Add a speculative execution pass

[broune]

Summary

This is a pass for speculative execution of instructions for simple if-then (triangle) control flow. It's aimed at GPUs, but could perhaps be used in other contexts. Enabling this pass gives us a 1.0% geomean improvement on Google benchmark suites, with one benchmark improving 33%.

Credit goes to Jingyue Wu for writing an earlier version of this pass.

Patched by Bjarke Roune.

[majnemer]

It seems this pass has a good deal in common with SimplifyCFG. How is this
pass different from bumping up the cost threshold in that pass?

[broune]

In D9360#163642, @majnemer wrote:

It seems this pass has a good deal in common with SimplifyCFG. How is this
pass different from bumping up the cost threshold in that pass?

I assume you are referring to the functionality in the function SpeculativelyExecuteBB in SimplifyCFG. As I read what it does, SimplifyCFG will not speculate if no selects are introduced and it will speculate at most one instruction. It also will not speculate if there is a value defined in the if-block that is only used in the then-block. These restrictions make sense since the speculation in SimplifyCFG seems aimed at introducing cheap selects, while this pass is intended to do more aggressive speculation while counting on later passes to either capitalize on that or clean it up.

What this pass does could be merged into SimplifyCFG, though at first look it doesn't appear to me to be an easy fit. We'd also need two different configurations of SimplifyCFG since aggressive speculation at every point where SimplifyCFG runs would probably be too much; I've experimented with adding this pass in multiple places simultaneously and that hasn't yielded good results so far.

[jingyue]

lib/Transforms/Scalar/SpeculativeExecution.cpp

60

Comment on why this threshold is necessary. My understanding is the NVIDIA driver only speculates a limited number of instructions. If too many instructions are left behind, the conditional basic block won't be executed in the SASS level.

69

Add a constructor that takes spec-exec-max-speculation-cost and spec-exec-max-not-hoisted as parameters. This allows embedded uses of LLVM (e.g., users can programmatically create these passes with their desired thresholds).

113

Can we make a new interface getSingleSuccessor()?

141

Should we consider ConstantExpr free?

180

TotalSpeculationCost

181

for (auto &I : FromBlock)

[jingyue]

lib/Transforms/Scalar/SpeculativeExecution.cpp

60

I meant "won't be speculatively executed".

[meheff]

lib/Transforms/Scalar/SpeculativeExecution.cpp

64

The double negative at the end of this description confused me as written. Maybe: "Speculative execution is not be applied to basic blocks where the number of ... exceeds this limit."

Could change the other option description to match.

146

Stale comment?

159

Instruction::And? Can you use TTI instruction costs here?

[hfinkel]

include/llvm/Transforms/Scalar.h

426

This pass does not speculatively execute anything. Please reword.

lib/Transforms/IPO/PassManagerBuilder.cpp

236

This is pretty early in the pipeline, why? Do you want LICM to run first?

lib/Transforms/Scalar/SpeculativeExecution.cpp

147

And should this depend on the number of non-constant indices?

[broune]

Added and used BasicBlock::getSingleSuccessor().
Improved flag descriptions.

[broune]

Use getSingleSuccessor() more.

include/llvm/Transforms/Scalar.h

426

I'm very happy to correct any mistaken terminology. I'm unclear on whether you object to "speculation" or "execution" or both and what you would prefer instead? I note that SimplifyCFG has a function SpeculativelyExecuteBB that also moves instructions out of branches and I did see some sources with a similar terminology, e.g.

"Software Speculative Execution [...]
When the compiler generates speculative code, it moves instructions in front of a branch that previously had protected them from causing exceptions."
http://techpubs.sgi.com/library/dynaweb_docs/0650/SGI_Developer/books/OrOn2_PfTune/sgi_html/ch05.html

lib/Transforms/IPO/PassManagerBuilder.cpp

236

I did start with a late placement, right before the nvptx backend runs. I also tried right before LICM and a bit after LICM and other places. The current placement is what worked best out of those things that I tried.

My intuition of why this early placement works well is that we speculate, optimize and then soon after those instructions that were speculated but did not enable further optimization get put back by InstCombine.

lib/Transforms/Scalar/SpeculativeExecution.cpp

60

Done. The base reason for this limit is that speculating just a few instructions from a larger block tends not to be profitable. That could in part be due to an interaction with the NVIDIA driver, even though this pass is run quite early in the compiler, putting it later makes our benchmarks worse, and speculated instructions that are not further optimized tend to be sunk back by InstCombine to where they were before speculation. In the best case that I've seen for this pass, speculation allowed further follow-on optimizations, collapsing a lot of control flow and bit-fiddling instructions into fewer instructions; that sort of thing is less likely to happen when speculating just a few instructions from a larger block, so that's another reason for the limit. The comment I added fits both reasons.

64

Done.

69

This doesn't seem to be common for the other passes that I looked at in Scalar/ that have flags and it seems like it would be hard to maintain when options are added or removed, so I'd prefer to postpone adding that until someone uses it. I could add it now if you'll be using it right away?

113

Nice idea. Done.

141

Do you mean should we return 0 if all operands to an instruction are ConstantExpr? I'd think most such cases would be folded already, though maybe not for GEP, since I believe that it's required for changing types, so that does seem reasonable to catch.

146

Done.

147

I'm especially keen to speculate GEPs, as that was part of the original motivation for the pass, but you're right that this might not be good. I'll try having the cost be the number of non-constant indices + 1 and get back to you.

159

Since the speculated instructions can be sunk back or optimized later, the optimal score isn't necessarily the time it would take to execute the instructions, it's more propensity to cause good things and not cause bad things when speculated. Though TTI could still be a better approximation to that than what I've got here, so I'll try it both ways and get back to you.

180

Done.

181

Done.

[jingyue]

lib/Transforms/Scalar/SpeculativeExecution.cpp

69

Fine with me. I've seen CFGSimplifyPass, JumpThreading and LoopUnrolling have pass parameters.

159

If TTI's cost model works for you, I'd prefer using that. No need to invent another cost model if an existing one already works.

[broune]

Use TTI for speculation cost of whitelisted instructions.

[broune]

I promised to get back to you on a few things. I tried these versions overnight:

1. no changes,
2. as 1, but with the cost of a GEP as the number of non-constant indices + 1,
3. as 1, but with the cost function entirely replaced with TTI->getUserCost,
4. as 3, but still returning UINT_MAX when 1 does.

1, 2 and 4 were about the same while 3 was worse than the others. So I propose to go with 4 and I have updated the patch to that.

[jingyue]

LGTM with a minor

lib/Transforms/Scalar/SpeculativeExecution.cpp

141

I was thinking of the case where I is a ConstantExpr because I is a User which can be Instruction or ConstantExpr. However, I later noticed you only call ComputeSpeculationCost with instructions. So we should make this function to more restrictively take only Instruction.

[meheff]

LGTM

[broune]

Change first parameter type of ComputeSpeculationCost to Instruction *.

lib/Transforms/Scalar/SpeculativeExecution.cpp

141

Done.

[hfinkel]

include/llvm/Transforms/Scalar.h

426

Please reword like this:

// SpeculativeExecution - Aggressively hoist instructions to enable speculatively execution on targets where branches are expensive.

(the import part here is that the verb is 'hoist', not 'execute').

lib/Transforms/IPO/PassManagerBuilder.cpp

236

How exactly do you intend this to work? There are two or three options:

1. Don't put it in the standard pipeline (assuming that GPU-compilers have their own pipelines anyway and will use it in those custom pipelines only).
2. Add some TTI hook to control whether or not the pass does anything.
3. Add a variable to the pass manager controlling this (so that the frontend must decide).

I prefer (2), but just leaving it here, as a something intended to be dead (except for use of some command-line flag) is not a reasonable plan. The command-line flags are for debugging and testing, not for production pass-manager control.

lib/Transforms/Scalar/SpeculativeExecution.cpp

11

Same problem here: the pass does not execute anything, it hoists instructions to cause speculative execution. Please reword.

[broune]

Remove speculation flag and call from PassManagerBuilder.
Update comments on speculation.

[hfinkel]

LGTM.

[broune]

Fix typo.

[broune]

include/llvm/Transforms/Scalar.h

426

Done, thank you for the clarification.

lib/Transforms/IPO/PassManagerBuilder.cpp

236

I removed the flag and the code it controls, going for option 1 for now. Let me know if anyone interested in this pass needs another option.

include/llvm/InitializePasses.h

 void initializeSimpleInlinerPass(PassRegistry&);
 void initializeShadowStackGCLoweringPass(PassRegistry&);  
 void initializeRegisterCoalescerPass(PassRegistry&);
 void initializeSingleLoopExtractorPass(PassRegistry&);
 void initializeSinkingPass(PassRegistry&);
 void initializeSeparateConstOffsetFromGEPPass(PassRegistry &);
 void initializeSlotIndexesPass(PassRegistry&);
 void initializeSpillPlacementPass(PassRegistry&);
+void initializeSpeculativeExecutionPass(PassRegistry&);
 void initializeStackProtectorPass(PassRegistry&);
 void initializeStackColoringPass(PassRegistry&);
 void initializeStackSlotColoringPass(PassRegistry&);
 void initializeStraightLineStrengthReducePass(PassRegistry &);
 void initializeStripDeadDebugInfoPass(PassRegistry&);
 void initializeStripDeadPrototypesPassPass(PassRegistry&);
 void initializeStripDebugDeclarePass(PassRegistry&);
 void initializeStripNonDebugSymbolsPass(PassRegistry&);

include/llvm/LinkAllPasses.h

ForcePassLinking() {
       (void) llvm::createMemDepPrinter();
       (void) llvm::createInstructionSimplifierPass();
       (void) llvm::createLoopVectorizePass();
       (void) llvm::createSLPVectorizerPass();
       (void) llvm::createBBVectorizePass();
       (void) llvm::createPartiallyInlineLibCallsPass();
       (void) llvm::createScalarizerPass();
       (void) llvm::createSeparateConstOffsetFromGEPPass();
+      (void) llvm::createSpeculativeExecutionPass();
       (void) llvm::createRewriteSymbolsPass();
       (void) llvm::createStraightLineStrengthReducePass();
       (void) llvm::createMemDerefPrinter();
       (void) llvm::createFloat2IntPass();
 
       (void)new llvm::IntervalPartition();
       (void)new llvm::ScalarEvolution();
       ((llvm::Function*)nullptr)->viewCFGOnly();

include/llvm/Transforms/Scalar.h

 // SeparateConstOffsetFromGEP - Split GEPs for better CSE
 //
 FunctionPass *
 createSeparateConstOffsetFromGEPPass(const TargetMachine *TM = nullptr,
                                      bool LowerGEP = false);
 
 //===----------------------------------------------------------------------===//
 //
+// SpeculativeExecution - Speculatively execute instructions.
+//
+FunctionPass *createSpeculativeExecutionPass();
+
+//===----------------------------------------------------------------------===//
+//
 // LoadCombine - Combine loads into bigger loads.
 //
 BasicBlockPass *createLoadCombinePass();
 
 //===----------------------------------------------------------------------===//
 //
 // StraightLineStrengthReduce - This pass strength-reduces some certain
 // instruction patterns in straight-line code.

lib/Transforms/IPO/PassManagerBuilder.cpp

 static cl::opt<bool> ExtraVectorizerPasses(
     "extra-vectorizer-passes", cl::init(false), cl::Hidden,
     cl::desc("Run cleanup optimization passes after vectorization."));
 
 static cl::opt<bool> UseNewSROA("use-new-sroa",
   cl::init(true), cl::Hidden,
   cl::desc("Enable the new, experimental SROA pass"));
 
+static cl::opt<bool> RunSpeculation(
+    "run-speculative-execution", cl::Hidden, cl::init(false),
+    cl::desc("Enable the experimental speculative execution pass."));
+
 static cl::opt<bool>
 RunLoopRerolling("reroll-loops", cl::Hidden,
                  cl::desc("Run the loop rerolling pass"));
 
 static cl::opt<bool>
 RunFloat2Int("float-to-int", cl::Hidden, cl::init(true),
              cl::desc("Run the float2int (float demotion) pass"));
 
if (OptLevel > 2)
     MPM.add(createArgumentPromotionPass());   // Scalarize uninlined fn args
 
   // Start of function pass.
   // Break up aggregate allocas, using SSAUpdater.
   if (UseNewSROA)
     MPM.add(createSROAPass(/*RequiresDomTree*/ false));
   else
     MPM.add(createScalarReplAggregatesPass(-1, false));
+
+  if (RunSpeculation)
+    MPM.add(createSpeculativeExecutionPass());
+
   MPM.add(createEarlyCSEPass());              // Catch trivial redundancies
   MPM.add(createJumpThreadingPass());         // Thread jumps.
   MPM.add(createCorrelatedValuePropagationPass()); // Propagate conditionals
   MPM.add(createCFGSimplificationPass());     // Merge & remove BBs
   MPM.add(createInstructionCombiningPass());  // Combine silly seq's
   addExtensionsToPM(EP_Peephole, MPM);
 
   if (!DisableTailCalls)

lib/Transforms/Scalar/CMakeLists.txt

add_llvm_library(LLVMScalarOpts
   SROA.cpp
   SampleProfile.cpp
   Scalar.cpp
   ScalarReplAggregates.cpp
   Scalarizer.cpp
   SeparateConstOffsetFromGEP.cpp
   SimplifyCFGPass.cpp
   Sink.cpp
+  SpeculativeExecution.cpp
   StraightLineStrengthReduce.cpp
   StructurizeCFG.cpp
   TailRecursionElimination.cpp
 
   ADDITIONAL_HEADER_DIRS
   ${LLVM_MAIN_INCLUDE_DIR}/llvm/Transforms
   ${LLVM_MAIN_INCLUDE_DIR}/llvm/Transforms/Scalar
   )
 
 add_dependencies(LLVMScalarOpts intrinsics_gen)

lib/Transforms/Scalar/Scalar.cpp

void llvm::initializeScalarOpts(PassRegistry &Registry) {
   initializeSROAPass(Registry);
   initializeSROA_DTPass(Registry);
   initializeSROA_SSAUpPass(Registry);
   initializeCFGSimplifyPassPass(Registry);
   initializeStructurizeCFGPass(Registry);
   initializeSinkingPass(Registry);
   initializeTailCallElimPass(Registry);
   initializeSeparateConstOffsetFromGEPPass(Registry);
+  initializeSpeculativeExecutionPass(Registry);
   initializeStraightLineStrengthReducePass(Registry);
   initializeLoadCombinePass(Registry);
   initializePlaceBackedgeSafepointsImplPass(Registry);
   initializePlaceSafepointsPass(Registry);
   initializeFloat2IntPass(Registry);
 }
 
 void LLVMInitializeScalarOpts(LLVMPassRegistryRef R) {

lib/Transforms/Scalar/SpeculativeExecution.cpp

+//===- SpeculativeExecution.cpp ---------------------------------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This pass does speculative execution aimed at GPU targets where branches
+// are expensive. It currently works on simple if-then and if-then-else
+// patterns. E.g. consider this code and assume that there is no addressing
+// mode for multiplying by sizeof(*a):
+//
+//   if (b > 0)
+//     c = a[i + 1]
+//   if (d > 0)
+//     e = a[i + 2]
+//
+// turns into
+//
+//   p = &a[i + 1];
+//   if (b > 0)
+//     c = *p;
+//   q = &a[i + 2];
+//   if (d > 0)
+//     e = *q;
+//
+// which could later be optimized to
+//
+//   r = &a[i];
+//   if (b > 0)
+//     c = r[1];
+//   if (d > 0)
+//     e = r[2];
+//
+// Later passes sink back much of the speculated code that did not enable
+// further optimization.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/ADT/SmallSet.h"
+#include "llvm/Analysis/ValueTracking.h"
+#include "llvm/IR/Instructions.h"
+#include "llvm/IR/Module.h"
+#include "llvm/IR/Operator.h"
+#include "llvm/Support/CommandLine.h"
+#include "llvm/Support/Debug.h"
+
+using namespace llvm;
+
+#define DEBUG_TYPE "speculative-execution"
+
+static cl::opt<unsigned> SpecExecMaxSpeculationCost(
+    "spec-exec-max-speculation-cost", cl::init(7), cl::Hidden,
+    cl::desc("Speculative execution will only be applied to basic blocks where "
+             "the cost of the instructions to speculatively execute do not "
+             "exceed this limit."));
+
+static cl::opt<unsigned> SpecExecMaxNotHoisted(
+    "spec-exec-max-not-hoisted", cl::init(5), cl::Hidden,
+    cl::desc("Speculative execution will only be applied to basic blocks where "
+             "the number of instructions that cannot be speculatively executed "
+             "do not exceed this limit."));
+
+class SpeculativeExecution : public FunctionPass {
+ public:
+  static char ID;
+  SpeculativeExecution(): FunctionPass(ID) {}
+
+  bool runOnFunction(Function &F) override;
+
+ private:
+  bool runOnBasicBlock(BasicBlock &B);
+  bool considerHoistingFromTo(BasicBlock &FromBlock, BasicBlock &ToBlock);
+};
+
+char SpeculativeExecution::ID = 0;
+INITIALIZE_PASS(SpeculativeExecution, "speculative-execution",
+                "Speculatively execute instructions", false, false)
+
+bool SpeculativeExecution::runOnFunction(Function &F) {
+  bool Changed = false;
+  for (auto& B : F) {
+    Changed |= runOnBasicBlock(B);
+  }
+  return Changed;
+}
+
+bool SpeculativeExecution::runOnBasicBlock(BasicBlock &B) {
+  BranchInst *BI = dyn_cast<BranchInst>(B.getTerminator());
+  if (BI == nullptr)
+    return false;
+
+  if (BI->getNumSuccessors() != 2)
+    return false;
+  BasicBlock &Succ0 = *BI->getSuccessor(0);
+  BasicBlock &Succ1 = *BI->getSuccessor(1);
+
+  if (&B == &Succ0 || &B == &Succ1 || &Succ0 == &Succ1) {
+    return false;
+  }
+
+  // Hoist from if-then (triangle).
+  if (Succ0.getSinglePredecessor() != nullptr &&
+      Succ0.getTerminator()->getNumSuccessors() == 1 &&
+      Succ0.getTerminator()->getSuccessor(0) == &Succ1) {
+    return considerHoistingFromTo(Succ0, B);
+  }
+
+  // Hoist from if-else (triangle).
+  if (Succ1.getSinglePredecessor() != nullptr &&
+      Succ1.getTerminator()->getNumSuccessors() == 1 &&
+      Succ1.getTerminator()->getSuccessor(0) == &Succ0) {
+    return considerHoistingFromTo(Succ1, B);
+  }
+
+  // Hoist from if-then-else (diamond), but only if it is equivalent to
+  // an if-else or if-then due to one of the branches doing nothing.
+  if (Succ0.getSinglePredecessor() != nullptr &&
+      Succ1.getSinglePredecessor() != nullptr &&
+      Succ0.getTerminator()->getNumSuccessors() == 1 &&
+      Succ1.getTerminator()->getNumSuccessors() == 1 &&
+      Succ0.getTerminator()->getSuccessor(0) ==
+        Succ1.getTerminator()->getSuccessor(0) &&
+      Succ1.getTerminator()->getSuccessor(0) != &B) {
+    // If a block has only one instruction, then that is a terminator
+    // instruction so that the block does nothing. This does happen.
+    if (Succ1.size() == 1) // equivalent to if-then
+      return considerHoistingFromTo(Succ0, B);
+    if (Succ0.size() == 1) // equivalent to if-else
+      return considerHoistingFromTo(Succ1, B);
+  }
+
+  return false;
+}
+
+static unsigned ComputeSpeculationCost(const User *I) {
+  // This selection of instructions and their cost is based on informal
+  // benchmarking. There is likely room for improvement.
+  switch (Operator::getOpcode(I)) {
+    case Instruction::GetElementPtr:
+      // GEPs are cheap if all indices are constant.
+      if (cast<GEPOperator>(I)->hasAllConstantIndices())
+        return 1;
+      return 2; // Changed by Jingyue.
+
+    case Instruction::Add:
+    case Instruction::Mul:
+    case Instruction::And:
+    case Instruction::Or:
+    case Instruction::Select:
+    case Instruction::Shl:
+    case Instruction::Sub:
+    case Instruction::LShr:
+    case Instruction::AShr:
+    case Instruction::Xor:
+    case Instruction::ZExt:
+    case Instruction::SExt:
+      return 1; // These are all cheap.
+
+    default:
+      return UINT_MAX; // Disallow anything not whitelisted.
+  }
+}
+
+bool SpeculativeExecution::considerHoistingFromTo(BasicBlock &FromBlock,
+                                                  BasicBlock &ToBlock) {
+  SmallSet<Instruction *, 8> NotHoisted;
+  const auto AllPrecedingUsesFromBlockHoisted = [&NotHoisted](User *U) {
+    for (auto V : U->operand_values()) {
+      if (Instruction *I = dyn_cast<Instruction>(V)) {
+        if (NotHoisted.count(I) > 0)
+          return false;
+      }
+    }
+    return true;
+  };
+
+  unsigned TotalCost = 0;
+  for (auto I = FromBlock.begin(), End = FromBlock.end(); I != End; ++I) {
+    const unsigned Cost = ComputeSpeculationCost(I);
+    if (Cost != UINT_MAX && isSafeToSpeculativelyExecute(I) &&
+        AllPrecedingUsesFromBlockHoisted(I)) {
+      TotalCost += Cost;
+      if (TotalCost > SpecExecMaxSpeculationCost)
+        return false;  // too much to hoist
+    } else {
+      NotHoisted.insert(I);
+      if (NotHoisted.size() > SpecExecMaxNotHoisted)
+        return false; // too much left behind
+    }
+  }
+
+  if (TotalCost == 0)
+    return false; // nothing to hoist
+
+  for (auto I = FromBlock.begin(); I != FromBlock.end();) {
+    // We have to increment I before moving Current as moving Current
+    // changes the list that I is iterating through.
+    auto Current = I;
+    ++I;
+    if (!NotHoisted.count(Current)) {
+      Current->moveBefore(ToBlock.getTerminator());
+    }
+  }
+  return true;
+}
+
+namespace llvm {
+
+FunctionPass *createSpeculativeExecutionPass() {
+  return new SpeculativeExecution();
+}
+
+}  // namespace llvm

test/Transforms/SpeculativeExecution/spec.ll

+; RUN: opt < %s -S -speculative-execution \
+; RUN:   -spec-exec-max-speculation-cost 4 -spec-exec-max-not-hoisted 3 \
+; RUN:   | FileCheck %s
+
+target datalayout = "e-i64:64-v16:16-v32:32-n16:32:64"
+
+; Hoist in if-then pattern.
+define void @ifThen() {
+; CHECK-LABEL: @ifThen(
+; CHECK: %x = add i32 2, 3
+; CHECK: br i1 true
+  br i1 true, label %a, label %b
+; CHECK: a:
+a:
+  %x = add i32 2, 3
+; CHECK: br label
+  br label %b
+; CHECK: b:
+b:
+; CHECK: ret void
+  ret void
+}
+
+; Hoist in if-else pattern.
+define void @ifElse() {
+; CHECK-LABEL: @ifElse(
+; CHECK: %x = add i32 2, 3
+; CHECK: br i1 true
+  br i1 true, label %b, label %a
+; CHECK: a:
+a:
+  %x = add i32 2, 3
+; CHECK: br label
+  br label %b
+; CHECK: b:
+b:
+; CHECK: ret void
+  ret void
+}
+
+; Hoist in if-then-else pattern if it is equivalent to if-then.
+define void @ifElseThenAsIfThen() {
+; CHECK-LABEL: @ifElseThenAsIfThen(
+; CHECK: %x = add i32 2, 3
+; CHECK: br
+  br i1 true, label %a, label %b
+; CHECK: a:
+a:
+  %x = add i32 2, 3
+; CHECK: br label
+  br label %c
+; CHECK: b:
+b:
+  br label %c
+; CHECK: c
+c:
+  ret void
+}
+
+; Hoist in if-then-else pattern if it is equivalent to if-else.
+define void @ifElseThenAsIfElse() {
+; CHECK-LABEL: @ifElseThenAsIfElse(
+; CHECK: %x = add i32 2, 3
+; CHECK: br
+  br i1 true, label %b, label %a
+; CHECK: a:
+a:
+  %x = add i32 2, 3
+; CHECK: br label
+  br label %c
+; CHECK: b:
+b:
+  br label %c
+; CHECK: c
+c:
+  ret void
+}
+
+; Do not hoist if-then-else pattern if it is not equivalent to if-then
+; or if-else.
+define void @ifElseThen() {
+; CHECK-LABEL: @ifElseThen(
+; CHECK: br
+  br i1 true, label %a, label %b
+; CHECK: a:
+a:
+; CHECK: %x = add
+  %x = add i32 2, 3
+; CHECK: br label
+  br label %c
+; CHECK: b:
+b:
+; CHECK: %y = add
+  %y = add i32 2, 3
+  br label %c
+; CHECK: c
+c:
+  ret void
+}
+
+; Do not hoist loads and do not hoist an instruction past a definition of
+; an operand.
+define void @doNotHoistPastDef() {
+; CHECK-LABEL: @doNotHoistPastDef(
+  br i1 true, label %b, label %a
+; CHECK-NOT: load
+; CHECK-NOT: add
+; CHECK: a:
+a:
+; CHECK: %def = load
+  %def = load i32, i32* null
+; CHECK: %use = add
+  %use = add i32 %def, 0
+  br label %b
+; CHECK: b:
+b:
+  ret void
+}
+
+; Case with nothing to speculate.
+define void @nothingToSpeculate() {
+; CHECK-LABEL: @nothingToSpeculate(
+  br i1 true, label %b, label %a
+; CHECK: a:
+a:
+; CHECK: %def = load
+  %def = load i32, i32* null
+  br label %b
+; CHECK: b:
+b:
+  ret void
+}
+
+; Still hoist if an operand is defined before the block or is itself hoisted.
+define void @hoistIfNotPastDef() {
+; CHECK-LABEL: @hoistIfNotPastDef(
+; CHECK: %x = load
+  %x = load i32, i32* null
+; CHECK: %y = add i32 %x, 1
+; CHECK: %z = add i32 %y, 1
+; CHECK: br
+  br i1 true, label %b, label %a
+; CHECK: a:
+a:
+  %y = add i32 %x, 1
+  %z = add i32 %y, 1
+  br label %b
+; CHECK: b:
+b:
+  ret void
+}
+
+; Do not hoist if the speculation cost is too high.
+define void @costTooHigh() {
+; CHECK-LABEL: @costTooHigh(
+; CHECK: br
+  br i1 true, label %b, label %a
+; CHECK: a:
+a:
+; CHECK: %r1 = add
+  %r1 = add i32 1, 1
+; CHECK: %r2 = add
+  %r2 = add i32 1, 1
+; CHECK: %r3 = add
+  %r3 = add i32 1, 1
+; CHECK: %r4 = add
+  %r4 = add i32 1, 1
+; CHECK: %r5 = add
+  %r5 = add i32 1, 1
+  br label %b
+; CHECK: b:
+b:
+  ret void
+}
+
+; Do not hoist if too many instructions are left behind.
+define void @tooMuchLeftBehind() {
+; CHECK-LABEL: @tooMuchLeftBehind(
+; CHECK: br
+  br i1 true, label %b, label %a
+; CHECK: a:
+a:
+; CHECK: %x = load
+  %x = load i32, i32* null
+; CHECK: %r1 = add
+  %r1 = add i32 %x, 1
+; CHECK: %r2 = add
+  %r2 = add i32 %x, 1
+; CHECK: %r3 = add
+  %r3 = add i32 %x, 1
+  br label %b
+; CHECK: b:
+b:
+  ret void
+}