I've been long pained that we have so much duplicated
code in instcombine and dagcombine.  I know this is way beyond the scope
of this work but do you think the basic concepts could be applied to
produce TableGen-generated instcombine passes?  It would be nice to
re-use many of the rules, for example, except they'd match LLVM IR
rather than MIR.  As you go about implementation, maybe keep this idea
in mind?

> Here's a simple example that eliminates a redundant G_TRUNC.
>
> def : GICombineRule<(defs root:$D, operand:$S),
> (match [{MIR %1(s32) = G_ZEXT %S(s8)
> %D(s16) = G_TRUNC %1(s32) }]),
> (apply [{MIR %D(s16) = G_ZEXT %S(s8) }])>;

The use of '%' vs. '$' here really threw me for a loop.  I would have
expected '$D' and '$S' everywhere.  What's the significance of '%'
vs. '$'?  I know MIR uses '%' for names but must that be the case in
these match blocks?

Of course if we go with Nicolai's use of DAGs this issue seems to go
away.

> * defs declares the interface for the rule. The definitions in the
>   defs section are the glue between the Combine algorithm and the
>   rule, as well as between the match and apply sections. In this case
>   we only define the root of the match and that we have a register
>   variable called $S.

My understanding is that "root" means the sink here, but later in the
"upside-down" example, "root" means the source.  It seems to me the use
of "root" here is a misnomer/confusing.  I liked Nicolai's ideas about
specifying the insert point.  Why do users need to know about "root" at
all?  Is there some other special meaning attached to it?

                               -David