[llvm][misexpect] Enable diagnostics for profitable llvm.expect annotations

ilovepi · ilovepi · commit 1d3847e6b3f3 · 2024-03-26T20:46:45.000Z
Issue llvm#56502 describes an enhancement related to the use of llvm.expect. The request is for a diagnostic mode that can identify branches that would benefit from the use of llvm.expect based on the branch_weights assigned from a PGO or sample profile. To support identify branches(or switches) that would benefit from the use of an llvm.expect intrinsic, we follow a similar checking pattern to that used in MisExpect, but only in cases where MisExpect diagnostics would not be used (i.e., when an llvm.expect intrinsic has already been used).
diff --git a/llvm/include/llvm/IR/LLVMContext.h b/llvm/include/llvm/IR/LLVMContext.h
@@ -208,6 +208,8 @@ class LLVMContext {
   void setMisExpectWarningRequested(bool Requested);
   void setDiagnosticsMisExpectTolerance(std::optional<uint32_t> Tolerance);
   uint32_t getDiagnosticsMisExpectTolerance() const;
+  bool getAnnotationDiagsRequested() const;
+  void setAnnotationDiagsRequested(bool Requested);
 
   /// Return the minimum hotness value a diagnostic would need in order
   /// to be included in optimization diagnostics.
diff --git a/llvm/include/llvm/Target/TargetOptions.h b/llvm/include/llvm/Target/TargetOptions.h
@@ -369,6 +369,10 @@ namespace llvm {
     /// By default, it is set to false
     unsigned MisExpect : 1;
 
+    /// When set to true, enable MissingAnnotations diagnostics
+    /// By default, it is set to false
+    unsigned MissingAnnotations : 1;
+
     /// When set to true, const objects with relocatable address values are put
     /// into the RO data section.
     unsigned XCOFFReadOnlyPointers : 1;
diff --git a/llvm/include/llvm/Transforms/Utils/MisExpect.h b/llvm/include/llvm/Transforms/Utils/MisExpect.h
@@ -76,6 +76,9 @@ void checkExpectAnnotations(Instruction &I,
                             const ArrayRef<uint32_t> ExistingWeights,
                             bool IsFrontend);
 
+void checkMissingAnnotations(Instruction &I,
+                             const ArrayRef<uint32_t> ExistingWeights,
+                             bool IsFrontendInstr);
 } // namespace misexpect
 } // namespace llvm
 
diff --git a/llvm/lib/IR/LLVMContext.cpp b/llvm/lib/IR/LLVMContext.cpp
@@ -165,6 +165,13 @@ uint32_t LLVMContext::getDiagnosticsMisExpectTolerance() const {
   return pImpl->DiagnosticsMisExpectTolerance.value_or(0);
 }
 
+void LLVMContext::setAnnotationDiagsRequested(bool Requested) {
+  pImpl->AnnotationsDiagsRequested = Requested;
+}
+bool LLVMContext::getAnnotationDiagsRequested() const {
+  return pImpl->AnnotationsDiagsRequested;
+}
+
 bool LLVMContext::isDiagnosticsHotnessThresholdSetFromPSI() const {
   return !pImpl->DiagnosticsHotnessThreshold.has_value();
 }
diff --git a/llvm/lib/IR/LLVMContextImpl.h b/llvm/lib/IR/LLVMContextImpl.h
@@ -1483,6 +1483,10 @@ class LLVMContextImpl {
   std::optional<uint32_t> DiagnosticsMisExpectTolerance = 0;
   bool MisExpectWarningRequested = false;
 
+  /// Enables Diagnostics for Missing llvm.expect annotations on extremely hot
+  /// branches
+  bool AnnotationsDiagsRequested = false;
+
   /// The specialized remark streamer used by LLVM's OptimizationRemarkEmitter.
   std::unique_ptr<LLVMRemarkStreamer> LLVMRS;
 
diff --git a/llvm/lib/Transforms/Instrumentation/PGOInstrumentation.cpp b/llvm/lib/Transforms/Instrumentation/PGOInstrumentation.cpp
@@ -2191,6 +2191,7 @@ void llvm::setProfMetadata(Module *M, Instruction *TI,
   } dbgs() << "\n";);
 
   misexpect::checkExpectAnnotations(*TI, Weights, /*IsFrontend=*/false);
+  misexpect::checkMissingAnnotations(*TI, Weights, /*IsFrontend=*/false);
 
   setBranchWeights(*TI, Weights, /*IsExpected=*/false);
   if (EmitBranchProbability) {
diff --git a/llvm/lib/Transforms/Utils/MisExpect.cpp b/llvm/lib/Transforms/Utils/MisExpect.cpp
@@ -65,9 +65,60 @@ static cl::opt<uint32_t> MisExpectTolerance(
     cl::desc("Prevents emitting diagnostics when profile counts are "
              "within N% of the threshold.."));
 
+// Command line option to enable/disable the Remparks when profile data suggests
+// that the llvm.expect intrinsic may be profitable
+static cl::opt<bool>
+    PGOMissingAnnotations("pgo-missing-annotations", cl::init(false),
+                          cl::Hidden,
+                          cl::desc("Use this option to turn on/off suggestions "
+                                   "of missing llvm.expect intrinsics."));
 } // namespace llvm
 
 namespace {
+struct ProfDataSummary {
+  uint64_t Likely;
+  uint64_t Unlikely;
+  uint64_t RealTotal;
+  uint64_t NumUnlikely;
+};
+
+enum class DiagKind {
+  MisExpect,     // Reports when llvm.expect usage is contradicted by PGO data
+  MissingExpect, // Reports when llvm.expect would be profitable
+  Unsupported,   // An error
+};
+
+std::optional<uint64_t> getScaledThreshold(const ProfDataSummary &PDS) {
+
+  uint64_t TotalBranchWeight = PDS.Likely + (PDS.Unlikely * PDS.NumUnlikely);
+
+  LLVM_DEBUG(dbgs() << "Total Branch Weight = " << TotalBranchWeight << "\n"
+                    << "Likely Branch Weight = " << PDS.Likely << "\n");
+
+  // FIXME: When we've addressed sample profiling, restore the assertion
+  //
+  // We cannot calculate branch probability if either of these invariants aren't
+  // met. However, MisExpect diagnostics should not prevent code from compiling,
+  // so we simply forgo emitting diagnostics here, and return early.
+  // assert((TotalBranchWeight >= LikelyBranchWeight) && (TotalBranchWeight > 0)
+  //              && "TotalBranchWeight is less than the Likely branch weight");
+  if ((TotalBranchWeight == 0) || (TotalBranchWeight <= PDS.Likely))
+    return std::nullopt;
+
+  // To determine our threshold value we need to obtain the branch probability
+  // for the weights added by llvm.expect and use that proportion to calculate
+  // our threshold based on the collected profile data.
+  BranchProbability LikelyProbablilty =
+      BranchProbability::getBranchProbability(PDS.Likely, TotalBranchWeight);
+
+  return LikelyProbablilty.scale(PDS.RealTotal);
+}
+
+bool isAnnotationDiagEnabled(LLVMContext &Ctx) {
+  LLVM_DEBUG(dbgs() << "PGOMissingAnnotations = " << PGOMissingAnnotations
+                    << "\n");
+  return PGOMissingAnnotations || Ctx.getAnnotationDiagsRequested();
+}
 
 bool isMisExpectDiagEnabled(LLVMContext &Ctx) {
   return PGOWarnMisExpect || Ctx.getMisExpectWarningRequested();
@@ -112,10 +163,69 @@ void emitMisexpectDiagnostic(Instruction *I, LLVMContext &Ctx,
   Instruction *Cond = getInstCondition(I);
   if (isMisExpectDiagEnabled(Ctx))
     Ctx.diagnose(DiagnosticInfoMisExpect(Cond, Msg));
-  OptimizationRemarkEmitter ORE(I->getParent()->getParent());
+  OptimizationRemarkEmitter ORE(I->getFunction());
   ORE.emit(OptimizationRemark(DEBUG_TYPE, "misexpect", Cond) << RemStr.str());
 }
 
+
+void emitMissingAnnotationDiag(Instruction *I) {
+  const auto *RemStr =
+      "Extremely hot condition. Consider adding llvm.expect intrinsic";
+  Instruction *Cond = getInstCondition(I);
+  OptimizationRemarkEmitter ORE(I->getParent()->getParent());
+  ORE.emit(
+      OptimizationRemark("missing-annotations", "missing-annotations", Cond)
+      << RemStr);
+}
+
+uint64_t totalWeight(const ArrayRef<uint32_t> Weights) {
+  return std::accumulate(Weights.begin(), Weights.end(), (uint64_t)0,
+                         std::plus<uint64_t>());
+}
+
+void scaleByTollerance(const Instruction &I, uint64_t &ScaledThreshold) {
+  // clamp tolerance range to [0, 100)
+  uint32_t Tolerance = getMisExpectTolerance(I.getContext());
+  Tolerance = std::clamp(Tolerance, 0u, 99u);
+
+  // Allow users to relax checking by N%  i.e., if they use a 5% tolerance,
+  // then we check against 0.95*ScaledThreshold
+  if (Tolerance > 0)
+    ScaledThreshold *= (1.0 - Tolerance / 100.0);
+
+  LLVM_DEBUG(dbgs() << "Scaled Threshold = " << ScaledThreshold << "\n");
+}
+
+void reportDiagnostics(Instruction &I, const ProfDataSummary &PDS,
+                       uint32_t ProfiledWeight, DiagKind Kind) {
+  std::optional<uint64_t> ScaledOpt = getScaledThreshold(PDS);
+  if (!ScaledOpt)
+    return;
+  uint64_t ScaledThreshold = ScaledOpt.value();
+  scaleByTollerance(I, ScaledThreshold);
+
+  LLVM_DEBUG(dbgs() << "Total Branch Weight = " << PDS.RealTotal << "\n"
+                    << "Scaled Threshold = " << ScaledThreshold << "\n"
+                    << "Profiled Weight = " << ProfiledWeight << "\n"
+                    << "Likely Branch Weight = " << PDS.Likely << "\n");
+  // When the profile weight is outside the range, we emit the diagnostic
+  switch (Kind) {
+  case DiagKind::MisExpect:
+    if (ProfiledWeight < ScaledThreshold) {
+      emitMisexpectDiagnostic(&I, I.getContext(), ProfiledWeight,
+                              PDS.RealTotal);
+    }
+    return;
+  case DiagKind::MissingExpect:
+    if (ProfiledWeight > ScaledThreshold) {
+      emitMissingAnnotationDiag(&I);
+    }
+    return;
+  default:
+    llvm_unreachable("Unsupported diagnostic type used in PGO based analysis");
+  };
+}
+
 } // namespace
 
 namespace llvm {
@@ -143,39 +253,10 @@ void verifyMisExpect(Instruction &I, ArrayRef<uint32_t> RealWeights,
   }
 
   const uint64_t ProfiledWeight = RealWeights[MaxIndex];
-  const uint64_t RealWeightsTotal =
-      std::accumulate(RealWeights.begin(), RealWeights.end(), (uint64_t)0,
-                      std::plus<uint64_t>());
-  const uint64_t NumUnlikelyTargets = RealWeights.size() - 1;
-
-  uint64_t TotalBranchWeight =
-      LikelyBranchWeight + (UnlikelyBranchWeight * NumUnlikelyTargets);
-
-  // Failing this assert means that we have corrupted metadata.
-  assert((TotalBranchWeight >= LikelyBranchWeight) && (TotalBranchWeight > 0) &&
-         "TotalBranchWeight is less than the Likely branch weight");
-
-  // To determine our threshold value we need to obtain the branch probability
-  // for the weights added by llvm.expect and use that proportion to calculate
-  // our threshold based on the collected profile data.
-  auto LikelyProbablilty = BranchProbability::getBranchProbability(
-      LikelyBranchWeight, TotalBranchWeight);
-
-  uint64_t ScaledThreshold = LikelyProbablilty.scale(RealWeightsTotal);
-
-  // clamp tolerance range to [0, 100)
-  auto Tolerance = getMisExpectTolerance(I.getContext());
-  Tolerance = std::clamp(Tolerance, 0u, 99u);
-
-  // Allow users to relax checking by N%  i.e., if they use a 5% tolerance,
-  // then we check against 0.95*ScaledThreshold
-  if (Tolerance > 0)
-    ScaledThreshold *= (1.0 - Tolerance / 100.0);
-
-  // When the profile weight is below the threshold, we emit the diagnostic
-  if (ProfiledWeight < ScaledThreshold)
-    emitMisexpectDiagnostic(&I, I.getContext(), ProfiledWeight,
-                            RealWeightsTotal);
+  const ProfDataSummary PDS = {LikelyBranchWeight, UnlikelyBranchWeight,
+                               totalWeight(RealWeights),
+                               RealWeights.size() - 1};
+  reportDiagnostics(I,PDS, ProfiledWeight, DiagKind::MisExpect);
 }
 
 void checkBackendInstrumentation(Instruction &I,
@@ -211,6 +292,47 @@ void checkExpectAnnotations(Instruction &I,
   }
 }
 
+void verifyMissingAnnotations(Instruction &I, ArrayRef<uint32_t> RealWeights) {
+  // To determine if we emit a diagnostic, we need to compare the branch weights
+  // from the profile to those that would be added by the llvm.expect intrinsic.
+  // And compare it to the real profile to see if it would be profitable.
+  uint32_t ProfiledWeight =
+      *std::max_element(RealWeights.begin(), RealWeights.end());
+
+  const uint64_t LikelyBranchWeight = 2000;
+  const uint64_t UnlikelyBranchWeight = 1;
+  const ProfDataSummary PDS = {LikelyBranchWeight, UnlikelyBranchWeight,
+                               totalWeight(RealWeights),
+                               RealWeights.size() - 1};
+  reportDiagnostics(I, PDS, ProfiledWeight, DiagKind::MissingExpect);
+}
+
+void checkMissingAnnotations(Instruction &I,
+                             const ArrayRef<uint32_t> ExistingWeights,
+                             bool IsFrontendInstr) {
+
+  // TODO: Ironically, this is probably a branch that should be marked UNLIKELY
+  //  exit early if these diagnostics weren't requested
+  if (!isAnnotationDiagEnabled(I.getContext()))
+    return;
+
+  if (IsFrontendInstr) {
+    // TODO: Fronend checking will have to be thought through, since we need
+    // to do the check on branches that don't have expect intrinsics
+
+    // auto RealWeightsOpt = extractWeights(&I, I.getContext());
+    // if (!RealWeightsOpt)
+    // return;
+    // auto RealWeights = RealWeightsOpt.getValue();
+    // verifyMissingAnnotations(I, RealWeights, ExistingWeights);
+  } else {
+    SmallVector<uint32_t> ExpectedWeights;
+    if (extractBranchWeights(I, ExpectedWeights))
+      return;
+    verifyMissingAnnotations(I, ExistingWeights);
+  }
+}
+
 } // namespace misexpect
 } // namespace llvm
 #undef DEBUG_TYPE
diff --git a/llvm/test/Transforms/PGOProfile/missing-annotation.ll b/llvm/test/Transforms/PGOProfile/missing-annotation.ll
@@ -0,0 +1,110 @@
+; Test misexpect checks do not issue diagnostics when profiling weights and
+; branch weights added by llvm.expect agree
+
+; RUN: llvm-profdata merge %S/Inputs/misexpect-branch-correct.proftext -o %t.profdata
+
+; RUN: opt < %s -passes="function(lower-expect),pgo-instr-use" -pgo-test-profile-file=%t.profdata -pgo-missing-annotations -pass-remarks=missing-annotation -S 2>&1 | FileCheck %s --check-prefix=MISSING_ANNOTATION
+
+; MISSING_ANNOTATION: remark: misexpect-branch.c:22:0:  Extremely hot condition. Consider adding llvm.expect intrinsic
+
+; CHECK-NOT: warning: {{.*}}
+; CHECK-NOT: remark: {{.*}}
+; CHECK: !{!"branch_weights", i32 0, i32 200000}
+
+
+; ModuleID = 'misexpect-branch.c'
+source_filename = "misexpect-branch.c"
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+@inner_loop = constant i32 100, align 4
+@outer_loop = constant i32 2000, align 4
+
+; Function Attrs: nounwind
+define i32 @bar() #0 !dbg !6 {
+entry:
+  %rando = alloca i32, align 4
+  %x = alloca i32, align 4
+  %0 = bitcast i32* %rando to i8*, !dbg !9
+  call void @llvm.lifetime.start.p0i8(i64 4, i8* %0) #4, !dbg !9
+  %call = call i32 (...) @buzz(), !dbg !9
+  store i32 %call, i32* %rando, align 4, !dbg !9, !tbaa !10
+  %1 = bitcast i32* %x to i8*, !dbg !14
+  call void @llvm.lifetime.start.p0i8(i64 4, i8* %1) #4, !dbg !14
+  store i32 0, i32* %x, align 4, !dbg !14, !tbaa !10
+  %2 = load i32, i32* %rando, align 4, !dbg !15, !tbaa !10
+  %rem = srem i32 %2, 200000, !dbg !15
+  %cmp = icmp eq i32 %rem, 0, !dbg !15
+  %lnot = xor i1 %cmp, true, !dbg !15
+  %lnot1 = xor i1 %lnot, true, !dbg !15
+  %lnot.ext = zext i1 %lnot1 to i32, !dbg !15
+  %conv = sext i32 %lnot.ext to i64, !dbg !15
+  %tobool = icmp ne i64 %conv, 0, !dbg !15
+  br i1 %tobool, label %if.then, label %if.else, !dbg !15
+
+if.then:                                          ; preds = %entry
+  %3 = load i32, i32* %rando, align 4, !dbg !16, !tbaa !10
+  %call2 = call i32 @baz(i32 %3), !dbg !16
+  store i32 %call2, i32* %x, align 4, !dbg !16, !tbaa !10
+  br label %if.end, !dbg !17
+
+if.else:                                          ; preds = %entry
+  %call3 = call i32 @foo(i32 50), !dbg !18
+  store i32 %call3, i32* %x, align 4, !dbg !18, !tbaa !10
+  br label %if.end
+
+if.end:                                           ; preds = %if.else, %if.then
+  %4 = load i32, i32* %x, align 4, !dbg !19, !tbaa !10
+  %5 = bitcast i32* %x to i8*, !dbg !20
+  call void @llvm.lifetime.end.p0i8(i64 4, i8* %5) #4, !dbg !20
+  %6 = bitcast i32* %rando to i8*, !dbg !20
+  call void @llvm.lifetime.end.p0i8(i64 4, i8* %6) #4, !dbg !20
+  ret i32 %4, !dbg !19
+}
+
+; Function Attrs: argmemonly nounwind willreturn
+declare void @llvm.lifetime.start.p0i8(i64 immarg, i8* nocapture) #1
+
+declare i32 @buzz(...) #2
+
+; Function Attrs: nounwind readnone willreturn
+declare i64 @llvm.expect.i64(i64, i64) #3
+
+declare i32 @baz(i32) #2
+
+declare i32 @foo(i32) #2
+
+; Function Attrs: argmemonly nounwind willreturn
+declare void @llvm.lifetime.end.p0i8(i64 immarg, i8* nocapture) #1
+
+attributes #0 = { nounwind "correctly-rounded-divide-sqrt-fp-math"="false" "disable-tail-calls"="false" "frame-pointer"="none" "less-precise-fpmad"="false" "min-legal-vector-width"="0" "no-infs-fp-math"="false" "no-jump-tables"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "no-trapping-math"="false" "stack-protector-buffer-size"="8" "target-features"="+cx8,+mmx,+sse,+sse2,+x87" "unsafe-fp-math"="false" "use-soft-float"="false" }
+attributes #1 = { argmemonly nounwind willreturn }
+attributes #2 = { "correctly-rounded-divide-sqrt-fp-math"="false" "disable-tail-calls"="false" "frame-pointer"="none" "less-precise-fpmad"="false" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "no-trapping-math"="false" "stack-protector-buffer-size"="8" "target-features"="+cx8,+mmx,+sse,+sse2,+x87" "unsafe-fp-math"="false" "use-soft-float"="false" }
+attributes #3 = { nounwind readnone willreturn }
+attributes #4 = { nounwind }
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4}
+!llvm.ident = !{!5}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 10.0.0 (trunk c20270bfffc9d6965219de339d66c61e9fe7d82d)", isOptimized: true, runtimeVersion: 0, emissionKind: LineTablesOnly, enums: !2, nameTableKind: None)
+!1 = !DIFile(filename: "<stdin>", directory: ".")
+!2 = !{}
+!3 = !{i32 2, !"Debug Info Version", i32 3}
+!4 = !{i32 1, !"wchar_size", i32 4}
+!5 = !{!"clang version 10.0.0 (trunk c20270bfffc9d6965219de339d66c61e9fe7d82d)"}
+!6 = distinct !DISubprogram(name: "bar", scope: !7, file: !7, line: 19, type: !8, scopeLine: 19, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !2)
+!7 = !DIFile(filename: "misexpect-branch.c", directory: ".")
+!8 = !DISubroutineType(types: !2)
+!9 = !DILocation(line: 20, scope: !6)
+!10 = !{!11, !11, i64 0}
+!11 = !{!"int", !12, i64 0}
+!12 = !{!"omnipotent char", !13, i64 0}
+!13 = !{!"Simple C/C++ TBAA"}
+!14 = !DILocation(line: 21, scope: !6)
+!15 = !DILocation(line: 22, scope: !6)
+!16 = !DILocation(line: 23, scope: !6)
+!17 = !DILocation(line: 24, scope: !6)
+!18 = !DILocation(line: 25, scope: !6)
+!19 = !DILocation(line: 27, scope: !6)
+!20 = !DILocation(line: 28, scope: !6)

Original file line number	Diff line number	Diff line change
`@@ -165,6 +165,13 @@ uint32_t LLVMContext::getDiagnosticsMisExpectTolerance() const {`
`165`	`165`	`return pImpl->DiagnosticsMisExpectTolerance.value_or(0);`
`166`	`166`	`}`
`167`	`167`
	`168`	`+void LLVMContext::setAnnotationDiagsRequested(bool Requested) {`
	`169`	`+ pImpl->AnnotationsDiagsRequested = Requested;`
	`170`	`+}`
	`171`	`+bool LLVMContext::getAnnotationDiagsRequested() const {`
	`172`	`+ return pImpl->AnnotationsDiagsRequested;`
	`173`	`+}`
	`174`	`+`
`168`	`175`	`bool LLVMContext::isDiagnosticsHotnessThresholdSetFromPSI() const {`
`169`	`176`	`return !pImpl->DiagnosticsHotnessThreshold.has_value();`
`170`	`177`	`}`