An introduction and some questions

[Doug Simon]

Welcome Jackson!

> On 30 Oct 2017, at 18:27, Jackson Davis <jackson at jcdav.is> wrote:
> 
> Hello,
> 
> I've been trying to finally introduce myself to graal over the last few
> days, with a hope I can maybe sneak in a few changes when I have time - its
> an interesting project :)
> 
> On the non-technical side: Whats with the [GR-XXXX] in commit messages, is
> this a private issue tracker? I don't see anything on the graal project on
> openjdk jira, or anywhere else for that matter.

These are indeed related to an internal jira instance Graal shares with a number of other projects at Oracle Labs, some of which are not open source.

We also respond to issues and pull requests at https://github.com/graalvm/graal.

> On the technical side: I've been looking into optimizing the code
> generation on x86 for add/sub/shl. Currently, if reg1 and reg2 are
> different, reg2 = add reg1, C generates
> mov %reg1, %reg2
> add C, %reg2
> 
> This can be folded into a single lea C(%reg1), %reg2 (if there is no
> resulting operation dependent on flags being set). We can do the same for
> sub, and for shl if C=1/2/3. I have a very basic attempt of this
> working-ish here:
> https://github.com/jcdavis/graal/commit/6d27c585726b64259635f36542263ca0cde3dcf1
> . This currently always uses leaq, which is at best 1 byte longer than it
> needs to be for 32 bit operands (since the REX prefix isn't necessary), and
> at worst possibly wrong because over/underflow will set the high order 32
> bits (which might be ok? But unsure). It also misses add/sub by 1 (since
> those are lowered to MOp, not ConstOp), and also doesn't consider %reg3 =
> %reg1 + %reg2 (which can also be represented as an lea)
> 
> Where I am a little lost is how to clean this up properly. I was thinking a
> new emit function that also takes a source register x could be added, which
> be default would still call the same move + emit, and then, individual
> opcodes could override that behavior as necessary?

I would suggest implementing and testing your suggestion then submitting a PR on github to allow someone more familiar than I am with this part of the compiler to offer feedback.

-Doug