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-rw-r--r--llvm/lib/IR/BasicBlock.cpp8
-rw-r--r--llvm/lib/Transforms/IPO/PassManagerBuilder.cpp1
-rw-r--r--llvm/lib/Transforms/Scalar/CMakeLists.txt1
-rw-r--r--llvm/lib/Transforms/Scalar/Scalar.cpp1
-rw-r--r--llvm/lib/Transforms/Scalar/SpeculativeExecution.cpp232
5 files changed, 243 insertions, 0 deletions
diff --git a/llvm/lib/IR/BasicBlock.cpp b/llvm/lib/IR/BasicBlock.cpp
index 23ec7052954..e6a4585a936 100644
--- a/llvm/lib/IR/BasicBlock.cpp
+++ b/llvm/lib/IR/BasicBlock.cpp
@@ -238,6 +238,14 @@ BasicBlock *BasicBlock::getUniquePredecessor() {
return PredBB;
}
+BasicBlock *BasicBlock::getSingleSuccessor() {
+ succ_iterator SI = succ_begin(this), E = succ_end(this);
+ if (SI == E) return nullptr; // no successors
+ BasicBlock *TheSucc = *SI;
+ ++SI;
+ return (SI == E) ? TheSucc : nullptr /* multiple successors */;
+}
+
BasicBlock *BasicBlock::getUniqueSuccessor() {
succ_iterator SI = succ_begin(this), E = succ_end(this);
if (SI == E) return NULL; // No successors
diff --git a/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp b/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
index 3b21d3f0cb1..0936022af83 100644
--- a/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
+++ b/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
@@ -231,6 +231,7 @@ void PassManagerBuilder::populateModulePassManager(
MPM.add(createSROAPass(/*RequiresDomTree*/ false));
else
MPM.add(createScalarReplAggregatesPass(-1, false));
+
MPM.add(createEarlyCSEPass()); // Catch trivial redundancies
MPM.add(createJumpThreadingPass()); // Thread jumps.
MPM.add(createCorrelatedValuePropagationPass()); // Propagate conditionals
diff --git a/llvm/lib/Transforms/Scalar/CMakeLists.txt b/llvm/lib/Transforms/Scalar/CMakeLists.txt
index 7ecd4c036b8..7ee279a5660 100644
--- a/llvm/lib/Transforms/Scalar/CMakeLists.txt
+++ b/llvm/lib/Transforms/Scalar/CMakeLists.txt
@@ -45,6 +45,7 @@ add_llvm_library(LLVMScalarOpts
SeparateConstOffsetFromGEP.cpp
SimplifyCFGPass.cpp
Sink.cpp
+ SpeculativeExecution.cpp
StraightLineStrengthReduce.cpp
StructurizeCFG.cpp
TailRecursionElimination.cpp
diff --git a/llvm/lib/Transforms/Scalar/Scalar.cpp b/llvm/lib/Transforms/Scalar/Scalar.cpp
index 483a2997eb8..d5d360571f8 100644
--- a/llvm/lib/Transforms/Scalar/Scalar.cpp
+++ b/llvm/lib/Transforms/Scalar/Scalar.cpp
@@ -74,6 +74,7 @@ void llvm::initializeScalarOpts(PassRegistry &Registry) {
initializeSinkingPass(Registry);
initializeTailCallElimPass(Registry);
initializeSeparateConstOffsetFromGEPPass(Registry);
+ initializeSpeculativeExecutionPass(Registry);
initializeStraightLineStrengthReducePass(Registry);
initializeLoadCombinePass(Registry);
initializePlaceBackedgeSafepointsImplPass(Registry);
diff --git a/llvm/lib/Transforms/Scalar/SpeculativeExecution.cpp b/llvm/lib/Transforms/Scalar/SpeculativeExecution.cpp
new file mode 100644
index 00000000000..0e51019ccce
--- /dev/null
+++ b/llvm/lib/Transforms/Scalar/SpeculativeExecution.cpp
@@ -0,0 +1,232 @@
+//===- 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 hoists instructions to enable speculative execution on
+// targets where branches are expensive. This is aimed at GPUs. It
+// currently works on simple if-then and if-then-else
+// patterns.
+//
+// Removing branches is not the only motivation for this
+// pass. 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/TargetTransformInfo.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"
+
+// The risk that speculation will not pay off increases with the
+// number of instructions speculated, so we put a limit on that.
+static cl::opt<unsigned> SpecExecMaxSpeculationCost(
+ "spec-exec-max-speculation-cost", cl::init(7), cl::Hidden,
+ cl::desc("Speculative execution is not applied to basic blocks where "
+ "the cost of the instructions to speculatively execute "
+ "exceeds this limit."));
+
+// Speculating just a few instructions from a larger block tends not
+// to be profitable and this limit prevents that. A reason for that is
+// that small basic blocks are more likely to be candidates for
+// further optimization.
+static cl::opt<unsigned> SpecExecMaxNotHoisted(
+ "spec-exec-max-not-hoisted", cl::init(5), cl::Hidden,
+ cl::desc("Speculative execution is not applied to basic blocks where the "
+ "number of instructions that would not be speculatively executed "
+ "exceeds this limit."));
+
+class SpeculativeExecution : public FunctionPass {
+ public:
+ static char ID;
+ SpeculativeExecution(): FunctionPass(ID) {}
+
+ void getAnalysisUsage(AnalysisUsage &AU) const override;
+ bool runOnFunction(Function &F) override;
+
+ private:
+ bool runOnBasicBlock(BasicBlock &B);
+ bool considerHoistingFromTo(BasicBlock &FromBlock, BasicBlock &ToBlock);
+
+ const TargetTransformInfo *TTI = nullptr;
+};
+
+char SpeculativeExecution::ID = 0;
+INITIALIZE_PASS_BEGIN(SpeculativeExecution, "speculative-execution",
+ "Speculatively execute instructions", false, false)
+INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
+INITIALIZE_PASS_END(SpeculativeExecution, "speculative-execution",
+ "Speculatively execute instructions", false, false)
+
+void SpeculativeExecution::getAnalysisUsage(AnalysisUsage &AU) const {
+ AU.addRequired<TargetTransformInfoWrapperPass>();
+}
+
+bool SpeculativeExecution::runOnFunction(Function &F) {
+ if (skipOptnoneFunction(F))
+ return false;
+
+ TTI = &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(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.getSingleSuccessor() == &Succ1) {
+ return considerHoistingFromTo(Succ0, B);
+ }
+
+ // Hoist from if-else (triangle).
+ if (Succ1.getSinglePredecessor() != nullptr &&
+ Succ1.getSingleSuccessor() == &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 &&
+ Succ1.getSingleSuccessor() != nullptr &&
+ Succ1.getSingleSuccessor() != &B &&
+ Succ1.getSingleSuccessor() == Succ0.getSingleSuccessor()) {
+ // 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 Instruction *I,
+ const TargetTransformInfo &TTI) {
+ switch (Operator::getOpcode(I)) {
+ case Instruction::GetElementPtr:
+ 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 TTI.getUserCost(I);
+
+ default:
+ return UINT_MAX; // Disallow anything not whitelisted.
+ }
+}
+
+bool SpeculativeExecution::considerHoistingFromTo(BasicBlock &FromBlock,
+ BasicBlock &ToBlock) {
+ SmallSet<const Instruction *, 8> NotHoisted;
+ const auto AllPrecedingUsesFromBlockHoisted = [&NotHoisted](User *U) {
+ for (Value* V : U->operand_values()) {
+ if (Instruction *I = dyn_cast<Instruction>(V)) {
+ if (NotHoisted.count(I) > 0)
+ return false;
+ }
+ }
+ return true;
+ };
+
+ unsigned TotalSpeculationCost = 0;
+ for (auto& I : FromBlock) {
+ const unsigned Cost = ComputeSpeculationCost(&I, *TTI);
+ if (Cost != UINT_MAX && isSafeToSpeculativelyExecute(&I) &&
+ AllPrecedingUsesFromBlockHoisted(&I)) {
+ TotalSpeculationCost += Cost;
+ if (TotalSpeculationCost > SpecExecMaxSpeculationCost)
+ return false; // too much to hoist
+ } else {
+ NotHoisted.insert(&I);
+ if (NotHoisted.size() > SpecExecMaxNotHoisted)
+ return false; // too much left behind
+ }
+ }
+
+ if (TotalSpeculationCost == 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
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