1 files changed, 90 insertions, 17 deletions

diff --git a/llvm/lib/Target/RISCV/RISCVISelLowering.cpp b/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
index 9b4193e53e1..94bd948c694 100644
--- a/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
+++ b/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
@@ -17,6 +17,7 @@
 #include "RISCVRegisterInfo.h"
 #include "RISCVSubtarget.h"
 #include "RISCVTargetMachine.h"
+#include "llvm/ADT/SmallSet.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/CodeGen/CallingConvLower.h"
 #include "llvm/CodeGen/MachineFrameInfo.h"
@@ -787,10 +788,21 @@ static MachineBasicBlock *emitBuildPairF64Pseudo(MachineInstr &MI,
   return BB;
 }
 
+static bool isSelectPseudo(MachineInstr &MI) {
+  switch (MI.getOpcode()) {
+  default:
+    return false;
+  case RISCV::Select_GPR_Using_CC_GPR:
+  case RISCV::Select_FPR32_Using_CC_GPR:
+  case RISCV::Select_FPR64_Using_CC_GPR:
+    return true;
+  }
+}
+
 static MachineBasicBlock *emitSelectPseudo(MachineInstr &MI,
                                            MachineBasicBlock *BB) {
-  // To "insert" a SELECT instruction, we actually have to insert the triangle
-  // control-flow pattern.  The incoming instruction knows the destination vreg
+  // To "insert" Select_* instructions, we actually have to insert the triangle
+  // control-flow pattern.  The incoming instructions know the destination vreg
   // to set, the condition code register to branch on, the true/false values to
   // select between, and the condcode to use to select the appropriate branch.
   //
@@ -800,6 +812,54 @@ static MachineBasicBlock *emitSelectPseudo(MachineInstr &MI,
   //     |  IfFalseMBB
   //     | /
   //    TailMBB
+  //
+  // When we find a sequence of selects we attempt to optimize their emission
+  // by sharing the control flow. Currently we only handle cases where we have
+  // multiple selects with the exact same condition (same LHS, RHS and CC).
+  // The selects may be interleaved with other instructions if the other
+  // instructions meet some requirements we deem safe:
+  // - They are debug instructions. Otherwise,
+  // - They do not have side-effects, do not access memory and their inputs do
+  //   not depend on the results of the select pseudo-instructions.
+  // The TrueV/FalseV operands of the selects cannot depend on the result of
+  // previous selects in the sequence.
+  // These conditions could be further relaxed. See the X86 target for a
+  // related approach and more information.
+  unsigned LHS = MI.getOperand(1).getReg();
+  unsigned RHS = MI.getOperand(2).getReg();
+  auto CC = static_cast<ISD::CondCode>(MI.getOperand(3).getImm());
+
+  SmallVector<MachineInstr *, 4> SelectDebugValues;
+  SmallSet<unsigned, 4> SelectDests;
+  SelectDests.insert(MI.getOperand(0).getReg());
+
+  MachineInstr *LastSelectPseudo = &MI;
+
+  for (auto E = BB->end(), SequenceMBBI = MachineBasicBlock::iterator(MI);
+       SequenceMBBI != E; ++SequenceMBBI) {
+    if (SequenceMBBI->isDebugInstr())
+      continue;
+    else if (isSelectPseudo(*SequenceMBBI)) {
+      if (SequenceMBBI->getOperand(1).getReg() != LHS ||
+          SequenceMBBI->getOperand(2).getReg() != RHS ||
+          SequenceMBBI->getOperand(3).getImm() != CC ||
+          SelectDests.count(SequenceMBBI->getOperand(4).getReg()) ||
+          SelectDests.count(SequenceMBBI->getOperand(5).getReg()))
+        break;
+      LastSelectPseudo = &*SequenceMBBI;
+      SequenceMBBI->collectDebugValues(SelectDebugValues);
+      SelectDests.insert(SequenceMBBI->getOperand(0).getReg());
+    } else {
+      if (SequenceMBBI->hasUnmodeledSideEffects() ||
+          SequenceMBBI->mayLoadOrStore())
+        break;
+      if (llvm::any_of(SequenceMBBI->operands(), [&](MachineOperand &MO) {
+            return MO.isReg() && MO.isUse() && SelectDests.count(MO.getReg());
+          }))
+        break;
+    }
+  }
+
   const TargetInstrInfo &TII = *BB->getParent()->getSubtarget().getInstrInfo();
   const BasicBlock *LLVM_BB = BB->getBasicBlock();
   DebugLoc DL = MI.getDebugLoc();
@@ -812,20 +872,23 @@ static MachineBasicBlock *emitSelectPseudo(MachineInstr &MI,
 
   F->insert(I, IfFalseMBB);
   F->insert(I, TailMBB);
-  // Move all remaining instructions to TailMBB.
-  TailMBB->splice(TailMBB->begin(), HeadMBB, std::next(MI.getIterator()),
-                  HeadMBB->end());
+
+  // Transfer debug instructions associated with the selects to TailMBB.
+  for (MachineInstr *DebugInstr : SelectDebugValues) {
+    TailMBB->push_back(DebugInstr->removeFromParent());
+  }
+
+  // Move all instructions after the sequence to TailMBB.
+  TailMBB->splice(TailMBB->end(), HeadMBB,
+                  std::next(LastSelectPseudo->getIterator()), HeadMBB->end());
   // Update machine-CFG edges by transferring all successors of the current
-  // block to the new block which will contain the Phi node for the select.
+  // block to the new block which will contain the Phi nodes for the selects.
   TailMBB->transferSuccessorsAndUpdatePHIs(HeadMBB);
   // Set the successors for HeadMBB.
   HeadMBB->addSuccessor(IfFalseMBB);
   HeadMBB->addSuccessor(TailMBB);
 
   // Insert appropriate branch.
-  unsigned LHS = MI.getOperand(1).getReg();
-  unsigned RHS = MI.getOperand(2).getReg();
-  auto CC = static_cast<ISD::CondCode>(MI.getOperand(3).getImm());
   unsigned Opcode = getBranchOpcodeForIntCondCode(CC);
 
   BuildMI(HeadMBB, DL, TII.get(Opcode))
@@ -836,15 +899,25 @@ static MachineBasicBlock *emitSelectPseudo(MachineInstr &MI,
   // IfFalseMBB just falls through to TailMBB.
   IfFalseMBB->addSuccessor(TailMBB);
 
-  // %Result = phi [ %TrueValue, HeadMBB ], [ %FalseValue, IfFalseMBB ]
-  BuildMI(*TailMBB, TailMBB->begin(), DL, TII.get(RISCV::PHI),
-          MI.getOperand(0).getReg())
-      .addReg(MI.getOperand(4).getReg())
-      .addMBB(HeadMBB)
-      .addReg(MI.getOperand(5).getReg())
-      .addMBB(IfFalseMBB);
+  // Create PHIs for all of the select pseudo-instructions.
+  auto SelectMBBI = MI.getIterator();
+  auto SelectEnd = std::next(LastSelectPseudo->getIterator());
+  auto InsertionPoint = TailMBB->begin();
+  while (SelectMBBI != SelectEnd) {
+    auto Next = std::next(SelectMBBI);
+    if (isSelectPseudo(*SelectMBBI)) {
+      // %Result = phi [ %TrueValue, HeadMBB ], [ %FalseValue, IfFalseMBB ]
+      BuildMI(*TailMBB, InsertionPoint, SelectMBBI->getDebugLoc(),
+              TII.get(RISCV::PHI), SelectMBBI->getOperand(0).getReg())
+          .addReg(SelectMBBI->getOperand(4).getReg())
+          .addMBB(HeadMBB)
+          .addReg(SelectMBBI->getOperand(5).getReg())
+          .addMBB(IfFalseMBB);
+      SelectMBBI->eraseFromParent();
+    }
+    SelectMBBI = Next;
+  }
 
-  MI.eraseFromParent(); // The pseudo instruction is gone now.
   return TailMBB;
 }