Peephole optimization for ABS on ARM.

Patch by Ana Pazos!

llvm-svn: 141365

Author: asl

llvm/lib/Target/ARM/ARMISelDAGToDAG.cpp

@@ -47,6 +47,11 @@ CheckVMLxHazard("check-vmlx-hazard", cl::Hidden,
   cl::desc("Check fp vmla / vmls hazard at isel time"),
   cl::init(true));
 
+static cl::opt<bool>
+DisableARMIntABS("disable-arm-int-abs", cl::Hidden,
+  cl::desc("Enable / disable ARM integer abs transform"),
+  cl::init(false));
+
 //===--------------------------------------------------------------------===//
 /// ARMDAGToDAGISel - ARM specific code to select ARM machine
 /// instructions for SelectionDAG operations.
@@ -252,6 +257,9 @@ class ARMDAGToDAGISel : public SelectionDAGISel {
                                ARMCC::CondCodes CCVal, SDValue CCR,
                                SDValue InFlag);
 
+  // Select special operations if node forms integer ABS pattern
+  SDNode *SelectABSOp(SDNode *N);
+
   SDNode *SelectConcatVector(SDNode *N);
 
   SDNode *SelectAtomic64(SDNode *Node, unsigned Opc);
@@ -2295,6 +2303,53 @@ SDNode *ARMDAGToDAGISel::SelectCMOVOp(SDNode *N) {
   return CurDAG->SelectNodeTo(N, Opc, VT, Ops, 5);
 }
 
+/// Target-specific DAG combining for ISD::XOR.
+/// Target-independent combining lowers SELECT_CC nodes of the form
+/// select_cc setg[ge] X,  0,  X, -X
+/// select_cc setgt    X, -1,  X, -X
+/// select_cc setl[te] X,  0, -X,  X
+/// select_cc setlt    X,  1, -X,  X
+/// which represent Integer ABS into:
+/// Y = sra (X, size(X)-1); xor (add (X, Y), Y)
+/// ARM instruction selection detects the latter and matches it to
+/// ARM::ABS or ARM::t2ABS machine node.
+SDNode *ARMDAGToDAGISel::SelectABSOp(SDNode *N){
+  SDValue XORSrc0 = N->getOperand(0);
+  SDValue XORSrc1 = N->getOperand(1);
+  DebugLoc DL = N->getDebugLoc();
+  EVT VT = N->getValueType(0);
+
+  if (DisableARMIntABS)
+    return NULL;
+
+  if (XORSrc0.getOpcode() != ISD::ADD ||
+    XORSrc1.getOpcode() != ISD::SRA)
+    return NULL;
+
+  SDValue ADDSrc0 = XORSrc0.getOperand(0);
+  SDValue ADDSrc1 = XORSrc0.getOperand(1);
+  SDValue SRASrc0 = XORSrc1.getOperand(0);
+  SDValue SRASrc1 = XORSrc1.getOperand(1);
+  ConstantSDNode *SRAConstant =  dyn_cast<ConstantSDNode>(SRASrc1);
+  EVT XType = SRASrc0.getValueType();
+  unsigned Size = XType.getSizeInBits() - 1;
+
+  if (ADDSrc1 == XORSrc1  &&
+      ADDSrc0 == SRASrc0 &&
+      XType.isInteger() &&
+      SRAConstant != NULL &&
+      Size == SRAConstant->getZExtValue()) {
+
+    unsigned Opcode = ARM::ABS;
+    if (Subtarget->isThumb2())
+      Opcode = ARM::t2ABS;
+
+    return CurDAG->SelectNodeTo(N, Opcode, VT, ADDSrc0);
+  }
+
+  return NULL;
+}
+
 SDNode *ARMDAGToDAGISel::SelectConcatVector(SDNode *N) {
   // The only time a CONCAT_VECTORS operation can have legal types is when
   // two 64-bit vectors are concatenated to a 128-bit vector.
@@ -2331,6 +2386,14 @@ SDNode *ARMDAGToDAGISel::Select(SDNode *N) {
 
   switch (N->getOpcode()) {
   default: break;
+  case ISD::XOR: {
+    // Select special operations if XOR node forms integer ABS pattern
+    SDNode *ResNode = SelectABSOp(N);
+    if (ResNode)
+      return ResNode;
+    // Other cases are autogenerated.
+    break;
+  }
   case ISD::Constant: {
     unsigned Val = cast<ConstantSDNode>(N)->getZExtValue();
     bool UseCP = true;

llvm/lib/Target/ARM/ARMISelLowering.cpp

@@ -6100,6 +6100,86 @@ ARMTargetLowering::EmitInstrWithCustomInserter(MachineInstr *MI,
     MI->eraseFromParent();   // The pseudo instruction is gone now.
     return BB;
   }
+
+  case ARM::ABS:
+  case ARM::t2ABS: {
+    // To insert an ABS instruction, we have to insert the
+    // diamond control-flow pattern.  The incoming instruction knows the
+    // source vreg to test against 0, the destination vreg to set,
+    // the condition code register to branch on, the
+    // true/false values to select between, and a branch opcode to use. 
+    // It transforms
+    //     V1 = ABS V0
+    // into
+    //     V2 = MOVS V0
+    //     BCC                      (branch to SinkBB if V0 >= 0)
+    //     RSBBB: V3 = RSBri V2, 0  (compute ABS if V2 < 0)
+    //     SinkBB: V1 = PHI(V2, V3)     
+    const BasicBlock *LLVM_BB = BB->getBasicBlock();
+    MachineFunction::iterator BBI = BB;
+    ++BBI;
+    MachineFunction *Fn = BB->getParent();
+    MachineBasicBlock *RSBBB = Fn->CreateMachineBasicBlock(LLVM_BB);
+    MachineBasicBlock *SinkBB  = Fn->CreateMachineBasicBlock(LLVM_BB);
+    Fn->insert(BBI, RSBBB);
+    Fn->insert(BBI, SinkBB);
+
+    unsigned int ABSSrcReg = MI->getOperand(1).getReg();
+    unsigned int ABSDstReg = MI->getOperand(0).getReg();
+    bool isThumb2 = Subtarget->isThumb2();
+    MachineRegisterInfo &MRI = Fn->getRegInfo();
+    // In Thumb mode S must not be specified if source register is the SP or
+    // PC and if destination register is the SP, so restrict register class
+    unsigned NewMovDstReg = MRI.createVirtualRegister(
+      isThumb2 ? ARM::rGPRRegisterClass : ARM::GPRRegisterClass);
+    unsigned NewRsbDstReg = MRI.createVirtualRegister(
+      isThumb2 ? ARM::rGPRRegisterClass : ARM::GPRRegisterClass);
+
+    // Transfer the remainder of BB and its successor edges to sinkMBB.
+    SinkBB->splice(SinkBB->begin(), BB,
+      llvm::next(MachineBasicBlock::iterator(MI)),
+      BB->end());
+    SinkBB->transferSuccessorsAndUpdatePHIs(BB);
+
+    BB->addSuccessor(RSBBB);
+    BB->addSuccessor(SinkBB);
+
+    // fall through to SinkMBB
+    RSBBB->addSuccessor(SinkBB);
+
+    // insert a movs at the end of BB
+    BuildMI(BB, dl, TII->get(isThumb2 ? ARM::t2MOVr : ARM::MOVr),
+      NewMovDstReg)
+      .addReg(ABSSrcReg, RegState::Kill)
+      .addImm((unsigned)ARMCC::AL).addReg(0)
+      .addReg(ARM::CPSR, RegState::Define);
+
+    // insert a bcc with opposite CC to ARMCC::MI at the end of BB
+    BuildMI(BB, dl, 
+      TII->get(isThumb2 ? ARM::t2Bcc : ARM::Bcc)).addMBB(SinkBB)
+      .addImm(ARMCC::getOppositeCondition(ARMCC::MI)).addReg(ARM::CPSR);
+
+    // insert rsbri in RSBBB
+    // Note: BCC and rsbri will be converted into predicated rsbmi
+    // by if-conversion pass
+    BuildMI(*RSBBB, RSBBB->begin(), dl, 
+      TII->get(isThumb2 ? ARM::t2RSBri : ARM::RSBri), NewRsbDstReg)
+      .addReg(NewMovDstReg, RegState::Kill)
+      .addImm(0).addImm((unsigned)ARMCC::AL).addReg(0).addReg(0);
+
+    // insert PHI in SinkBB, 
+    // reuse ABSDstReg to not change uses of ABS instruction
+    BuildMI(*SinkBB, SinkBB->begin(), dl,
+      TII->get(ARM::PHI), ABSDstReg)
+      .addReg(NewRsbDstReg).addMBB(RSBBB)
+      .addReg(NewMovDstReg).addMBB(BB);
+
+    // remove ABS instruction
+    MI->eraseFromParent(); 
+
+    // return last added BB
+    return SinkBB;
+  }
   }
 }
 

llvm/lib/Target/ARM/ARMInstrInfo.td

@@ -2848,6 +2848,9 @@ def MOVr : AsI1<0b1101, (outs GPR:$Rd), (ins GPR:$Rm), DPFrm, IIC_iMOVr,
   let Inst{15-12} = Rd;
 }
 
+def : ARMInstAlias<"movs${p} $Rd, $Rm", 
+                   (MOVr GPR:$Rd, GPR:$Rm, pred:$p, CPSR)>;
+
 // A version for the smaller set of tail call registers.
 let neverHasSideEffects = 1 in
 def MOVr_TC : AsI1<0b1101, (outs tcGPR:$Rd), (ins tcGPR:$Rm), DPFrm,
@@ -4025,6 +4028,14 @@ def ISB : AInoP<(outs), (ins memb_opt:$opt), MiscFrm, NoItinerary,
   let Inst{3-0} = opt;
 }
 
+// Pseudo isntruction that combines movs + predicated rsbmi
+// to implement integer ABS
+let usesCustomInserter = 1, Defs = [CPSR] in {
+def ABS : ARMPseudoInst<
+  (outs GPR:$dst), (ins GPR:$src),
+  8, NoItinerary, []>;
+}
+
 let usesCustomInserter = 1 in {
   let Defs = [CPSR] in {
     def ATOMIC_LOAD_ADD_I8 : PseudoInst<

llvm/lib/Target/ARM/ARMInstrThumb2.td

@@ -3433,6 +3433,14 @@ def t2LDRpci_pic : PseudoInst<(outs rGPR:$dst), (ins i32imm:$addr, pclabel:$cp),
               [(set rGPR:$dst, (ARMpic_add (load (ARMWrapper tconstpool:$addr)),
                                            imm:$cp))]>,
                Requires<[IsThumb2]>;
+
+// Pseudo isntruction that combines movs + predicated rsbmi 
+// to implement integer ABS
+let usesCustomInserter = 1, Defs = [CPSR] in {
+def t2ABS : PseudoInst<(outs rGPR:$dst), (ins rGPR:$src),
+                       NoItinerary, []>, Requires<[IsThumb2]>;
+}
+
 //===----------------------------------------------------------------------===//
 // Coprocessor load/store -- for disassembly only
 //

llvm/test/CodeGen/ARM/iabs.ll

@@ -1,16 +1,16 @@
 ; RUN: llc < %s -march=arm -mattr=+v4t | FileCheck %s
 
 ;; Integer absolute value, should produce something as good as: ARM:
-;;   add r3, r0, r0, asr #31
-;;   eor r0, r3, r0, asr #31
+;;   movs r0, r0
+;;   rsbmi r0, r0, #0
 ;;   bx lr
 
 define i32 @test(i32 %a) {
         %tmp1neg = sub i32 0, %a
         %b = icmp sgt i32 %a, -1
         %abs = select i1 %b, i32 %a, i32 %tmp1neg
         ret i32 %abs
-; CHECK:   add r1, r0, r0, asr #31
-; CHECK:   eor r0, r1, r0, asr #31
+; CHECK:  movs r0, r0
+; CHECK:  rsbmi r0, r0, #0
 ; CHECK:  bx lr
 }

llvm/test/CodeGen/Thumb/iabs.ll

@@ -3,15 +3,20 @@
 
 ;; Integer absolute value, should produce something as good as:
 ;; Thumb:
-;;   asr r2, r0, #31
-;;   add r0, r0, r2
-;;   eor r0, r2
+;;   movs r0, r0
+;;   bpl
+;;   rsb r0, r0, #0 (with opitmization, bpl + rsb is if-converted into rsbmi)
 ;;   bx lr
 
 define i32 @test(i32 %a) {
         %tmp1neg = sub i32 0, %a
         %b = icmp sgt i32 %a, -1
         %abs = select i1 %b, i32 %a, i32 %tmp1neg
         ret i32 %abs
+; CHECK:  movs r0, r0
+; CHECK:  bpl
+; CHECK:  rsb r0, r0, #0
+; CHECK:  bx lr
 }
 
+