[llvm-dev] [LLVM] New Dead Code Elimination

[Nadav Rotem via llvm-dev]

Hi David,

Thanks for working on this.  I’d like to help reviewing your work!

How much code from the original patch are you leaving around? If you are reusing a lot of code from the original pass then you can refactor, clean and add new functionality. But if you are not keeping much then maybe it would be useful to develop a new pass (with small incremental reviewable changes), and when the work is done flip the switch and start using the new pass. What do you prefer?

I think that introducing the new flag that allows the removal of infinite loops is a great idea. LLVM is used for compilation of non-C languages, like Swift, Java, Python, etc, and these languages may benefit from the new functionality.

-Nadav

From: David Callahan <dcallahan at fb.com>
Date: Monday, August 1, 2016 at 10:45 AM
To: LLVM Dev Mailing list <llvm-dev at lists.llvm.org>
Cc: Kevin Frei <freik at fb.com>, Nadav Rotem <nrotem at fb.com>, Chandler Carruth <chandlerc at gmail.com>
Subject: [LLVM] New Dead Code Elimination

I have a rewrite of the aggressive dead code elimination pass which handles control flow and allows may-be-infinite loops to be removed under optional flag. Chandler suggested rather than a large change a series of small changes and, while that may not get us to small changes easily, there has been no further commentary on the diff: https://reviews.llvm.org/D18762.  Given that, I will plan on proposing a series of diffs and would like to recruit some people who are interested in the change to help review those changes.

I expect to stage as follows:


1.      Rewrite base algorithm to introduce data structures needed to hold extra information (no functional change)

2.      Rewrite base algorithm to allow removing of control flow (under option, off by default). This variant will not be correct in the general case.

3.      Code to rewrite the control flow graph and remove dead basic blocks

4.      Identify Phi nodes with non-instruction reaching definitions and keep the associated source block live. Identify Phi nodes with multiple-live values from dead blocks

5.      Remove unreachable code discovered by post-order traversal to avoid.
 (code is functionally correct at this point)

6.      Use post-order traversal to identify loop bottoms. By default this will be kept live but include switch to allow loops to be removed.

7.      Improve handling of live values out of dead regions

Please respond if you are willing to help or have suggestions on staging or approach.

Thanks
David
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