From 103299abab6922f450671baa76e34ad746ac4312 Mon Sep 17 00:00:00 2001
From: Richard-Qin-X <richard.qin.001@gmail.com>
Date: Mon, 17 Aug 2026 02:46:40 +0800
Subject: [RISCV][GlobalISel] Constrain core copies during selection

---
 .../lib/CodeGen/GlobalISel/InstructionSelect.cpp   | 24 +++++++++++++---------
 .../RISCV/GISel/RISCVInstructionSelector.cpp       | 15 ++++++++++++++
 2 files changed, 29 insertions(+), 10 deletions(-)

diff --git a/backend/lib/CodeGen/GlobalISel/InstructionSelect.cpp b/backend/lib/CodeGen/GlobalISel/InstructionSelect.cpp
index 1048843..8cfb9c7 100644
--- a/backend/lib/CodeGen/GlobalISel/InstructionSelect.cpp
+++ b/backend/lib/CodeGen/GlobalISel/InstructionSelect.cpp
@@ -291,7 +291,9 @@ Expected<MachinePassResult> InstructionSelect::runImplementation(MachineFunction
     if (CurrentDescriptor == nullptr)
       return std::unexpected(
           passFailure("instruction selector encountered an unknown instruction descriptor"));
-    if (!CurrentDescriptor->Flags.has(InstructionFlag::Generic))
+    const bool IsGeneric = CurrentDescriptor->Flags.has(InstructionFlag::Generic);
+    const bool IsCopy = Instruction->opcode() == opcode(GenericOpcode::COPY);
+    if (!IsGeneric && !IsCopy)
       continue;
     const OpcodeState Before = stateOf(*Instruction);
     if (!SeenStates.insert(Before).second) {
@@ -322,21 +324,23 @@ Expected<MachinePassResult> InstructionSelect::runImplementation(MachineFunction
     if (Selected == nullptr)
       return std::unexpected(
           passFailure("instruction selector produced an unknown instruction descriptor"));
-    auto FlagPropagation =
-        Info->propagateSelectionFlags(*Instruction, InputFlags, InputMayRaiseFPException, Target);
-    if (!FlagPropagation)
-      return std::unexpected(FlagPropagation.error());
-    auto Constraint = Info->constrainSelectedInstruction(*Instruction, Function, Target);
-    if (!Constraint) {
-      return std::unexpected(withErrorContext(
-          Constraint.error(), "instruction selector could not constrain selected registers: "));
+    if (IsGeneric) {
+      auto FlagPropagation =
+          Info->propagateSelectionFlags(*Instruction, InputFlags, InputMayRaiseFPException, Target);
+      if (!FlagPropagation)
+        return std::unexpected(FlagPropagation.error());
+      auto Constraint = Info->constrainSelectedInstruction(*Instruction, Function, Target);
+      if (!Constraint) {
+        return std::unexpected(withErrorContext(
+            Constraint.error(), "instruction selector could not constrain selected registers: "));
+      }
     }
     auto Validation = Target.validateInstruction(*Instruction, Function.registerInfo());
     if (!Validation)
       return std::unexpected(withErrorContext(Validation.error(),
                                               "instruction selector produced invalid MachineIR: "));
     const OpcodeState After = stateOf(*Instruction);
-    if (After == Before)
+    if (IsGeneric && After == Before)
       return std::unexpected(
           passFailure("instruction selector reported success without observable progress",
                       ErrorCode::InternalError));
diff --git a/backend/lib/Target/RISCV/GISel/RISCVInstructionSelector.cpp b/backend/lib/Target/RISCV/GISel/RISCVInstructionSelector.cpp
index 898b366..bd46144 100644
--- a/backend/lib/Target/RISCV/GISel/RISCVInstructionSelector.cpp
+++ b/backend/lib/Target/RISCV/GISel/RISCVInstructionSelector.cpp
@@ -421,6 +421,8 @@ public:
       return std::unexpected(Generated.error());
     if (*Generated)
       return {};
+    if (Instruction.opcode() == opcode(GenericOpcode::COPY))
+      return selectCopy(Instruction, Function, Target);
     auto Match = canSelect(Instruction, Function, Target);
     if (!Match)
       return std::unexpected(Match.error());
@@ -514,6 +516,19 @@ public:
   }
 
 private:
+  [[nodiscard]] Expected<void> selectCopy(MachineInstr &Instruction, MachineFunction &Function,
+                                          const TargetDescription &Target) const {
+    const MachineOperand *DestinationOperand = Instruction.operandAt(0);
+    const Register *Destination =
+        DestinationOperand == nullptr ? nullptr : DestinationOperand->getIf<Register>();
+    if (Destination == nullptr)
+      return makeUnexpected(ErrorCode::VerificationError,
+                            "RISC-V COPY selection requires a register destination");
+    if (!Destination->isVirtual())
+      return {};
+    return RegisterBanks.constrainRegister(Function, *Destination, Target);
+  }
+
   [[nodiscard]] Expected<Register>
   createTemporary(MachineFunction &Function, LowLevelType Type, const TargetDescription &Target,
                   std::optional<RegisterBankID> Bank = std::nullopt) const {
-- 
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