[Hexagon] Don't kill live registers when creating mux out of tfr

When a mux instruction is created from a pair of complementary conditional
transfers, it can be placed at the location of either the earlier or the
later of the transfers. Since it will use the operands of the original
transfers, putting it in the earlier location may hoist a kill of a source
register that was originally further down. Make sure the kill flag is
removed if the register is still used afterwards.

llvm-svn: 305300

llvm/lib/Target/Hexagon/HexagonGenMux.cpp

@@ -59,9 +59,7 @@ namespace {
   public:
     static char ID;
 
-    HexagonGenMux() : MachineFunctionPass(ID), HII(nullptr), HRI(nullptr) {
-      initializeHexagonGenMuxPass(*PassRegistry::getPassRegistry());
-    }
+    HexagonGenMux() : MachineFunctionPass(ID) {}
 
     StringRef getPassName() const override {
       return "Hexagon generate mux instructions";
@@ -79,8 +77,8 @@ namespace {
     }
 
   private:
-    const HexagonInstrInfo *HII;
-    const HexagonRegisterInfo *HRI;
+    const HexagonInstrInfo *HII = nullptr;
+    const HexagonRegisterInfo *HRI = nullptr;
 
     struct CondsetInfo {
       unsigned PredR = 0;
@@ -134,7 +132,7 @@ namespace {
 
 } // end anonymous namespace
 
-INITIALIZE_PASS(HexagonGenMux, "hexagon-mux",
+INITIALIZE_PASS(HexagonGenMux, "hexagon-gen-mux",
   "Hexagon generate mux instructions", false, false)
 
 void HexagonGenMux::getSubRegs(unsigned Reg, BitVector &SRs) const {
@@ -297,12 +295,15 @@ bool HexagonGenMux::genMuxInBlock(MachineBasicBlock &B) {
     unsigned SR1 = Src1->isReg() ? Src1->getReg() : 0;
     unsigned SR2 = Src2->isReg() ? Src2->getReg() : 0;
     bool Failure = false, CanUp = true, CanDown = true;
+    bool Used1 = false, Used2 = false;
     for (unsigned X = MinX+1; X < MaxX; X++) {
       const DefUseInfo &DU = DUM.lookup(X);
       if (DU.Defs[PR] || DU.Defs[DR] || DU.Uses[DR]) {
         Failure = true;
         break;
       }
+      Used1 |= DU.Uses[SR1];
+      Used2 |= DU.Uses[SR2];
       if (CanDown && DU.Defs[SR1])
         CanDown = false;
       if (CanUp && DU.Defs[SR2])
@@ -316,6 +317,15 @@ bool HexagonGenMux::genMuxInBlock(MachineBasicBlock &B) {
     // Prefer "down", since this will move the MUX farther away from the
     // predicate definition.
     MachineBasicBlock::iterator At = CanDown ? Def2 : Def1;
+    if (!CanDown) {
+      // If the MUX is placed "up", it shouldn't kill any source registers
+      // that are still used afterwards. We can reset the kill flags directly
+      // on the operands, because the source instructions will be erased.
+      if (Used1 && Src1->isReg())
+        Src1->setIsKill(false);
+      if (Used2 && Src2->isReg())
+        Src2->setIsKill(false);
+    }
     ML.push_back(MuxInfo(At, DR, PR, SrcT, SrcF, Def1, Def2));
   }
 

llvm/lib/Target/Hexagon/HexagonTargetMachine.cpp

@@ -111,6 +111,7 @@ namespace llvm {
   extern char &HexagonExpandCondsetsID;
   void initializeHexagonExpandCondsetsPass(PassRegistry&);
   void initializeHexagonLoopIdiomRecognizePass(PassRegistry&);
+  void initializeHexagonGenMuxPass(PassRegistry&);
   void initializeHexagonOptAddrModePass(PassRegistry&);
   Pass *createHexagonLoopIdiomPass();
 
@@ -152,8 +153,11 @@ static Reloc::Model getEffectiveRelocModel(Optional<Reloc::Model> RM) {
 extern "C" void LLVMInitializeHexagonTarget() {
   // Register the target.
   RegisterTargetMachine<HexagonTargetMachine> X(getTheHexagonTarget());
-  initializeHexagonLoopIdiomRecognizePass(*PassRegistry::getPassRegistry());
-  initializeHexagonOptAddrModePass(*PassRegistry::getPassRegistry());
+
+  PassRegistry &PR = *PassRegistry::getPassRegistry();
+  initializeHexagonLoopIdiomRecognizePass(PR);
+  initializeHexagonGenMuxPass(PR);
+  initializeHexagonOptAddrModePass(PR);
 }
 
 HexagonTargetMachine::HexagonTargetMachine(const Target &T, const Triple &TT,

llvm/test/CodeGen/Hexagon/mux-kill.mir

@@ -0,0 +1,15 @@
+# RUN: llc -march=hexagon -run-pass hexagon-gen-mux -o - %s -verify-machineinstrs | FileCheck %s
+# CHECK: %r2 = C2_mux %p0, %r0, %r1
+---
+name: fred
+tracksRegLiveness: true
+
+body: |
+  bb.0:
+    liveins: %d0, %p0
+
+    %r2 = A2_tfrt %p0, %r0
+    %r0 = A2_tfr %r1
+    %r2 = A2_tfrf %p0, killed %r1
+...
+