[BPI] Use a safer constructor to calculate branch probabilities

BPI may trigger signed overflow UB while computing branch probabilities
for cold calls or to unreachables. For example, with our current choice
of weights, we'll crash if there are >= 2^12 branches to an unreachable.

Use a safer BranchProbability constructor which is better at handling
fractions with large denominators.

rdar://problem/29368161

Differential Revision: https://reviews.llvm.org/D27862

llvm-svn: 290016

1 files changed, 88 insertions, 0 deletions

diff --git a/llvm/unittests/Analysis/BranchProbabilityInfoTest.cpp b/llvm/unittests/Analysis/BranchProbabilityInfoTest.cpp
new file mode 100644
index 00000000000..cbf8b50c762
--- /dev/null
+++ b/llvm/unittests/Analysis/BranchProbabilityInfoTest.cpp
@@ -0,0 +1,88 @@
+//===- BranchProbabilityInfoTest.cpp - BranchProbabilityInfo unit tests ---===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Analysis/BranchProbabilityInfo.h"
+#include "llvm/Analysis/LoopInfo.h"
+#include "llvm/AsmParser/Parser.h"
+#include "llvm/IR/BasicBlock.h"
+#include "llvm/IR/Constants.h"
+#include "llvm/IR/Dominators.h"
+#include "llvm/IR/Instructions.h"
+#include "llvm/IR/Function.h"
+#include "llvm/IR/LLVMContext.h"
+#include "llvm/IR/Module.h"
+#include "llvm/Support/DataTypes.h"
+#include "llvm/Support/SourceMgr.h"
+#include "llvm/Support/raw_ostream.h"
+#include "gtest/gtest.h"
+
+namespace llvm {
+namespace {
+
+struct BranchProbabilityInfoTest : public testing::Test {
+  std::unique_ptr<BranchProbabilityInfo> BPI;
+  std::unique_ptr<DominatorTree> DT;
+  std::unique_ptr<LoopInfo> LI;
+  LLVMContext C;
+
+  BranchProbabilityInfo &buildBPI(Function &F) {
+    DT.reset(new DominatorTree(F));
+    LI.reset(new LoopInfo(*DT));
+    BPI.reset(new BranchProbabilityInfo(F, *LI));
+    return *BPI;
+  }
+
+  std::unique_ptr<Module> makeLLVMModule() {
+    const char *ModuleString = "define void @f() { exit: ret void }\n";
+    SMDiagnostic Err;
+    return parseAssemblyString(ModuleString, Err, C);
+  }
+};
+
+TEST_F(BranchProbabilityInfoTest, StressUnreachableHeuristic) {
+  auto M = makeLLVMModule();
+  Function *F = M->getFunction("f");
+
+  // define void @f() {
+  // entry:
+  //   switch i32 undef, label %exit, [
+  //      i32 0, label %preexit
+  //      ...                   ;;< Add lots of cases to stress the heuristic.
+  //   ]
+  // preexit:
+  //   unreachable
+  // exit:
+  //   ret void
+  // }
+
+  auto *ExitBB = &F->back();
+  auto *EntryBB = BasicBlock::Create(C, "entry", F, /*insertBefore=*/ExitBB);
+
+  auto *PreExitBB =
+      BasicBlock::Create(C, "preexit", F, /*insertBefore=*/ExitBB);
+  new UnreachableInst(C, PreExitBB);
+
+  unsigned NumCases = 4096;
+  auto *I32 = IntegerType::get(C, 32);
+  auto *Undef = UndefValue::get(I32);
+  auto *Switch = SwitchInst::Create(Undef, ExitBB, NumCases, EntryBB);
+  for (unsigned I = 0; I < NumCases; ++I)
+    Switch->addCase(ConstantInt::get(I32, I), PreExitBB);
+
+  BranchProbabilityInfo &BPI = buildBPI(*F);
+
+  // FIXME: This doesn't seem optimal. Since all of the cases handled by the
+  // switch have the *same* destination block ("preexit"), shouldn't it be the
+  // hot one? I'd expect the results to be reversed here...
+  EXPECT_FALSE(BPI.isEdgeHot(EntryBB, PreExitBB));
+  EXPECT_TRUE(BPI.isEdgeHot(EntryBB, ExitBB));
+}
+
+} // end anonymous namespace
+} // end namespace llvm