[X86] Replace LEAs with INC/DEC

[anadolskiy]

Summary

If possible and profitable, replace lea %reg, 1(%reg) and lea %reg, -1(%reg) with inc %reg and dec %reg respectively.

[nadav]

I don’t see a problem with this commit but please email llvm-commit and allow other developers to review this patch.

[anadolskiy]

ping

lib/Target/X86/X86FixupLEAs.cpp

 //
 // This file defines the pass that finds instructions that can be
 // re-written as LEA instructions in order to reduce pipeline delays.
+// When optimizing for size it replaces suitable LEAs with INC or DEC.
 //
 //===----------------------------------------------------------------------===//
 

   void processInstructionForSLM(MachineBasicBlock::iterator &I,
                                 MachineFunction::iterator MFI);
 
+  /// \brief Look for LEAs that add 1 to reg or subtract 1 from reg
+  /// and convert them to INC or DEC respectively.
+  bool fixupIncDec(MachineBasicBlock::iterator &I,
+                   MachineFunction::iterator MFI) const;
+
   /// \brief Determine if an instruction references a machine register
   /// and, if so, whether it reads or writes the register.
   RegUsageState usesRegister(MachineOperand &p, MachineBasicBlock::iterator I);

 private:
   MachineFunction *MF;
   const X86InstrInfo *TII; // Machine instruction info.
+  bool OptIncDec;
+  bool OptLEA;
 };
 char FixupLEAPass::ID = 0;
 }

 bool FixupLEAPass::runOnMachineFunction(MachineFunction &Func) {
   MF = &Func;
   const X86Subtarget &ST = Func.getSubtarget<X86Subtarget>();
-  if (!ST.LEAusesAG() && !ST.slowLEA())
+  OptIncDec = !ST.slowIncDec() || Func.getFunction()->optForMinSize();
+  OptLEA = ST.LEAusesAG() || ST.slowLEA();
+
+  if (!OptLEA && !OptIncDec)
     return false;
 
   TII = ST.getInstrInfo();

   return nullptr;
 }
 
+static inline bool isLEA(const int opcode) {
+  return opcode == X86::LEA16r || opcode == X86::LEA32r ||
+         opcode == X86::LEA64r || opcode == X86::LEA64_32r;
+}
+
+/// isLEASimpleIncOrDec - Does this LEA have one these forms:
+/// lea  %reg, 1(%reg)
+/// lea  %reg, -1(%reg)
+static inline bool isLEASimpleIncOrDec(MachineInstr *LEA) {
+  unsigned SrcReg = LEA->getOperand(1 + X86::AddrBaseReg).getReg();
+  unsigned DstReg = LEA->getOperand(0).getReg();
+  unsigned AddrDispOp = 1 + X86::AddrDisp;
+  return SrcReg == DstReg &&
+         LEA->getOperand(1 + X86::AddrIndexReg).getReg() == 0 &&
+         LEA->getOperand(1 + X86::AddrSegmentReg).getReg() == 0 &&
+         LEA->getOperand(AddrDispOp).isImm() &&
+         (LEA->getOperand(AddrDispOp).getImm() == 1 ||
+          LEA->getOperand(AddrDispOp).getImm() == -1);
+}
+
+bool FixupLEAPass::fixupIncDec(MachineBasicBlock::iterator &I,
+                               MachineFunction::iterator MFI) const {
+  MachineInstr *MI = I;
+  int Opcode = MI->getOpcode();
+  if (!isLEA(Opcode))
+    return false;
+
+  if (isLEASimpleIncOrDec(MI) && TII->isSafeToClobberEFLAGS(*MFI, I)) {
+    int NewOpcode;
+    bool isINC = MI->getOperand(4).getImm() == 1;
+    switch (Opcode) {
+    case X86::LEA16r:
+      NewOpcode = isINC ? X86::INC16r : X86::DEC16r;
+      break;
+    case X86::LEA32r:
+    case X86::LEA64_32r:
+      NewOpcode = isINC ? X86::INC32r : X86::DEC32r;
+      break;
+    case X86::LEA64r:
+      NewOpcode = isINC ? X86::INC64r : X86::DEC64r;
+      break;
+    }
+
+    MachineInstr *NewMI =
+        BuildMI(*MFI, I, MI->getDebugLoc(), TII->get(NewOpcode))
+            .addOperand(MI->getOperand(0))
+            .addOperand(MI->getOperand(1));
+    MFI->erase(I);
+    I = static_cast<MachineBasicBlock::iterator>(NewMI);
+    return true;
+  }
+  return false;
+}
+
 void FixupLEAPass::processInstruction(MachineBasicBlock::iterator &I,
                                       MachineFunction::iterator MFI) {
   // Process a load, store, or LEA instruction.

                                             MachineFunction::iterator MFI) {
   MachineInstr *MI = I;
   const int opcode = MI->getOpcode();
-  if (opcode != X86::LEA16r && opcode != X86::LEA32r && opcode != X86::LEA64r &&
-      opcode != X86::LEA64_32r)
+  if (!isLEA(opcode))
     return;
   if (MI->getOperand(5).getReg() != 0 || !MI->getOperand(4).isImm() ||
       !TII->isSafeToClobberEFLAGS(*MFI, I))

     return;
   int addrr_opcode, addri_opcode;
   switch (opcode) {
-  default: llvm_unreachable("Unexpected LEA instruction");
+  default:
+    llvm_unreachable("Unexpected LEA instruction");
   case X86::LEA16r:
     addrr_opcode = X86::ADD16rr;
     addri_opcode = X86::ADD16ri;

                                      MachineFunction::iterator MFI) {
 
   for (MachineBasicBlock::iterator I = MFI->begin(); I != MFI->end(); ++I) {
-    if (MF.getSubtarget<X86Subtarget>().isSLM())
-      processInstructionForSLM(I, MFI);
-    else
-      processInstruction(I, MFI);
+    if (OptIncDec)
+      if (fixupIncDec(I, MFI))
+        continue;
+
+    if (OptLEA) {
+      if (MF.getSubtarget<X86Subtarget>().isSLM())
+        processInstructionForSLM(I, MFI);
+      else
+        processInstruction(I, MFI);
+    }
   }
   return false;
 }

test/CodeGen/X86/fixup-lea.ll

+;RUN: llc < %s -march=x86 | FileCheck %s
+
+define void @foo(i32 inreg %dns) minsize {
+entry:
+; CHECK-LABEL: foo
+; CHECK: dec
+  br label %for.body
+
+for.body:
+  %i.05 = phi i16 [ %dec, %for.body ], [ 0, %entry ]
+  %dec = add i16 %i.05, -1
+  %conv = zext i16 %dec to i32
+  %cmp = icmp slt i32 %conv, %dns
+  br i1 %cmp, label %for.body, label %for.end
+
+for.end:
+  ret void
+}
+
+define void @bar(i32 inreg %dns) minsize {
+entry:
+; CHECK-LABEL: bar
+; CHECK: inc
+  br label %for.body
+
+for.body:
+  %i.05 = phi i16 [ %inc, %for.body ], [ 0, %entry ]
+  %inc = add i16 %i.05, 1
+  %conv = zext i16 %inc to i32
+  %cmp = icmp slt i32 %conv, %dns
+  br i1 %cmp, label %for.body, label %for.end
+for.end:
+  ret void
+}

test/CodeGen/X86/lsr-static-addr.ll

 ; ATOM-NEXT: movsd A(,%rax,8)
 ; ATOM-NEXT: mulsd
 ; ATOM-NEXT: movsd
-; ATOM-NEXT: leaq 1(%rax), %rax
+; ATOM-NEXT: incq %rax
 
 @A = external global [0 x double]