[llvm-dev] [GlobalISel] Quick Status

Quentin Colombet via llvm-dev llvm-dev at lists.llvm.org
Fri Jan 20 17:19:08 PST 2017

Hi all,

Following the thread from http://lists.llvm.org/pipermail/llvm-dev/2017-January/109029.html, I am sending this email to give a status on GlobalISel progress and situation.

We are pushing GlobalISel from the state of prototype to a production quality framework. We welcome help with patches, reviews, feedbacks and so on.

As explained during the last developer meeting, we are aiming at enabling GISel by default for AArch64 at -O0 for this year (See http://llvm.org/devmtg/2016-11/#talk16).

Note: That does not mean the design is settle nor that we won’t change the APIs.
Note: A lot of the things listed in this email is a reminder of what we said during the dev meeting talk.

*** High Level View ***

As of r292481, we compile and run correctly with GISel (without fall back to SDISel) 63% of the LLVM test suite. If you are interested in detailed numbers, please see the attachments (courtesy of Kristof Beyls).

Note: The compile time numbers are probably noisy (compiled in parallel on the same machine), and not relevant at this point of the project anyway.

*** Per Pass Status ***

** IRTranslator **

Mostly done.

* What’s Left? *

Some instructions are not yet supported.

** Legalizer **

Core logic is present.

* What’s Left? *

- A lot of instructions are missing, in particular the vector ones.
- Legalization of G_SEQUENCE/G_EXTRACT still up in the air for complex cases.

Note: The lack of broad vector support is one on the reason we target O0, i.e., the vectorizer doesn’t run and we are less likely to hit the missing implementation.

** RegBankSelect **

- Core logic is present, no optimizations yet, or more accurately, the greedy mode is still pretty silly.
- TableGen support for RegisterBanks description.

* What’s Left *

- TableGen the instruction mapping from the existing SDISel patterns.
- Improve the optimization heuristic.

** InstructionSelect **

- Core logic present.
- TableGen support for simple SDISel patterns (i.e., GISel reuses SDISel patterns)

* What’s Left *

- Teach TableGen how to reuse more complex patterns:
— Patterns with combines in them (e.g., (mull (add)) => madd)
— Patterns with complex patterns (e.g., SelectAddressModXR0)

*** On Going Work ***

- General approach: use AArch64 O0 on the LLVM test suite as a driving vehicle to guide what to support next in the various passes.
- Extend TableGen support to reuse more and more SDISel patterns.
- ARM port.
- AMDGPU port.
- X86 port.

If you have questions, don’t hesitate!

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