[llvm] [RISCV][TII] Add and use new hook to optimize/canonicalize instructions after MachineCopyPropagation (PR #137973)

[Matt Arsenault via llvm-commits]

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@@ -510,6 +510,16 @@ class TargetInstrInfo : public MCInstrInfo {
     return false;
   }
 
+  /// If possible, converts the instruction to a more 'optimized'/canonical
+  /// form. Returns true if the instruction was modified.
+  ///
+  /// This function is only called after register allocation. The MI will be
+  /// modified in place. This is called by passes such as
+  /// MachineCopyPropagation, where their mutation of the MI operands may
+  /// expose opportunities to convert the instruction to a simpler form (e.g.
+  /// a load of 0).
+  virtual bool optimizeInstruction(MachineInstr &MI) const { return false; }
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arsenm wrote:

Name is way too general for a very narrowly applied optimization. Can't this just go in a separate post-RA pass 

https://github.com/llvm/llvm-project/pull/137973