[ARM] Revert WLSTP to DLSTP if the target block is out of range

[dmgreen]

Summary

If the block target for a WLSTP instruction is known to be out of range, and cannot be fixed by the ARMBlockPlacementPass, we can relax it to a DLSTP (and cmp/branch) to still allow the creation of tail predicated loops. That is what this patch does, adding extra revert code to the fallback path of ARMBlockPlacementPass.

Due to the code produced when reverting, this creates a DLSTP between a Bcc and a Br. As a DLS isn't necessarily a terminator we need to split the block to move the DLS/Br into.

[dmgreen]

Now splits the block to keep terminators happy.

[samtebbs]

llvm/lib/Target/ARM/ARMBlockPlacement.cpp

104

Will you need to mark them as killed in the new DLS or is that done automatically?

120

DPS -> DLS ?

[dmgreen]

Thanks. This also now updates some block offsets, which need to be kept uptodate.

[dmgreen]

llvm/lib/Target/ARM/ARMBlockPlacement.cpp

104

Yeah it should done automatically if it's needed. The kill flags only need to be conservative, not precise.

[samtebbs]

Nice one. Looks good to me.

llvm/lib/Target/ARM/ARMBlockPlacement.cpp

public:
   static char ID;
   ARMBlockPlacement() : MachineFunctionPass(ID) {}
 
   bool runOnMachineFunction(MachineFunction &MF) override;
   void moveBasicBlock(MachineBasicBlock *BB, MachineBasicBlock *Before);
   bool blockIsBefore(MachineBasicBlock *BB, MachineBasicBlock *Other);
   bool fixBackwardsWLS(MachineLoop *ML);
   bool processPostOrderLoops(MachineLoop *ML);
+  bool revertWhileToDo(MachineInstr *WLS, MachineLoop *ML);
 
   void getAnalysisUsage(AnalysisUsage &AU) const override {
     AU.setPreservesCFG();
     AU.addRequired<MachineLoopInfo>();
     MachineFunctionPass::getAnalysisUsage(AU);
   }
 };
 
static MachineInstr *findWLS(MachineLoop *ML) {
   MachineInstr *WlsInstr = findWLSInBlock(Predecessor);
   if (WlsInstr)
     return WlsInstr;
   if (Predecessor->pred_size() == 1)
     return findWLSInBlock(*Predecessor->pred_begin());
   return nullptr;
 }
 
+// Revert a WhileLoopStart to an equivalent DoLoopStart and branch. Note that
+// because of the branches this requires an extra block to be created.
+bool ARMBlockPlacement::revertWhileToDo(MachineInstr *WLS, MachineLoop *ML) {
+  //   lr = t2WhileLoopStartTP r0, r1, TgtBB
+  //   t2Br Ph
+  // ->
+  //   cmp r0, 0
+  //   brcc TgtBB
+  // block2:
+  //   LR = t2DoLoopStartTP r0, r1
+  //   t2Br Ph
+  MachineBasicBlock *Preheader = WLS->getParent();
+  assert(WLS != &Preheader->back());
+  assert(WLS->getNextNode() == &Preheader->back());
+  MachineInstr *Br = &Preheader->back();
+  assert(Br->getOpcode() == ARM::t2B);
+  assert(Br->getOperand(1).getImm() == 14);
+
+  // Clear the kill flags, as the cmp/bcc will no longer kill any operands.
+  WLS->getOperand(1).setIsKill(false);
+  if (WLS->getOpcode() == ARM::t2WhileLoopStartTP)
+    WLS->getOperand(2).setIsKill(false);
+
+  // Create the new block
+  MachineBasicBlock *NewBlock = Preheader->getParent()->CreateMachineBasicBlock(
+      Preheader->getBasicBlock());
+  Preheader->getParent()->insert(++Preheader->getIterator(), NewBlock);
+  // Move the Br to it
+  Br->removeFromParent();
+  NewBlock->insert(NewBlock->end(), Br);
+  // And setup the successors correctly.
+  Preheader->replaceSuccessor(Br->getOperand(0).getMBB(), NewBlock);
+  NewBlock->addSuccessor(Br->getOperand(0).getMBB());
+
+  // Create a new DLS to replace the WLS
+  MachineInstrBuilder MIB =
+      BuildMI(*NewBlock, Br, WLS->getDebugLoc(),
+              TII->get(WLS->getOpcode() == ARM::t2WhileLoopStartTP
+                           ? ARM::t2DoLoopStartTP
+                           : ARM::t2DoLoopStart));
+  MIB.add(WLS->getOperand(0));
+  MIB.add(WLS->getOperand(1));
+  if (WLS->getOpcode() == ARM::t2WhileLoopStartTP)
+    MIB.add(WLS->getOperand(2));
+
+  LLVM_DEBUG(dbgs() << DEBUG_PREFIX
+                    << "Reverting While Loop to Do Loop: " << *WLS << "\n");
+
+  RevertWhileLoopStartLR(WLS, TII, ARM::t2Bcc, true);
+
+  LivePhysRegs LiveRegs;
+  computeAndAddLiveIns(LiveRegs, *NewBlock);
+
+  Preheader->getParent()->RenumberBlocks();
+  BBUtils->computeAllBlockSizes();
+  BBUtils->adjustBBOffsetsAfter(Preheader);
+
+  return true;
+}
+
 /// Checks if loop has a backwards branching WLS, and if possible, fixes it.
 /// This requires checking the predecessor (ie. preheader or it's predecessor)
 /// for a WLS and if its loopExit/target is before it.
 /// If moving the predecessor won't convert a WLS (to the predecessor) from
 /// a forward to a backward branching WLS, then move the predecessor block
 /// to before the loopExit/target.
 bool ARMBlockPlacement::fixBackwardsWLS(MachineLoop *ML) {
   MachineInstr *WlsInstr = findWLS(ML);
for (auto &Terminator : MBB->terminators()) {
       // TODO: Analyse the blocks to make a decision if it would be worth
       // moving Preheader even if we'd introduce a backwards WLS
       if (WLSTarget == Predecessor) {
         LLVM_DEBUG(
             dbgs() << DEBUG_PREFIX
                    << "Can't move Predecessor"
                       "block as it would convert a WLS from forward to a "
                       "backwards branching WLS\n");
-        return false;
+        return revertWhileToDo(WlsInstr, ML);
       }
     }
   }
 
   moveBasicBlock(Predecessor, LoopExit);
   return true;
 }
 
void ARMBlockPlacement::moveBasicBlock(MachineBasicBlock *BB,
   if (BeforePrev->isSuccessor(Before))
     FixFallthrough(BeforePrev, Before);
   // Fix fall through from the moved BB to the one that used to follow.
   if (BBNext && BB->isSuccessor(BBNext))
     FixFallthrough(BB, BBNext);
 
   F->RenumberBlocks();
   BBUtils->computeAllBlockSizes();
-  BBUtils->adjustBBOffsetsAfter(&F->front());
+  BBUtils->adjustBBOffsetsAfter(BB);
 }

llvm/test/CodeGen/Thumb2/mve-memtp-branch.ll

 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
 ; RUN: llc -mtriple=thumbv8.1m.main-none-none-eabi -mattr=+mve.fp -verify-machineinstrs -arm-memtransfer-tploop=force-enabled %s -o - | FileCheck %s
 
 ; In this test, the successors of various blocks were becoming invalid after
 ; ifcvt as the blocks did not properly fall through to the successor after a
 ; WhileLoopStart
 
 @arr_183 = external dso_local local_unnamed_addr global [20 x [23 x [19 x i8]]], align 1
 define i32 @a(i8 zeroext %b, [3 x i8]* nocapture readonly %c, [3 x i32]* nocapture readonly %d) {
 ; CHECK-LABEL: a:
 ; CHECK:       @ %bb.0: @ %entry
 ; CHECK-NEXT:    .save {r4, r5, r7, lr}
 ; CHECK-NEXT:    push {r4, r5, r7, lr}
 ; CHECK-NEXT:    cmp r0, #1
-; CHECK-NEXT:    bls.w .LBB0_11
+; CHECK-NEXT:    bls.w .LBB0_12
 ; CHECK-NEXT:  @ %bb.1: @ %for.body.us.preheader
 ; CHECK-NEXT:    movw r5, :lower16:arr_183
 ; CHECK-NEXT:    movs r3, #0
 ; CHECK-NEXT:    movt r5, :upper16:arr_183
 ; CHECK-NEXT:    mov.w r12, #19
 ; CHECK-NEXT:    vmov.i32 q0, #0x0
 ; CHECK-NEXT:    vmov.i32 q1, #0x0
 ; CHECK-NEXT:    vmov.i32 q2, #0x0
 ; CHECK-NEXT:    vmov.i32 q3, #0x0
 ; CHECK-NEXT:    b .LBB0_3
 ; CHECK-NEXT:  .LBB0_2: @ %land.end.us.3
 ; CHECK-NEXT:    @ in Loop: Header=BB0_3 Depth=1
 ; CHECK-NEXT:    movs r3, #1
 ; CHECK-NEXT:  .LBB0_3: @ %for.body.us
 ; CHECK-NEXT:    @ =>This Loop Header: Depth=1
 ; CHECK-NEXT:    @ Child Loop BB0_4 Depth 2
 ; CHECK-NEXT:    @ Child Loop BB0_6 Depth 2
 ; CHECK-NEXT:    @ Child Loop BB0_8 Depth 2
-; CHECK-NEXT:    @ Child Loop BB0_10 Depth 2
+; CHECK-NEXT:    @ Child Loop BB0_11 Depth 2
 ; CHECK-NEXT:    ldr.w r0, [r2, r3, lsl #2]
 ; CHECK-NEXT:    cmp r0, #0
 ; CHECK-NEXT:    ite ne
 ; CHECK-NEXT:    ldrbne r0, [r1, r3]
 ; CHECK-NEXT:    moveq r0, #0
 ; CHECK-NEXT:    mla r3, r3, r12, r5
 ; CHECK-NEXT:    add r3, r0
 ; CHECK-NEXT:    rsb.w r0, r0, #108

 ; CHECK-NEXT:    ite ne
 ; CHECK-NEXT:    ldrbne r0, [r1, #1]
 ; CHECK-NEXT:    moveq r0, #0
 ; CHECK-NEXT:    adds r3, r5, r0
 ; CHECK-NEXT:    rsb.w r0, r0, #108
 ; CHECK-NEXT:    add.w r4, r0, #15
 ; CHECK-NEXT:    adds r3, #19
 ; CHECK-NEXT:    lsrs r4, r4, #4
-; CHECK-NEXT:    subs.w lr, r4, #0
+; CHECK-NEXT:    cmp.w r4, #0
 ; CHECK-NEXT:    beq .LBB0_2
-; CHECK-NEXT:    b .LBB0_10
-; CHECK-NEXT:  .LBB0_10: @ Parent Loop BB0_3 Depth=1
+; CHECK-NEXT:  @ %bb.10: @ %land.end.us.2
+; CHECK-NEXT:    @ in Loop: Header=BB0_3 Depth=1
+; CHECK-NEXT:    dlstp.8 lr, r0
+; CHECK-NEXT:  .LBB0_11: @ Parent Loop BB0_3 Depth=1
 ; CHECK-NEXT:    @ => This Inner Loop Header: Depth=2
-; CHECK-NEXT:    vctp.8 r0
-; CHECK-NEXT:    subs r0, #16
-; CHECK-NEXT:    vpst
-; CHECK-NEXT:    vstrbt.8 q3, [r3], #16
-; CHECK-NEXT:    subs.w lr, lr, #1
-; CHECK-NEXT:    bne .LBB0_10
+; CHECK-NEXT:    vstrb.8 q3, [r3], #16
+; CHECK-NEXT:    letp lr, .LBB0_11
 ; CHECK-NEXT:    b .LBB0_2
-; CHECK-NEXT:  .LBB0_11:
+; CHECK-NEXT:  .LBB0_12:
 ; CHECK-NEXT:    movw r12, :lower16:arr_183
 ; CHECK-NEXT:    vmov.i32 q0, #0x0
 ; CHECK-NEXT:    movt r12, :upper16:arr_183
 ; CHECK-NEXT:    vmov.i32 q1, #0x0
 ; CHECK-NEXT:    vmov.i32 q2, #0x0
 ; CHECK-NEXT:    vmov.i32 q3, #0x0
-; CHECK-NEXT:    b .LBB0_13
-; CHECK-NEXT:  .LBB0_12: @ %for.body.lr.ph.3
-; CHECK-NEXT:    @ in Loop: Header=BB0_13 Depth=1
+; CHECK-NEXT:    b .LBB0_14
+; CHECK-NEXT:  .LBB0_13: @ %for.body.lr.ph.3
+; CHECK-NEXT:    @ in Loop: Header=BB0_14 Depth=1
 ; CHECK-NEXT:    ldr r3, [r2, #4]
 ; CHECK-NEXT:    cmp r3, #0
 ; CHECK-NEXT:    ite ne
 ; CHECK-NEXT:    ldrbne r3, [r1, #1]
 ; CHECK-NEXT:    moveq r3, #0
 ; CHECK-NEXT:    add.w r5, r12, r3
 ; CHECK-NEXT:    rsb.w r3, r3, #108
 ; CHECK-NEXT:    add.w r4, r5, #19
-; CHECK-NEXT:    wlstp.8 lr, r3, .LBB0_13
-; CHECK-NEXT:    b .LBB0_23
-; CHECK-NEXT:  .LBB0_13: @ %for.cond
+; CHECK-NEXT:    wlstp.8 lr, r3, .LBB0_14
+; CHECK-NEXT:    b .LBB0_24
+; CHECK-NEXT:  .LBB0_14: @ %for.cond
 ; CHECK-NEXT:    @ =>This Loop Header: Depth=1
-; CHECK-NEXT:    @ Child Loop BB0_15 Depth 2
-; CHECK-NEXT:    @ Child Loop BB0_18 Depth 2
-; CHECK-NEXT:    @ Child Loop BB0_21 Depth 2
-; CHECK-NEXT:    @ Child Loop BB0_23 Depth 2
+; CHECK-NEXT:    @ Child Loop BB0_16 Depth 2
+; CHECK-NEXT:    @ Child Loop BB0_19 Depth 2
+; CHECK-NEXT:    @ Child Loop BB0_22 Depth 2
+; CHECK-NEXT:    @ Child Loop BB0_24 Depth 2
 ; CHECK-NEXT:    cmp r0, #2
-; CHECK-NEXT:    blo .LBB0_16
-; CHECK-NEXT:  @ %bb.14: @ %for.body.lr.ph
-; CHECK-NEXT:    @ in Loop: Header=BB0_13 Depth=1
+; CHECK-NEXT:    blo .LBB0_17
+; CHECK-NEXT:  @ %bb.15: @ %for.body.lr.ph
+; CHECK-NEXT:    @ in Loop: Header=BB0_14 Depth=1
 ; CHECK-NEXT:    ldr r3, [r2, #4]
 ; CHECK-NEXT:    cmp r3, #0
 ; CHECK-NEXT:    ite ne
 ; CHECK-NEXT:    ldrbne r3, [r1, #1]
 ; CHECK-NEXT:    moveq r3, #0
 ; CHECK-NEXT:    add.w r5, r12, r3
 ; CHECK-NEXT:    rsb.w r3, r3, #108
 ; CHECK-NEXT:    add.w r4, r5, #19
-; CHECK-NEXT:    wlstp.8 lr, r3, .LBB0_16
-; CHECK-NEXT:  .LBB0_15: @ Parent Loop BB0_13 Depth=1
+; CHECK-NEXT:    wlstp.8 lr, r3, .LBB0_17
+; CHECK-NEXT:  .LBB0_16: @ Parent Loop BB0_14 Depth=1
 ; CHECK-NEXT:    @ => This Inner Loop Header: Depth=2
 ; CHECK-NEXT:    vstrb.8 q0, [r4], #16
-; CHECK-NEXT:    letp lr, .LBB0_15
-; CHECK-NEXT:  .LBB0_16: @ %for.cond.backedge
-; CHECK-NEXT:    @ in Loop: Header=BB0_13 Depth=1
+; CHECK-NEXT:    letp lr, .LBB0_16
+; CHECK-NEXT:  .LBB0_17: @ %for.cond.backedge
+; CHECK-NEXT:    @ in Loop: Header=BB0_14 Depth=1
 ; CHECK-NEXT:    cmp r0, #2
-; CHECK-NEXT:    blo .LBB0_19
-; CHECK-NEXT:  @ %bb.17: @ %for.body.lr.ph.1
-; CHECK-NEXT:    @ in Loop: Header=BB0_13 Depth=1
+; CHECK-NEXT:    blo .LBB0_20
+; CHECK-NEXT:  @ %bb.18: @ %for.body.lr.ph.1
+; CHECK-NEXT:    @ in Loop: Header=BB0_14 Depth=1
 ; CHECK-NEXT:    ldr r3, [r2, #4]
 ; CHECK-NEXT:    cmp r3, #0
 ; CHECK-NEXT:    ite ne
 ; CHECK-NEXT:    ldrbne r3, [r1, #1]
 ; CHECK-NEXT:    moveq r3, #0
 ; CHECK-NEXT:    add.w r5, r12, r3
 ; CHECK-NEXT:    rsb.w r3, r3, #108
 ; CHECK-NEXT:    add.w r4, r5, #19
-; CHECK-NEXT:    wlstp.8 lr, r3, .LBB0_19
-; CHECK-NEXT:  .LBB0_18: @ Parent Loop BB0_13 Depth=1
+; CHECK-NEXT:    wlstp.8 lr, r3, .LBB0_20
+; CHECK-NEXT:  .LBB0_19: @ Parent Loop BB0_14 Depth=1
 ; CHECK-NEXT:    @ => This Inner Loop Header: Depth=2
 ; CHECK-NEXT:    vstrb.8 q1, [r4], #16
-; CHECK-NEXT:    letp lr, .LBB0_18
-; CHECK-NEXT:  .LBB0_19: @ %for.cond.backedge.1
-; CHECK-NEXT:    @ in Loop: Header=BB0_13 Depth=1
+; CHECK-NEXT:    letp lr, .LBB0_19
+; CHECK-NEXT:  .LBB0_20: @ %for.cond.backedge.1
+; CHECK-NEXT:    @ in Loop: Header=BB0_14 Depth=1
 ; CHECK-NEXT:    cmp r0, #2
-; CHECK-NEXT:    blo .LBB0_22
-; CHECK-NEXT:  @ %bb.20: @ %for.body.lr.ph.2
-; CHECK-NEXT:    @ in Loop: Header=BB0_13 Depth=1
+; CHECK-NEXT:    blo .LBB0_23
+; CHECK-NEXT:  @ %bb.21: @ %for.body.lr.ph.2
+; CHECK-NEXT:    @ in Loop: Header=BB0_14 Depth=1
 ; CHECK-NEXT:    ldr r3, [r2, #4]
 ; CHECK-NEXT:    cmp r3, #0
 ; CHECK-NEXT:    ite ne
 ; CHECK-NEXT:    ldrbne r3, [r1, #1]
 ; CHECK-NEXT:    moveq r3, #0
 ; CHECK-NEXT:    add.w r5, r12, r3
 ; CHECK-NEXT:    rsb.w r3, r3, #108
 ; CHECK-NEXT:    add.w r4, r5, #19
-; CHECK-NEXT:    wlstp.8 lr, r3, .LBB0_22
-; CHECK-NEXT:  .LBB0_21: @ Parent Loop BB0_13 Depth=1
+; CHECK-NEXT:    wlstp.8 lr, r3, .LBB0_23
+; CHECK-NEXT:  .LBB0_22: @ Parent Loop BB0_14 Depth=1
 ; CHECK-NEXT:    @ => This Inner Loop Header: Depth=2
 ; CHECK-NEXT:    vstrb.8 q2, [r4], #16
-; CHECK-NEXT:    letp lr, .LBB0_21
-; CHECK-NEXT:  .LBB0_22: @ %for.cond.backedge.2
-; CHECK-NEXT:    @ in Loop: Header=BB0_13 Depth=1
+; CHECK-NEXT:    letp lr, .LBB0_22
+; CHECK-NEXT:  .LBB0_23: @ %for.cond.backedge.2
+; CHECK-NEXT:    @ in Loop: Header=BB0_14 Depth=1
 ; CHECK-NEXT:    cmp r0, #2
-; CHECK-NEXT:    blo .LBB0_13
-; CHECK-NEXT:    b .LBB0_12
-; CHECK-NEXT:  .LBB0_23: @ Parent Loop BB0_13 Depth=1
+; CHECK-NEXT:    blo .LBB0_14
+; CHECK-NEXT:    b .LBB0_13
+; CHECK-NEXT:  .LBB0_24: @ Parent Loop BB0_14 Depth=1
 ; CHECK-NEXT:    @ => This Inner Loop Header: Depth=2
 ; CHECK-NEXT:    vstrb.8 q3, [r4], #16
-; CHECK-NEXT:    letp lr, .LBB0_23
-; CHECK-NEXT:    b .LBB0_13
+; CHECK-NEXT:    letp lr, .LBB0_24
+; CHECK-NEXT:    b .LBB0_14
 entry:
   %cmp = icmp ugt i8 %b, 1
   br i1 %cmp, label %for.body.us.preheader, label %for.cond.preheader
 
 for.cond.preheader:                               ; preds = %entry
   %cmp43 = icmp ugt i8 %b, 1
   %arrayidx6 = getelementptr inbounds [3 x i32], [3 x i32]* %d, i32 0, i32 1
   %arrayidx12 = getelementptr inbounds [3 x i8], [3 x i8]* %c, i32 0, i32 1

llvm/test/CodeGen/Thumb2/mve-memtp-loop.ll

 ; CHECK-NEXT:  .LBB18_1: @ %loop.preheader
 ; CHECK-NEXT:    mov.w r4, #-1
 ; CHECK-NEXT:    vmov.i32 q0, #0x0
 ; CHECK-NEXT:    b .LBB18_3
 ; CHECK-NEXT:  .LBB18_2: @ %loop
 ; CHECK-NEXT:    @ in Loop: Header=BB18_3 Depth=1
 ; CHECK-NEXT:    adds r4, #1
 ; CHECK-NEXT:    cmp.w r4, #1024
-; CHECK-NEXT:    bge .LBB18_11
+; CHECK-NEXT:    bge .LBB18_12
 ; CHECK-NEXT:  .LBB18_3: @ %loop
 ; CHECK-NEXT:    @ =>This Loop Header: Depth=1
 ; CHECK-NEXT:    @ Child Loop BB18_4 Depth 2
 ; CHECK-NEXT:    @ Child Loop BB18_6 Depth 2
 ; CHECK-NEXT:    @ Child Loop BB18_8 Depth 2
-; CHECK-NEXT:    @ Child Loop BB18_10 Depth 2
+; CHECK-NEXT:    @ Child Loop BB18_11 Depth 2
 ; CHECK-NEXT:    movw r3, :lower16:arr_56
 ; CHECK-NEXT:    add.w r1, r0, #15
 ; CHECK-NEXT:    movt r3, :upper16:arr_56
 ; CHECK-NEXT:    lsr.w r12, r1, #4
 ; CHECK-NEXT:    mov r2, r3
 ; CHECK-NEXT:    wlstp.8 lr, r0, .LBB18_5
 ; CHECK-NEXT:  .LBB18_4: @ Parent Loop BB18_3 Depth=1
 ; CHECK-NEXT:    @ => This Inner Loop Header: Depth=2

 ; CHECK-NEXT:    mov r2, r3
 ; CHECK-NEXT:    wlstp.8 lr, r0, .LBB18_9
 ; CHECK-NEXT:  .LBB18_8: @ Parent Loop BB18_3 Depth=1
 ; CHECK-NEXT:    @ => This Inner Loop Header: Depth=2
 ; CHECK-NEXT:    vstrb.8 q0, [r2], #16
 ; CHECK-NEXT:    letp lr, .LBB18_8
 ; CHECK-NEXT:  .LBB18_9: @ %loop
 ; CHECK-NEXT:    @ in Loop: Header=BB18_3 Depth=1
-; CHECK-NEXT:    mov r1, r0
-; CHECK-NEXT:    subs.w lr, r12, #0
+; CHECK-NEXT:    cmp.w r12, #0
 ; CHECK-NEXT:    beq .LBB18_2
-; CHECK-NEXT:    b .LBB18_10
-; CHECK-NEXT:  .LBB18_10: @ Parent Loop BB18_3 Depth=1
+; CHECK-NEXT:  @ %bb.10: @ %loop
+; CHECK-NEXT:    @ in Loop: Header=BB18_3 Depth=1
+; CHECK-NEXT:    dlstp.8 lr, r0
+; CHECK-NEXT:  .LBB18_11: @ Parent Loop BB18_3 Depth=1
 ; CHECK-NEXT:    @ => This Inner Loop Header: Depth=2
-; CHECK-NEXT:    vctp.8 r1
-; CHECK-NEXT:    subs r1, #16
-; CHECK-NEXT:    vpst
-; CHECK-NEXT:    vstrbt.8 q0, [r3], #16
-; CHECK-NEXT:    subs.w lr, lr, #1
-; CHECK-NEXT:    bne .LBB18_10
+; CHECK-NEXT:    vstrb.8 q0, [r3], #16
+; CHECK-NEXT:    letp lr, .LBB18_11
 ; CHECK-NEXT:    b .LBB18_2
-; CHECK-NEXT:  .LBB18_11: @ %exit
+; CHECK-NEXT:  .LBB18_12: @ %exit
 ; CHECK-NEXT:    pop {r4, pc}
 entry:
   %cmp8 = icmp sgt i32 %b, 0
   br i1 %cmp8, label %loop, label %exit
 
 loop:
   %p = phi i32 [ 0, %entry ], [ %inc, %loop ]
   call void @llvm.memset.p0i8.i32(i8* align 1 getelementptr ([21 x [16 x [11 x i8]]], [21 x [16 x [11 x i8]]]* @arr_56, i32 0, i32 0, i32 undef, i32 0), i8 0, i32 %b, i1 false)

llvm/test/CodeGen/Thumb2/mve-wls-block-placement.mir

body:             |
   ; CHECK:   successors: %bb.2(0x80000000)
   ; CHECK:   liveins: $r0, $r1, $r2, $lr
   ; CHECK:   tCMPi8 renamable $r0, 1, 14 /* CC::al */, $noreg, implicit-def $cpsr
   ; CHECK:   t2IT 11, 8, implicit-def $itstate
   ; CHECK:   frame-destroy tPOP_RET 11 /* CC::lt */, killed $cpsr, def $r7, def $pc, implicit killed $itstate
   ; CHECK: bb.1:
   ; CHECK:   frame-destroy tPOP_RET 14 /* CC::al */, $noreg, def $r7, def $pc
   ; CHECK: bb.2:
-  ; CHECK:   successors: %bb.3(0x40000000), %bb.5(0x40000000)
+  ; CHECK:   successors: %bb.3(0x40000000), %bb.6(0x40000000)
   ; CHECK:   liveins: $r0, $r1, $r2
   ; CHECK:   $lr = t2WhileLoopStartLR killed renamable $r0, %bb.3, implicit-def dead $cpsr
-  ; CHECK:   t2B %bb.5, 14 /* CC::al */, $noreg
+  ; CHECK:   t2B %bb.6, 14 /* CC::al */, $noreg
   ; CHECK: bb.3:
   ; CHECK:   successors: %bb.1(0x7c000000), %bb.4(0x04000000)
   ; CHECK:   liveins: $lr, $r0, $r1, $r2
-  ; CHECK:   $lr = t2WhileLoopStartLR killed renamable $r0, %bb.1, implicit-def dead $cpsr
-  ; CHECK:   t2B %bb.4, 14 /* CC::al */, $noreg
+  ; CHECK:   t2CMPri renamable $r0, 0, 14 /* CC::al */, $noreg, implicit-def $cpsr
+  ; CHECK:   t2Bcc %bb.1, 0 /* CC::eq */, $cpsr
   ; CHECK: bb.4:
-  ; CHECK:   successors: %bb.1(0x40000000), %bb.4(0x40000000)
-  ; CHECK:   liveins: $lr, $r1, $r2
-  ; CHECK:   renamable $lr = t2LoopEndDec killed renamable $lr, %bb.4, implicit-def dead $cpsr
-  ; CHECK:   t2B %bb.1, 14 /* CC::al */, $noreg
+  ; CHECK:   successors: %bb.5(0x80000000)
+  ; CHECK:   liveins: $r2, $r1, $r0
+  ; CHECK:   $lr = t2DoLoopStart renamable $r0
+  ; CHECK:   t2B %bb.5, 14 /* CC::al */, $noreg
   ; CHECK: bb.5:
-  ; CHECK:   successors: %bb.5(0x40000000), %bb.3(0x40000000)
+  ; CHECK:   successors: %bb.1(0x40000000), %bb.5(0x40000000)
   ; CHECK:   liveins: $lr, $r1, $r2
   ; CHECK:   renamable $lr = t2LoopEndDec killed renamable $lr, %bb.5, implicit-def dead $cpsr
+  ; CHECK:   t2B %bb.1, 14 /* CC::al */, $noreg
+  ; CHECK: bb.6:
+  ; CHECK:   successors: %bb.6(0x40000000), %bb.3(0x40000000)
+  ; CHECK:   liveins: $lr, $r1, $r2
+  ; CHECK:   renamable $lr = t2LoopEndDec killed renamable $lr, %bb.6, implicit-def dead $cpsr
   ; CHECK:   t2B %bb.3, 14 /* CC::al */, $noreg
   bb.0:
     successors: %bb.2(0x80000000)
     liveins: $r0, $r1, $r2, $lr
 
     tCMPi8 renamable $r0, 1, 14 /* CC::al */, $noreg, implicit-def $cpsr
     t2IT 11, 8, implicit-def $itstate
     frame-destroy tPOP_RET 11 /* CC::lt */, killed $cpsr, def $r7, def $pc, implicit killed $itstate