[PATCH] D105516: [clang][PassManager] Add -falways-mem2reg to run mem2reg at -O0

[John McCall via Phabricator via cfe-commits]

rjmccall added a comment.

I agree with Eli: we should decide what the goals are here and then use those goals to decide if we can identify a desirable permanent feature and, if so, what the appropriate name for that feature is.

It sounds like your goal is to get readable assembly that still corresponds fairly literally to your original code, in the sense that the readability of -O0 assembly is often undermined by the sheer amount of code and all the extra, unnecessary work it seems to do.  However, I've found that a lot of the extra -O0 code is not actually from loads and stores to local variables, it's from the -O0 instruction selection and register allocation, which often combine to do very silly things.  Have you looked into whether you get more readable code by just running normal -O0 IR through the non-O0 codegen pipeline?  Because the problem with doing just mem2reg is that that's already a fairly major non-literal change to the code, and at some point it's tricky to say what exactly should be part of this new pipeline; whereas still emitting exactly what the abstract machine says to do, just with less nonsense from fast-isel, is a lot easier to define.


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D105516/new/

https://reviews.llvm.org/D105516