[llvm] Add Instruction selection support for x87 ld/st (PR #97016)

[Evgenii Kudriashov via llvm-commits]

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@@ -195,6 +195,14 @@ X86InstructionSelector::getRegClass(LLT Ty, const RegisterBank &RB) const {
       return &X86::VR512RegClass;
   }
 
+  if (RB.getID() == X86::PSRRegBankID) {
+    if (Ty.getSizeInBits() == 80)
+      return &X86::RFP80RegClass;
+    if (Ty.getSizeInBits() == 64)
+      return &X86::RFP64RegClass;
+    return &X86::RFP32RegClass;
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e-kud wrote:

This is about whether it is guaranteed that type sizes of `PSRRegBankID` are limited by `32`, `64` and `80`. Or do we have to check on `32` as well to hit `llvm_unreachable` if something out of these three types comes.

I'm also thinking about whether it is better to rewrite this function to use switches instead of if/else...

https://github.com/llvm/llvm-project/pull/97016