[PATCH] D159111: [TableGen][GlobalISel] Add support for matching/writing MIFlags in MIR Patterns

[Matt Arsenault via Phabricator via llvm-commits]

arsenm added inline comments.


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Comment at: llvm/docs/GlobalISel/MIRPatterns.rst:171
+mentionned multiple times, it's only checked/added once.
 
 Limitations
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Pierre-vh wrote:
> arsenm wrote:
> > Should mention the default behavior. Flags should be dropped unless explicitly preserved. Can you and/or the incoming flags?
> The current behavior is that the output instruction is a "blank canvas" so no flags are preserved, and no there's no way to fetch the flags from an instruction & preserve them (hence why I added the `PropagateFlags` bool).  
> 
> Is there any use case where we need to match only one flag and optionally preserve the other flags?
> 
> If the set of possibilities is relatively small, a PatFrag can do it - just explicitly match & rewrite them. Not ideal but it works. I'm sure it could all be written with some loop in TableGen as well.
> 
> Otherwise, maybe we could have a builtin for that, something like `GIMIFlagsAnd<$a, $b>` but we also need to be careful about `GIR_MutateOpcode` (which may erase the flags from the original inst), so it's not 100% trivial.
> 
Most cases involving flags are match one or two and preserve all. There are a fair number of match one or two and drop specific flag


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https://reviews.llvm.org/D159111