[llvm] [RISCV] Refactor GPRF64 register class to make it usable for Zacas. (PR #77408)

[Yingwei Zheng via llvm-commits]

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@@ -546,33 +549,36 @@ def DUMMY_REG_PAIR_WITH_X0 : RISCVReg<0, "0">;
 def GPRAll : GPRRegisterClass<(add GPR, DUMMY_REG_PAIR_WITH_X0)>;
 
 let RegAltNameIndices = [ABIRegAltName] in {
-  def X0_PD : RISCVRegWithSubRegs<0, X0.AsmName,
-                                     [X0, DUMMY_REG_PAIR_WITH_X0],
-                                     X0.AltNames> {
-    let SubRegIndices = [sub_32, sub_32_hi];
+  def X0_Pair : RISCVRegWithSubRegs<0, X0.AsmName,
+                                    [X0, DUMMY_REG_PAIR_WITH_X0],
+                                    X0.AltNames> {
+    let SubRegIndices = [sub_gpr_even, sub_gpr_odd];
     let CoveredBySubRegs = 1;
   }
   foreach I = 1-15 in {
     defvar Index = !shl(I, 1);
+    defvar IndexP1 = !add(Index, 1);
     defvar Reg = !cast<Register>("X"#Index);
-    defvar RegP1 = !cast<Register>("X"#!add(Index,1));
-    def X#Index#_PD : RISCVRegWithSubRegs<Index, Reg.AsmName,
-                                          [Reg, RegP1],
-                                          Reg.AltNames> {
-      let SubRegIndices = [sub_32, sub_32_hi];
+    defvar RegP1 = !cast<Register>("X"#IndexP1);
+    def "X" # Index #"_X" # IndexP1 : RISCVRegWithSubRegs<Index,
+                                                          Reg.AsmName,
+                                                          [Reg, RegP1],
+                                                          Reg.AltNames> {
+      let SubRegIndices = [sub_gpr_even, sub_gpr_odd];
       let CoveredBySubRegs = 1;
     }
   }
 }
 
-let RegInfos = RegInfoByHwMode<[RV64], [RegInfo<64, 64, 64>]> in
-def GPRPF64 : RegisterClass<"RISCV", [f64], 64, (add
-    X10_PD, X12_PD, X14_PD, X16_PD,
-    X6_PD,
-    X28_PD, X30_PD,
-    X8_PD,
-    X18_PD, X20_PD, X22_PD, X24_PD, X26_PD,
-    X0_PD, X2_PD, X4_PD
+let RegInfos = RegInfoByHwMode<[RV64], [RegInfo<64, 64, 64>]>,
+    DecoderMethod = "DecodeGPRPairRegisterClass" in
+def GPRPair : RegisterClass<"RISCV", [f64], 64, (add
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dtcxzyw wrote:

Emm, looks like it cannot be used by zacas in rv32.
```suggestion
def GPRPair : RegisterClass<"RISCV", [untyped], 64, (add
```
Is it OK to use `f64` as the `regTypes`? It is wired for me.


https://github.com/llvm/llvm-project/pull/77408