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

[Mehdi Amini via llvm-dev]

Hi David,

Feel free to add me as a reviewer (mehdi_amini on Phabricator). I’ll do my best to help with it.

—
Mehdi

> On Aug 1, 2016, at 10:45 AM, David Callahan via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> 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 <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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