[PATCH] D91513: [DeadMachineInstrctionElim] Iteratively run DeadMachineInstructionElim pass until nothing dead

[Michael Liao via Phabricator via llvm-commits]

hliao added a comment.

In D91513#2398717 <https://reviews.llvm.org/D91513#2398717>, @rampitec wrote:

> In D91513#2398697 <https://reviews.llvm.org/D91513#2398697>, @hliao wrote:
>
>> In D91513#2398528 <https://reviews.llvm.org/D91513#2398528>, @guopeilin wrote:
>>
>>> In D91513#2397313 <https://reviews.llvm.org/D91513#2397313>, @hliao wrote:
>>>
>>>> could you elaborate more on why we need to run that iteratively? since the original one runs bottom-up, supposedly it should find all.
>>>
>>> From the `iteratively-run-dead-mi-elim.mir` we can see that `bb.5` defines `%6`, and `%6` is used in `bb.2`. When we traverse the all basic blocks, that is, we runs bottome-up, we will meet `bb.5` first, for `%6`, we find that it is not dead cause  `%3` in `bb.2` use it. So `%6` surrive. Then we continue traverse other BBs, When we meet `bb.2`, we see that no one use `%5`, so we kill it. So as `%4`, `%3`. Right now, actually `%6` becomes dead cause we kill `%3` thus there is no longer any one uses `%6`. 
>>> However, cause we only traverse blocks once, we can't erase `%6` at the end. So if we iteratively visit all blocks until nothing change, then we can ensure that all dead mi is erased.
>>
>> Ah, I see. Even though we try to traverse basic blocks bottom-up, that's just the block placement order instead of block reachability. Could we replace that order with the post-order? So that, the use is always traversed before the defiine.
>
> That probably will not help if we have a loop?

It still works unless that value has a cyclic dependency through phi-node.


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