[PATCH] D112323: GlobalISel/Utils: Use incoming regbank while constraining the superclasses

[Christudasan Devadasan via Phabricator via llvm-commits]

cdevadas added a comment.

In D112323#3081078 <https://reviews.llvm.org/D112323#3081078>, @arsenm wrote:

> It's not accurate to say it would lead to incorrect codegen, we would just prefer to pick the classes beforehand rather than let the allocator decide until it needs to split live ranges

Yes, I was referring to the case when we fail to adjust the opcode for MOV/COPY to AV_* classes if we let regalloc to choose the register without fixing the class beforehand. 
I will modify the description.



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Comment at: llvm/lib/CodeGen/GlobalISel/Utils.cpp:117
+  // not be overriden.
+  if (MRI.getRegBankOrNull(Reg) && RegClass) {
+    const TargetRegisterClass *RC =
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arsenm wrote:
> I'm not sure there's any case where this shoudl be called where MRI.getRegBankOrNull(Reg) can be null?
It's been called from SelectionDAG too.


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Comment at: llvm/lib/CodeGen/GlobalISel/Utils.cpp:122
+               TRI.getCommonSubClass(RegClass, RC) == RC))
+      RegClass = RC;
+  }
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arsenm wrote:
> I would expect this to just directly assign getCommonSubClass's result without the comparison
Will do.


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  https://reviews.llvm.org/D112323/new/

https://reviews.llvm.org/D112323