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

[Carl Ritson 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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perlfu wrote:

Theoretical minimum size of worst case would be 8972*8972*2 -> ~154MiB?
The upfront cost of a 70KiB vector version without the risk of multiple megabyte map seems preferable.

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