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

[Pierre van Houtryve via Phabricator via llvm-commits]

Pierre-vh 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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arsenm wrote:
> 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, $b>` but we also need to be careful about `GIR_MutateOpcode` (which may erase the flags from the original inst), so it's not 100% trivial.
> > 
> Most cases involving flags are match one or two and preserve all. There are a fair number of match one or two and drop specific flag
So what we need is:
- A way to copy flags from a matched instruction to a new instruction
  - Is the old PropagateFlags system good enough? It only copies the flags from the root AFAIK. I'm also wondering if it's not better to make flags not preserved by default, and add something like `GICopyMIFlags<$mi>` to explicitly copy them)
- A way to remove a flag (e.g. `GIRemoveMIFlag`)


Repository:
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  https://reviews.llvm.org/D159111/new/

https://reviews.llvm.org/D159111