AMDGPU/SIInsertSkips: Fix the determination of whether early-exit-after-kill is possible

[nhaehnle]

Summary

The old code made some incorrect assumptions about the order in which
basic blocks are laid out in a function. This could lead to incorrect
early-exits, especially when kills occurred inside of loops.

The new approach is to check whether the point where the conditional
kill occurs dominates all reachable code. If that is the case, there
cannot be any other threads in the wave that are waiting to rejoin
at a later point in the CFG, i.e. if exec=0 at that point, then all
threads really are dead and we can exit the wave.

Make some other minor cleanups to the pass while we're at it.

Change-Id: Ia0d2b113ac944ad642d1c622b6da1b20aa1aabcc

[arsenm]

llvm/lib/Target/AMDGPU/SIInsertSkips.cpp

85

The update to also preserve it shouldn't be too difficult

[nhaehnle]

• preserve the dominator tree

[foad]

Could you pre-commit the complex_loop test case, if that's the one that really demonstrates the bug fix?

[nhaehnle]

The new testcase is now in master; rebase this change on top of it.

[arsenm]

lgtm except the dead code

llvm/lib/Target/AMDGPU/SIInsertSkips.cpp

344

Dead code

[nhaehnle]

Thanks.

llvm/lib/Target/AMDGPU/SIInsertSkips.cpp

344

Removed.

llvm/lib/Target/AMDGPU/SIInsertSkips.cpp

 //===-- SIInsertSkips.cpp - Use predicates for control flow ---------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 /// \file
 /// This pass inserts branches on the 0 exec mask over divergent branches
 /// branches when it's expected that jumping over the untaken control flow will
 /// be cheaper than having every workitem no-op through it.
 //
 //===----------------------------------------------------------------------===//
 
 #include "AMDGPU.h"
 #include "AMDGPUSubtarget.h"
 #include "SIInstrInfo.h"
 #include "SIMachineFunctionInfo.h"
 #include "MCTargetDesc/AMDGPUMCTargetDesc.h"
+#include "llvm/ADT/DepthFirstIterator.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/CodeGen/MachineBasicBlock.h"
+#include "llvm/CodeGen/MachineDominators.h"
 #include "llvm/CodeGen/MachineFunction.h"
 #include "llvm/CodeGen/MachineFunctionPass.h"
 #include "llvm/CodeGen/MachineInstr.h"
 #include "llvm/CodeGen/MachineInstrBuilder.h"
 #include "llvm/CodeGen/MachineOperand.h"
 #include "llvm/IR/CallingConv.h"
 #include "llvm/IR/DebugLoc.h"
+#include "llvm/InitializePasses.h"
 #include "llvm/MC/MCAsmInfo.h"
 #include "llvm/Pass.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Target/TargetMachine.h"
 #include <cassert>
 #include <cstdint>
 #include <iterator>
 
 using namespace llvm;
 
 #define DEBUG_TYPE "si-insert-skips"
 
 static cl::opt<unsigned> SkipThresholdFlag(
   "amdgpu-skip-threshold-legacy",
   cl::desc("Number of instructions before jumping over divergent control flow"),
   cl::init(12), cl::Hidden);
 
 namespace {
 
 class SIInsertSkips : public MachineFunctionPass {
 private:
   const SIRegisterInfo *TRI = nullptr;
   const SIInstrInfo *TII = nullptr;
   unsigned SkipThreshold = 0;
+  MachineDominatorTree *MDT = nullptr;
 
   bool shouldSkip(const MachineBasicBlock &From,
                   const MachineBasicBlock &To) const;
 
-  bool skipIfDead(MachineInstr &MI, MachineBasicBlock &NextBB);
+  bool dominatesAllReachable(MachineBasicBlock &MBB);
+  void skipIfDead(MachineBasicBlock &MBB, MachineBasicBlock::iterator I,
+                  DebugLoc DL);
 
-  void kill(MachineInstr &MI);
-
-  MachineBasicBlock *insertSkipBlock(MachineBasicBlock &MBB,
-                                     MachineBasicBlock::iterator I) const;
+  bool kill(MachineInstr &MI);
 
   bool skipMaskBranch(MachineInstr &MI, MachineBasicBlock &MBB);
 
   bool optimizeVccBranch(MachineInstr &MI) const;
 
 public:
   static char ID;
 
   SIInsertSkips() : MachineFunctionPass(ID) {}
 
   bool runOnMachineFunction(MachineFunction &MF) override;
 
   StringRef getPassName() const override {
     return "SI insert s_cbranch_execz instructions";
   }
 
   void getAnalysisUsage(AnalysisUsage &AU) const override {
+    AU.addRequired<MachineDominatorTree>();
+    AU.addPreserved<MachineDominatorTree>();
     MachineFunctionPass::getAnalysisUsage(AU);
   }
 };
 
 } // end anonymous namespace
 
 char SIInsertSkips::ID = 0;
 
-INITIALIZE_PASS(SIInsertSkips, DEBUG_TYPE,
+INITIALIZE_PASS_BEGIN(SIInsertSkips, DEBUG_TYPE,
+                      "SI insert s_cbranch_execz instructions", false, false)
+INITIALIZE_PASS_DEPENDENCY(MachineDominatorTree)
+INITIALIZE_PASS_END(SIInsertSkips, DEBUG_TYPE,
                     "SI insert s_cbranch_execz instructions", false, false)
 
 char &llvm::SIInsertSkipsPassID = SIInsertSkips::ID;
 
 static bool opcodeEmitsNoInsts(const MachineInstr &MI) {
   if (MI.isMetaInstruction())
     return true;
 
   // Handle target specific opcodes.
for (MachineBasicBlock::const_iterator I = MBB.begin(), E = MBB.end();
       if (NumInstr >= SkipThreshold)
         return true;
     }
   }
 
   return false;
 }
 
-bool SIInsertSkips::skipIfDead(MachineInstr &MI, MachineBasicBlock &NextBB) {
-  MachineBasicBlock &MBB = *MI.getParent();
-  MachineFunction *MF = MBB.getParent();
-
-  if (MF->getFunction().getCallingConv() != CallingConv::AMDGPU_PS ||
-      !shouldSkip(MBB, MBB.getParent()->back()))
+/// Check whether \p MBB dominates all blocks that are reachable from it.
+bool SIInsertSkips::dominatesAllReachable(MachineBasicBlock &MBB) {
+  for (MachineBasicBlock *Other : depth_first(&MBB)) {
+    if (!MDT->dominates(&MBB, Other))
       return false;
+  }
+  return true;
+}
 
-  MachineBasicBlock *SkipBB = insertSkipBlock(MBB, MI.getIterator());
+/// Insert an "if exec=0 { null export; s_endpgm }" sequence before the given
+/// iterator. Only applies to pixel shaders.
+void SIInsertSkips::skipIfDead(MachineBasicBlock &MBB,
+                               MachineBasicBlock::iterator I, DebugLoc DL) {
+  MachineFunction *MF = MBB.getParent();
+  assert(MF->getFunction().getCallingConv() == CallingConv::AMDGPU_PS);
 
-  const DebugLoc &DL = MI.getDebugLoc();
+  // Currently, SI_KILL_*_TERMINATOR is expected to occur only as the last
+  // terminator of a basic block. If this ever changes, we need to optionally
+  // split MBB here.
+  assert(I == MBB.end());
+
+  // It is possible for an SI_KILL_*_TERMINATOR to sit at the bottom of a
+  // basic block that has no further successors (e.g., there was an
+  // `unreachable` there in IR). This can happen with original source of the
+  // form:
+  //
+  //   if (uniform_condition) {
+  //     write_to_memory();
+  //     discard;
+  //   }
+  //
+  // In this case, we write the "null_export; s_endpgm" skip code in the
+  // already-existing basic block.
+  auto NextBBI = std::next(MBB.getIterator());
+  bool NoSuccessor = llvm::find(MBB.successors(), &*NextBBI) == MBB.succ_end();
+  MachineBasicBlock *SkipBB;
 
-  // If the exec mask is non-zero, skip the next two instructions
-  BuildMI(&MBB, DL, TII->get(AMDGPU::S_CBRANCH_EXECNZ))
-    .addMBB(&NextBB);
+  if (NoSuccessor) {
+    SkipBB = &MBB;
+  } else {
+    // Create a new basic block that will contain the "null export; s_endpgm"
+    // and set up the branching to go around it.
+    SkipBB = MF->CreateMachineBasicBlock();
+    MF->insert(NextBBI, SkipBB);
+
+    BuildMI(&MBB, DL, TII->get(AMDGPU::S_CBRANCH_EXECNZ)).addMBB(&*NextBBI);
+    MBB.addSuccessor(SkipBB);
 
-  MachineBasicBlock::iterator Insert = SkipBB->begin();
+    MDT->addNewBlock(SkipBB, &MBB);
+  }
 
-  // Exec mask is zero: Export to NULL target...
-  BuildMI(*SkipBB, Insert, DL, TII->get(AMDGPU::EXP_DONE))
+  // Generate "null export; s_endpgm".
+  BuildMI(SkipBB, DL, TII->get(AMDGPU::EXP_DONE))
       .addImm(0x09) // V_008DFC_SQ_EXP_NULL
       .addReg(AMDGPU::VGPR0, RegState::Undef)
       .addReg(AMDGPU::VGPR0, RegState::Undef)
       .addReg(AMDGPU::VGPR0, RegState::Undef)
       .addReg(AMDGPU::VGPR0, RegState::Undef)
       .addImm(1)  // vm
       .addImm(0)  // compr
       .addImm(0); // en
-
-  // ... and terminate wavefront.
-  BuildMI(*SkipBB, Insert, DL, TII->get(AMDGPU::S_ENDPGM)).addImm(0);
-
-  return true;
+  BuildMI(SkipBB, DL, TII->get(AMDGPU::S_ENDPGM)).addImm(0);
 }
 
-void SIInsertSkips::kill(MachineInstr &MI) {
+/// Translate a SI_KILL_*_TERMINATOR into exec-manipulating instructions.
+/// Return true unless the terminator is a no-op.
+bool SIInsertSkips::kill(MachineInstr &MI) {
   MachineBasicBlock &MBB = *MI.getParent();
   DebugLoc DL = MI.getDebugLoc();
 
   switch (MI.getOpcode()) {
   case AMDGPU::SI_KILL_F32_COND_IMM_TERMINATOR: {
     unsigned Opcode = 0;
 
     // The opcodes are inverted because the inline immediate has to be
if (TRI->isVGPR(MBB.getParent()->getRegInfo(),
 
       I.addImm(0)  // src0 modifiers
         .add(MI.getOperand(1))
         .addImm(0)  // src1 modifiers
         .add(MI.getOperand(0));
 
       I.addImm(0);  // omod
     }
-    break;
+    return true;
   }
   case AMDGPU::SI_KILL_I1_TERMINATOR: {
     const MachineFunction *MF = MI.getParent()->getParent();
     const GCNSubtarget &ST = MF->getSubtarget<GCNSubtarget>();
     unsigned Exec = ST.isWave32() ? AMDGPU::EXEC_LO : AMDGPU::EXEC;
     const MachineOperand &Op = MI.getOperand(0);
     int64_t KillVal = MI.getOperand(1).getImm();
     assert(KillVal == 0 || KillVal == -1);
 
     // Kill all threads if Op0 is an immediate and equal to the Kill value.
     if (Op.isImm()) {
       int64_t Imm = Op.getImm();
       assert(Imm == 0 || Imm == -1);
 
-      if (Imm == KillVal)
+      if (Imm == KillVal) {
         BuildMI(MBB, &MI, DL, TII->get(ST.isWave32() ? AMDGPU::S_MOV_B32
                                                      : AMDGPU::S_MOV_B64), Exec)
           .addImm(0);
-      break;
+        return true;
+      }
+      return false;
     }
 
     unsigned Opcode = KillVal ? AMDGPU::S_ANDN2_B64 : AMDGPU::S_AND_B64;
     if (ST.isWave32())
       Opcode = KillVal ? AMDGPU::S_ANDN2_B32 : AMDGPU::S_AND_B32;
     BuildMI(MBB, &MI, DL, TII->get(Opcode), Exec)
         .addReg(Exec)
         .add(Op);
-    break;
+    return true;
   }
   default:
     llvm_unreachable("invalid opcode, expected SI_KILL_*_TERMINATOR");
+    return false;
   }
 }
 
-MachineBasicBlock *SIInsertSkips::insertSkipBlock(
-  MachineBasicBlock &MBB, MachineBasicBlock::iterator I) const {
-  MachineFunction *MF = MBB.getParent();
-
-  MachineBasicBlock *SkipBB = MF->CreateMachineBasicBlock();
-  MachineFunction::iterator MBBI(MBB);
-  ++MBBI;
-
-  MF->insert(MBBI, SkipBB);
-  MBB.addSuccessor(SkipBB);
-
-  return SkipBB;
-}
-
 // Returns true if a branch over the block was inserted.
 bool SIInsertSkips::skipMaskBranch(MachineInstr &MI,
                                    MachineBasicBlock &SrcMBB) {
   MachineBasicBlock *DestBB = MI.getOperand(0).getMBB();
 
   if (!shouldSkip(**SrcMBB.succ_begin(), *DestBB))
     return false;
 
bool SIInsertSkips::optimizeVccBranch(MachineInstr &MI) const {
 
   return true;
 }
 
 bool SIInsertSkips::runOnMachineFunction(MachineFunction &MF) {
   const GCNSubtarget &ST = MF.getSubtarget<GCNSubtarget>();
   TII = ST.getInstrInfo();
   TRI = &TII->getRegisterInfo();
+  MDT = &getAnalysis<MachineDominatorTree>();
   SkipThreshold = SkipThresholdFlag;
 
-  bool HaveKill = false;
-  bool MadeChange = false;
-
-  // Track depth of exec mask, divergent branches.
-  SmallVector<MachineBasicBlock *, 16> ExecBranchStack;
-
-  MachineFunction::iterator NextBB;
-
   MachineBasicBlock *EmptyMBBAtEnd = nullptr;
+  SmallVector<MachineInstr *, 4> KillInstrs;
+  bool MadeChange = false;
 
-  for (MachineFunction::iterator BI = MF.begin(), BE = MF.end();
-       BI != BE; BI = NextBB) {
-    NextBB = std::next(BI);
-    MachineBasicBlock &MBB = *BI;
-    bool HaveSkipBlock = false;
-
-    if (!ExecBranchStack.empty() && ExecBranchStack.back() == &MBB) {
-      // Reached convergence point for last divergent branch.
-      ExecBranchStack.pop_back();
-    }
-
-    if (HaveKill && ExecBranchStack.empty()) {
-      HaveKill = false;
-
-      // TODO: Insert skip if exec is 0?
-    }
-
+  for (MachineBasicBlock &MBB : MF) {
     MachineBasicBlock::iterator I, Next;
     for (I = MBB.begin(); I != MBB.end(); I = Next) {
       Next = std::next(I);
-
       MachineInstr &MI = *I;
 
       switch (MI.getOpcode()) {
-      case AMDGPU::S_CBRANCH_EXECZ:
-        ExecBranchStack.push_back(MI.getOperand(0).getMBB());
-        break;
       case AMDGPU::SI_MASK_BRANCH:
-        ExecBranchStack.push_back(MI.getOperand(0).getMBB());
         MadeChange |= skipMaskBranch(MI, MBB);
         break;
 
       case AMDGPU::S_BRANCH:
         // Optimize out branches to the next block.
         // FIXME: Shouldn't this be handled by BranchFolding?
         if (MBB.isLayoutSuccessor(MI.getOperand(0).getMBB())) {
+          assert(&MI == &MBB.back());
           MI.eraseFromParent();
-        } else if (HaveSkipBlock) {
-          // Remove the given unconditional branch when a skip block has been
-          // inserted after the current one and let skip the two instructions
-          // performing the kill if the exec mask is non-zero.
-          MI.eraseFromParent();
+          MadeChange = true;
         }
         break;
 
       case AMDGPU::SI_KILL_F32_COND_IMM_TERMINATOR:
-      case AMDGPU::SI_KILL_I1_TERMINATOR:
+      case AMDGPU::SI_KILL_I1_TERMINATOR: {
         MadeChange = true;
-        kill(MI);
+        bool CanKill = kill(MI);
 
-        if (ExecBranchStack.empty()) {
-          if (NextBB != BE && skipIfDead(MI, *NextBB)) {
-            HaveSkipBlock = true;
-            NextBB = std::next(BI);
-            BE = MF.end();
-          }
+        // Check if we can add an early "if exec=0 { end shader }".
+        //
+        // Note that we _always_ do this if it is correct, even if the kill
+        // happens fairly late in the shader, because the null export should
+        // generally still be cheaper than normal export(s).
+        //
+        // TODO: The dominatesAllReachable check is conservative: if the
+        //       dominance is only missing due to _uniform_ branches, we could
+        //       in fact insert the early-exit as well.
+        if (CanKill &&
+            MF.getFunction().getCallingConv() == CallingConv::AMDGPU_PS &&
+            dominatesAllReachable(MBB)) {
+          // Mark the instruction for kill-if-dead insertion. We delay this
+          // change because it modifies the CFG.
+          KillInstrs.push_back(&MI);
         } else {
-          HaveKill = true;
-        }
-
           MI.eraseFromParent();
+        }
         break;
+      }
 
       case AMDGPU::SI_RETURN_TO_EPILOG:
         // FIXME: Should move somewhere else
         assert(!MF.getInfo<SIMachineFunctionInfo>()->returnsVoid());
 
         // Graphics shaders returning non-void shouldn't contain S_ENDPGM,
         // because external bytecode will be appended at the end.
-        if (BI != --MF.end() || I != MBB.getFirstTerminator()) {
+        if (&MBB != &MF.back() || &MI != &MBB.back()) {
           // SI_RETURN_TO_EPILOG is not the last instruction. Add an empty block at
           // the end and jump there.
           if (!EmptyMBBAtEnd) {
             EmptyMBBAtEnd = MF.CreateMachineBasicBlock();
             MF.insert(MF.end(), EmptyMBBAtEnd);
           }
 
           MBB.addSuccessor(EmptyMBBAtEnd);
-          BuildMI(*BI, I, MI.getDebugLoc(), TII->get(AMDGPU::S_BRANCH))
+          BuildMI(MBB, &MI, MI.getDebugLoc(), TII->get(AMDGPU::S_BRANCH))
             .addMBB(EmptyMBBAtEnd);
-          I->eraseFromParent();
+          MI.eraseFromParent();
         }
         break;
 
       case AMDGPU::S_CBRANCH_VCCZ:
       case AMDGPU::S_CBRANCH_VCCNZ:
         MadeChange |= optimizeVccBranch(MI);
         break;
 
       default:
         break;
       }
     }
   }
 
+  for (MachineInstr *Kill : KillInstrs) {
+    skipIfDead(*Kill->getParent(), std::next(Kill->getIterator()),
+               Kill->getDebugLoc());
+    Kill->eraseFromParent();
+  }
+  KillInstrs.clear();
+
   return MadeChange;
 }

llvm/test/CodeGen/AMDGPU/skip-if-dead.ll

 ; RUN: llc -march=amdgcn -verify-machineinstrs < %s | FileCheck %s
 
 ; CHECK-LABEL: {{^}}test_kill_depth_0_imm_pos:
 ; CHECK-NEXT: ; %bb.0:
 ; CHECK-NEXT: ; %bb.1:
 ; CHECK-NEXT: s_endpgm
 define amdgpu_ps void @test_kill_depth_0_imm_pos() #0 {
   call void @llvm.amdgcn.kill(i1 true)
   ret void
 }
 
 ; CHECK-LABEL: {{^}}test_kill_depth_0_imm_neg:
 ; CHECK-NEXT: ; %bb.0:
 ; CHECK-NEXT: s_mov_b64 exec, 0
+; CHECK-NEXT: s_cbranch_execnz BB1_2
 ; CHECK-NEXT: ; %bb.1:
+; CHECK-NEXT: exp null off, off, off, off done vm
+; CHECK-NEXT: s_endpgm
+; CHECK-NEXT: BB1_2:
 ; CHECK-NEXT: s_endpgm
 define amdgpu_ps void @test_kill_depth_0_imm_neg() #0 {
   call void @llvm.amdgcn.kill(i1 false)
   ret void
 }
 
 ; FIXME: Ideally only one would be emitted
 ; CHECK-LABEL: {{^}}test_kill_depth_0_imm_neg_x2:
 ; CHECK-NEXT: ; %bb.0:
 ; CHECK-NEXT: s_mov_b64 exec, 0
-; CHECK-NEXT: ; %bb.1:
+; CHECK-NEXT: s_cbranch_execnz BB2_2
+; CHECK:      exp null
+; CHECK-NEXT: s_endpgm
+; CHECK:      BB2_2:
 ; CHECK-NEXT: s_mov_b64 exec, 0
-; CHECK-NEXT: ; %bb.2:
+; CHECK-NEXT: s_cbranch_execnz BB2_4
+; CHECK:      exp null
+; CHECK-NEXT: s_endpgm
+; CHECK-NEXT: BB2_4:
 ; CHECK-NEXT: s_endpgm
 define amdgpu_ps void @test_kill_depth_0_imm_neg_x2() #0 {
   call void @llvm.amdgcn.kill(i1 false)
   call void @llvm.amdgcn.kill(i1 false)
   ret void
 }
 
 ; CHECK-LABEL: {{^}}test_kill_depth_var:
 ; CHECK-NEXT: ; %bb.0:
 ; CHECK-NEXT: v_cmpx_gt_f32_e32 vcc, 0, v0
-; CHECK-NEXT: ; %bb.1:
+; CHECK-NEXT: s_cbranch_execnz BB3_2
+; CHECK:      exp null
+; CHECK-NEXT: s_endpgm
+; CHECK-NEXT: BB3_2:
 ; CHECK-NEXT: s_endpgm
 define amdgpu_ps void @test_kill_depth_var(float %x) #0 {
   %cmp = fcmp olt float %x, 0.0
   call void @llvm.amdgcn.kill(i1 %cmp)
   ret void
 }
 
 ; FIXME: Ideally only one would be emitted
 ; CHECK-LABEL: {{^}}test_kill_depth_var_x2_same:
 ; CHECK-NEXT: ; %bb.0:
 ; CHECK-NEXT: v_cmpx_gt_f32_e32 vcc, 0, v0
-; CHECK-NEXT: ; %bb.1:
+; CHECK-NEXT: s_cbranch_execnz BB4_2
+; CHECK:      exp null
+; CHECK-NEXT: s_endpgm
+; CHECK-NEXT: BB4_2:
 ; CHECK-NEXT: v_cmpx_gt_f32_e32 vcc, 0, v0
-; CHECK-NEXT: ; %bb.2:
+; CHECK-NEXT: s_cbranch_execnz BB4_4
+; CHECK:      exp null
+; CHECK-NEXT: s_endpgm
+; CHECK-NEXT: BB4_4:
 ; CHECK-NEXT: s_endpgm
 define amdgpu_ps void @test_kill_depth_var_x2_same(float %x) #0 {
   %cmp = fcmp olt float %x, 0.0
   call void @llvm.amdgcn.kill(i1 %cmp)
   call void @llvm.amdgcn.kill(i1 %cmp)
   ret void
 }
 
+; FIXME: Ideally only one early-exit would be emitted
 ; CHECK-LABEL: {{^}}test_kill_depth_var_x2:
 ; CHECK-NEXT: ; %bb.0:
 ; CHECK-NEXT: v_cmpx_gt_f32_e32 vcc, 0, v0
-; CHECK-NEXT: ; %bb.1:
+; CHECK-NEXT: s_cbranch_execnz BB5_2
+; CHECK:      exp null
+; CHECK-NEXT: s_endpgm
+; CHECK-NEXT: BB5_2:
 ; CHECK-NEXT: v_cmpx_gt_f32_e32 vcc, 0, v1
-; CHECK-NEXT: ; %bb.2:
+; CHECK-NEXT: s_cbranch_execnz BB5_4
+; CHECK:      exp null
+; CHECK-NEXT: s_endpgm
+; CHECK-NEXT: BB5_4:
 ; CHECK-NEXT: s_endpgm
 define amdgpu_ps void @test_kill_depth_var_x2(float %x, float %y) #0 {
   %cmp.x = fcmp olt float %x, 0.0
   call void @llvm.amdgcn.kill(i1 %cmp.x)
   %cmp.y = fcmp olt float %y, 0.0
   call void @llvm.amdgcn.kill(i1 %cmp.y)
   ret void
 }

 ; CHECK: v_nop_e64
 ; CHECK: v_nop_e64
 ; CHECK: v_nop_e64
 ; CHECK: v_nop_e64
 ; CHECK: v_nop_e64
 ; CHECK: v_nop_e64
 
 ; CHECK: v_cmpx_gt_f32_e32 vcc, 0, v7
-; CHECK-NEXT: s_cbranch_execnz [[SPLIT_BB:BB[0-9]+_[0-9]+]]
-; CHECK-NEXT: ; %bb.2:
-; CHECK-NEXT: exp null off, off, off, off done vm
-; CHECK-NEXT: s_endpgm
 
-; CHECK-NEXT: {{^}}[[SPLIT_BB]]:
-; CHECK-NEXT: s_endpgm
-define amdgpu_ps void @test_kill_control_flow(i32 inreg %arg) #0 {
+; TODO: We could do an early-exit here (the branch above is uniform!)
+; CHECK-NOT: exp null
+
+; CHECK: v_mov_b32_e32 v0, 1.0
+define amdgpu_ps float @test_kill_control_flow(i32 inreg %arg) #0 {
 entry:
   %cmp = icmp eq i32 %arg, 0
   br i1 %cmp, label %bb, label %exit
 
 bb:
   %var = call float asm sideeffect "
     v_mov_b32_e64 v7, -1
     v_nop_e64
     v_nop_e64
     v_nop_e64
     v_nop_e64
     v_nop_e64
     v_nop_e64
     v_nop_e64
     v_nop_e64
     v_nop_e64
     v_nop_e64", "={v7}"()
   %cmp.var = fcmp olt float %var, 0.0
   call void @llvm.amdgcn.kill(i1 %cmp.var)
   br label %exit
 
 exit:
-  ret void
+  ret float 1.0
 }
 
 ; CHECK-LABEL: {{^}}test_kill_control_flow_remainder:
 ; CHECK: s_cmp_lg_u32 s{{[0-9]+}}, 0
 ; CHECK-NEXT: v_mov_b32_e32 v{{[0-9]+}}, 0
 ; CHECK-NEXT: s_cbranch_scc1 [[RETURN_BB:BB[0-9]+_[0-9]+]]
 
 ; CHECK-NEXT: ; %bb.1: ; %bb
 ; CHECK: v_mov_b32_e64 v7, -1
 ; CHECK: v_nop_e64
 ; CHECK: v_nop_e64
 ; CHECK: v_nop_e64
 ; CHECK: v_nop_e64
 ; CHECK: v_nop_e64
 ; CHECK: v_nop_e64
 ; CHECK: v_nop_e64
 ; CHECK: v_nop_e64
 ; CHECK: ;;#ASMEND
 ; CHECK: v_mov_b32_e64 v8, -1
 ; CHECK: ;;#ASMEND
 ; CHECK: v_cmpx_gt_f32_e32 vcc, 0, v7
-; CHECK-NEXT: s_cbranch_execnz [[SPLIT_BB:BB[0-9]+_[0-9]+]]
 
-; CHECK-NEXT: ; %bb.2:
-; CHECK-NEXT: exp null off, off, off, off done vm
-; CHECK-NEXT: s_endpgm
+; TODO: We could do an early-exit here (the branch above is uniform!)
+; CHECK-NOT: exp null
 
-; CHECK-NEXT: {{^}}[[SPLIT_BB]]:
 ; CHECK: buffer_store_dword v8
 ; CHECK: v_mov_b32_e64 v9, -2
 
 ; CHECK: {{^}}BB{{[0-9]+_[0-9]+}}:
 ; CHECK: buffer_store_dword v9
 ; CHECK-NEXT: s_endpgm
 define amdgpu_ps void @test_kill_control_flow_remainder(i32 inreg %arg) #0 {
 entry:
export:
   %out.0 = call <2 x half> @llvm.amdgcn.cvt.pkrtz(float %proxy.0.0, float %proxy.0.1) #2
   %out.1 = call <2 x half> @llvm.amdgcn.cvt.pkrtz(float %proxy.0.2, float %proxy.0.3) #2
   call void @llvm.amdgcn.exp.compr.v2f16(i32 immarg 0, i32 immarg 15, <2 x half> %out.0, <2 x half> %out.1, i1 immarg true, i1 immarg true) #3
   ret void
 }
 
 ; CHECK-LABEL: {{^}}complex_loop:
 ; CHECK: s_mov_b64 exec, 0
-; The following is an error, since it happens nested inside the loop:
-; CHECK-NEXT: s_cbranch_execnz
-; CHECK-NEXT: ; %bb.{{[0-9]+}}
-; CHECK-NEXT: exp null
+; CHECK-NOT: exp null
 define amdgpu_ps void @complex_loop(i32 inreg %cmpa, i32 %cmpb, i32 %cmpc) {
 .entry:
   %flaga = icmp sgt i32 %cmpa, 0
   br i1 %flaga, label %.lr.ph, label %._crit_edge
 
 .lr.ph:
   br label %hdr