[PATCH] D132347: [RISCV] Use analyzeBranch in RISCVRedundantCopyElimination.

[Craig Topper via Phabricator via llvm-commits]

craig.topper created this revision.
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The existing code was incorrect if we had more than one conditional
branch instruction in a basic block. Though I don't think that will
occur, using analyzeBranch detects that as an unsupported case.

Overall this results in simpler code in RISCVRedundantCopyElimination.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D132347

Files:
  llvm/lib/Target/RISCV/RISCVRedundantCopyElimination.cpp


Index: llvm/lib/Target/RISCV/RISCVRedundantCopyElimination.cpp
===================================================================
--- llvm/lib/Target/RISCV/RISCVRedundantCopyElimination.cpp
+++ llvm/lib/Target/RISCV/RISCVRedundantCopyElimination.cpp
@@ -24,6 +24,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "RISCV.h"
+#include "RISCVInstrInfo.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/CodeGen/MachineFunctionPass.h"
 #include "llvm/CodeGen/MachineRegisterInfo.h"
@@ -39,6 +40,7 @@
 class RISCVRedundantCopyElimination : public MachineFunctionPass {
   const MachineRegisterInfo *MRI;
   const TargetRegisterInfo *TRI;
+  const TargetInstrInfo *TII;
 
 public:
   static char ID;
@@ -68,16 +70,17 @@
 INITIALIZE_PASS(RISCVRedundantCopyElimination, "riscv-copyelim",
                 "RISCV redundant copy elimination pass", false, false)
 
-static bool guaranteesZeroRegInBlock(const MachineInstr &MI,
-                                     const MachineBasicBlock &MBB) {
-  unsigned Opc = MI.getOpcode();
-  if (Opc == RISCV::BEQ && MI.getOperand(1).getReg() == RISCV::X0 &&
-      &MBB == MI.getOperand(2).getMBB())
+static bool
+guaranteesZeroRegInBlock(MachineBasicBlock &MBB,
+                         const SmallVectorImpl<MachineOperand> &Cond,
+                         MachineBasicBlock *TBB) {
+  assert(Cond.size() == 3 && "Unexpected number of operands");
+  assert(TBB != nullptr && "Expected branch target basic block");
+  auto CC = static_cast<RISCVCC::CondCode>(Cond[0].getImm());
+  if (CC == RISCVCC::COND_EQ && Cond[2].getReg() == RISCV::X0 && TBB == &MBB)
     return true;
-  if (Opc == RISCV::BNE && MI.getOperand(1).getReg() == RISCV::X0 &&
-      &MBB != MI.getOperand(2).getMBB())
+  if (CC == RISCVCC::COND_NE && Cond[2].getReg() == RISCV::X0 && TBB != &MBB)
     return true;
-
   return false;
 }
 
@@ -92,24 +95,17 @@
   if (PredMBB->succ_size() != 2)
     return false;
 
-  MachineBasicBlock::iterator CondBr = PredMBB->getLastNonDebugInstr();
-  if (CondBr == PredMBB->end())
+  MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
+  SmallVector<MachineOperand, 3> Cond;
+  if (TII->analyzeBranch(*PredMBB, TBB, FBB, Cond, /*AllowModify*/ false) ||
+      Cond.empty())
     return false;
 
-  while (true) {
-    // If we run out of terminators, give up.
-    if (!CondBr->isTerminator())
-      return false;
-    // If we found a branch with X0, stop searching and try to remove copies.
-    if (guaranteesZeroRegInBlock(*CondBr, MBB))
-      break;
-    // If we reached the beginning of the basic block, give up.
-    if (CondBr == PredMBB->begin())
-      return false;
-    --CondBr;
-  }
+  // Is this a branch with X0?
+  if (!guaranteesZeroRegInBlock(MBB, Cond, TBB))
+    return false;
 
-  Register TargetReg = CondBr->getOperand(0).getReg();
+  Register TargetReg = Cond[1].getReg();
   if (!TargetReg)
     return false;
 
@@ -146,7 +142,7 @@
 
   // Otherwise, we have to fixup the use-def chain, starting with the
   // BEQ/BNE. Conservatively mark as much as we can live.
-  CondBr->clearRegisterKills(TargetReg, TRI);
+  Cond[1].setIsKill(false);
 
   // Add newly used reg to the block's live-in list if it isn't there already.
   if (!MBB.isLiveIn(TargetReg))
@@ -163,6 +159,7 @@
   if (skipFunction(MF.getFunction()))
     return false;
 
+  TII = MF.getSubtarget().getInstrInfo();
   TRI = MF.getSubtarget().getRegisterInfo();
   MRI = &MF.getRegInfo();
 


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