[llvm] [RFC][MC] Cache MCRegAliasIterator (PR #93510)

[Pierre van Houtryve via llvm-commits]

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@@ -187,6 +187,9 @@ class MCRegisterInfo {
   DenseMap<MCRegister, int> L2SEHRegs;        // LLVM to SEH regs mapping
   DenseMap<MCRegister, int> L2CVRegs;         // LLVM to CV regs mapping
 
+  mutable DenseMap<MCPhysReg, std::vector<MCPhysReg>> RegAliasesCache;
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Pierre-vh wrote:

Realistically I don't know if a program could use all registers available. I don't think so.
Theoretically, there are 9000 registers and I'm not sure how much space DenseMap needs, I think it's a lot more than what std::vector would use

https://github.com/llvm/llvm-project/pull/93510