[llvm] [RISCV][GISEL] legalize, regbankselect, and instruction-select for G_… (PR #73061)

[Michael Maitland via llvm-commits]

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@@ -353,6 +353,34 @@ RISCVRegisterBankInfo::getInstrMapping(const MachineInstr &MI) const {
     OpdsMapping[2] = OpdsMapping[3] = getFPValueMapping(Size);
     break;
   }
+  case TargetOpcode::G_MERGE_VALUES: {
+    // Use FPR64 for s64 merge on rv32.
+    assert(MI.getNumOperands() == 3 && "Unsupported G_MERGE_VALUES");
+    LLT Ty = MRI.getType(MI.getOperand(0).getReg());
+    if (GPRSize == 32 && Ty.getSizeInBits() == 64) {
+      assert(MF.getSubtarget<RISCVSubtarget>().hasStdExtD());
+      // FIXME: OpdsMapping[0, 1] should probably visit their uses to determine
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michaelmaitland wrote:

If we had `f32, f32 = UNMERGE f64`, could we use `SplitF64Pseudo`?
If we had `f64 = MERGE f32, f32`, could we use `BuildPairF64Pseudo`?

https://github.com/llvm/llvm-project/pull/73061