[NFC] [TargetRegisterInfo] add another API to get srcreg through copy.

Reviewed By: nemanjai, jsji

Differential Revision: https://reviews.llvm.org/D92069

Author: Chen Zheng

llvm/include/llvm/CodeGen/TargetRegisterInfo.h

@@ -415,6 +415,16 @@ class TargetRegisterInfo : public MCRegisterInfo {
   virtual Register lookThruCopyLike(Register SrcReg,
                                     const MachineRegisterInfo *MRI) const;
 
+  /// Find the original SrcReg unless it is the target of a copy-like operation,
+  /// in which case we chain backwards through all such operations to the
+  /// ultimate source register. If a physical register is encountered, we stop
+  /// the search.
+  /// Return the original SrcReg if all the definitions in the chain only have
+  /// one user and not a physical register.
+  virtual Register
+  lookThruSingleUseCopyChain(Register SrcReg,
+                             const MachineRegisterInfo *MRI) const;
+
   /// Return a null-terminated list of all of the callee-saved registers on
   /// this target. The register should be in the order of desired callee-save
   /// stack frame offset. The first register is closest to the incoming stack

llvm/lib/CodeGen/TargetRegisterInfo.cpp

@@ -533,6 +533,31 @@ TargetRegisterInfo::lookThruCopyLike(Register SrcReg,
   }
 }
 
+Register TargetRegisterInfo::lookThruSingleUseCopyChain(
+    Register SrcReg, const MachineRegisterInfo *MRI) const {
+  while (true) {
+    const MachineInstr *MI = MRI->getVRegDef(SrcReg);
+    // Found the real definition, return it if it has a single use.
+    if (!MI->isCopyLike())
+      return MRI->hasOneNonDBGUse(SrcReg) ? SrcReg : Register();
+
+    Register CopySrcReg;
+    if (MI->isCopy())
+      CopySrcReg = MI->getOperand(1).getReg();
+    else {
+      assert(MI->isSubregToReg() && "Bad opcode for lookThruCopyLike");
+      CopySrcReg = MI->getOperand(2).getReg();
+    }
+
+    // Continue only if the next definition in the chain is for a virtual
+    // register that has a single use.
+    if (!CopySrcReg.isVirtual() || !MRI->hasOneNonDBGUse(CopySrcReg))
+      return Register();
+
+    SrcReg = CopySrcReg;
+  }
+}
+
 void TargetRegisterInfo::getOffsetOpcodes(
     const StackOffset &Offset, SmallVectorImpl<uint64_t> &Ops) const {
   assert(!Offset.getScalable() && "Scalable offsets are not handled");