[PATCH] D153974: [RISCV] Don't include X1 in the X0_PD register pair

[Alex Bradbury via Phabricator via llvm-commits]

asb updated this revision to Diff 542013.
asb edited the summary of this revision.
asb added a comment.

Updated to incorporate suggested fixes from @craig.topper that means unwanted codegen changes are avoided.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D153974/new/

https://reviews.llvm.org/D153974

Files:
  llvm/lib/Target/RISCV/RISCVRegisterInfo.cpp
  llvm/lib/Target/RISCV/RISCVRegisterInfo.td


Index: llvm/lib/Target/RISCV/RISCVRegisterInfo.td
===================================================================
--- llvm/lib/Target/RISCV/RISCVRegisterInfo.td
+++ llvm/lib/Target/RISCV/RISCVRegisterInfo.td
@@ -535,8 +535,23 @@
 def GPRF32  : RegisterClass<"RISCV", [f32], 32, (add GPR)>;
 } // RegInfos = XLenRI
 
+// Dummy zero register for use in the register pair containing X0 (as X1 is
+// not read to or written when the X0 register pair is used).
+def DUMMY_REG_PAIR_WITH_X0 : RISCVReg<0, "0">;
+
+// Must add DUMMY_REG_PAIR_WITH_X0 to a separate register class to prevent the
+// register's existence from changing codegen (due to the regPressureSetLimit
+// for the GPR register class being altered).
+def GPRAll : GPRRegisterClass<(add GPR, DUMMY_REG_PAIR_WITH_X0)>;
+
 let RegAltNameIndices = [ABIRegAltName] in {
-  foreach I = 0-15 in {
+  def X0_PD : RISCVRegWithSubRegs<0, X0.AsmName,
+                                     [X0, DUMMY_REG_PAIR_WITH_X0],
+                                     X0.AltNames> {
+    let SubRegIndices = [sub_32, sub_32_hi];
+    let CoveredBySubRegs = 1;
+  }
+  foreach I = 1-15 in {
     defvar Index = !shl(I, 1);
     defvar Reg = !cast<Register>("X"#Index);
     defvar RegP1 = !cast<Register>("X"#!add(Index,1));
Index: llvm/lib/Target/RISCV/RISCVRegisterInfo.cpp
===================================================================
--- llvm/lib/Target/RISCV/RISCVRegisterInfo.cpp
+++ llvm/lib/Target/RISCV/RISCVRegisterInfo.cpp
@@ -103,6 +103,10 @@
   if (TFI->hasBP(MF))
     markSuperRegs(Reserved, RISCVABI::getBPReg()); // bp
 
+  // Additionally reserve dummy register used to form the register pair
+  // beginning with 'x0' for instructions that take register pairs.
+  markSuperRegs(Reserved, RISCV::DUMMY_REG_PAIR_WITH_X0);
+
   // V registers for code generation. We handle them manually.
   markSuperRegs(Reserved, RISCV::VL);
   markSuperRegs(Reserved, RISCV::VTYPE);


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