[PATCH] D114832: [SROA] Improve SROA to prevent generating redundant coalescing operations.

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri added a comment.

In D114832#3177534 <https://reviews.llvm.org/D114832#3177534>, @davidxl wrote:

> The bad code pattern generated is probably not general enough to be useful to introduce a cleanup pass or to enhance existing pass to do so -- it will probably just shift the complexity from one pass to another. Fixing this at the source (SROA) is reasonable (unlike other canonicalization pass, this code pattern from SROA actually makes IR much worse)

Who defines what is/isn't reasonable? :)

As i see it, the input IR may or may not already have such bad patterns that are similar to the bad patterns you are trying to avoid producing here.
Trying to avoid producing it has compile time cost: https://llvm-compile-time-tracker.com/compare.php?from=0850655da69a700b7def4fe8d9a44d1c8d55877c&to=8a4304c8f4253fb1944e5e5988a24285a14181c4&stat=instructions
So, you've paid for avoiding producing more bad patterns, but you are still left with the ones that were already there.

Alternatively, instead of paying for trying not to introduce bad patterns, you could pay for trying to improve all of the patterns afterwards, regardless of their source.
Then, you still end up paying, but end up with not just the SROA patterns improved.

This is a very standard logic behind IR changes - if something doesn't understand the pattern, generally don't try to workaround it elsewhere, just fix the missing piece.


Repository:
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  https://reviews.llvm.org/D114832/new/

https://reviews.llvm.org/D114832