1//===--- MisExpect.cpp - Check the use of llvm.expect with PGO data -------===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8//
9// This contains code to emit warnings for potentially incorrect usage of the
10// llvm.expect intrinsic. This utility extracts the threshold values from
11// metadata associated with the instrumented Branch or Switch instruction. The
12// threshold values are then used to determine if a warning should be emmited.
13//
14// MisExpect's implementation relies on two assumptions about how branch weights
15// are managed in LLVM.
16//
17// 1) Frontend profiling weights are always in place before llvm.expect is
18// lowered in LowerExpectIntrinsic.cpp. Frontend based instrumentation therefore
19// needs to extract the branch weights and then compare them to the weights
20// being added by the llvm.expect intrinsic lowering.
21//
22// 2) Sampling and IR based profiles will *only* have branch weight metadata
23// before profiling data is consulted if they are from a lowered llvm.expect
24// intrinsic. These profiles thus always extract the expected weights and then
25// compare them to the weights collected during profiling to determine if a
26// diagnostic message is warranted.
27//
28//===----------------------------------------------------------------------===//
29
30#include "llvm/Transforms/Utils/MisExpect.h"
31#include "llvm/ADT/Twine.h"
32#include "llvm/Analysis/OptimizationRemarkEmitter.h"
33#include "llvm/IR/DiagnosticInfo.h"
34#include "llvm/IR/Instruction.h"
35#include "llvm/IR/Instructions.h"
36#include "llvm/IR/LLVMContext.h"
37#include "llvm/IR/ProfDataUtils.h"
38#include "llvm/Support/BranchProbability.h"
39#include "llvm/Support/CommandLine.h"
40#include "llvm/Support/FormatVariadic.h"
41#include <algorithm>
42#include <cstdint>
43#include <functional>
44#include <numeric>
45
46#define DEBUG_TYPE "misexpect"
47
48using namespace llvm;
49using namespace misexpect;
50
51namespace llvm {
52
53// Command line option to enable/disable the warning when profile data suggests
54// a mismatch with the use of the llvm.expect intrinsic
55static cl::opt<bool> PGOWarnMisExpect(
56 "pgo-warn-misexpect", cl::init(Val: false), cl::Hidden,
57 cl::desc("Use this option to turn on/off "
58 "warnings about incorrect usage of llvm.expect intrinsics."));
59
60// Command line option for setting the diagnostic tolerance threshold
61static cl::opt<uint32_t> MisExpectTolerance(
62 "misexpect-tolerance", cl::init(Val: 0),
63 cl::desc("Prevents emitting diagnostics when profile counts are "
64 "within N% of the threshold.."));
65
66} // namespace llvm
67
68namespace {
69
70bool isMisExpectDiagEnabled(LLVMContext &Ctx) {
71 return PGOWarnMisExpect || Ctx.getMisExpectWarningRequested();
72}
73
74uint32_t getMisExpectTolerance(LLVMContext &Ctx) {
75 return std::max(a: static_cast<uint32_t>(MisExpectTolerance),
76 b: Ctx.getDiagnosticsMisExpectTolerance());
77}
78
79Instruction *getInstCondition(Instruction *I) {
80 assert(I != nullptr && "MisExpect target Instruction cannot be nullptr");
81 Instruction *Ret = nullptr;
82 if (auto *B = dyn_cast<BranchInst>(Val: I)) {
83 Ret = dyn_cast<Instruction>(Val: B->getCondition());
84 }
85 // TODO: Find a way to resolve condition location for switches
86 // Using the condition of the switch seems to often resolve to an earlier
87 // point in the program, i.e. the calculation of the switch condition, rather
88 // than the switch's location in the source code. Thus, we should use the
89 // instruction to get source code locations rather than the condition to
90 // improve diagnostic output, such as the caret. If the same problem exists
91 // for branch instructions, then we should remove this function and directly
92 // use the instruction
93 //
94 else if (auto *S = dyn_cast<SwitchInst>(Val: I)) {
95 Ret = dyn_cast<Instruction>(Val: S->getCondition());
96 }
97 return Ret ? Ret : I;
98}
99
100void emitMisexpectDiagnostic(Instruction *I, LLVMContext &Ctx,
101 uint64_t ProfCount, uint64_t TotalCount) {
102 double PercentageCorrect = (double)ProfCount / TotalCount;
103 auto PerString =
104 formatv(Fmt: "{0:P} ({1} / {2})", Vals&: PercentageCorrect, Vals&: ProfCount, Vals&: TotalCount);
105 auto RemStr = formatv(
106 Fmt: "Potential performance regression from use of the llvm.expect intrinsic: "
107 "Annotation was correct on {0} of profiled executions.",
108 Vals&: PerString);
109 Instruction *Cond = getInstCondition(I);
110 if (isMisExpectDiagEnabled(Ctx))
111 Ctx.diagnose(DI: DiagnosticInfoMisExpect(Cond, Twine(PerString)));
112 OptimizationRemarkEmitter ORE(I->getParent()->getParent());
113 ORE.emit(OptDiag: OptimizationRemark(DEBUG_TYPE, "misexpect", Cond) << RemStr.str());
114}
115
116} // namespace
117
118namespace llvm {
119namespace misexpect {
120
121void verifyMisExpect(Instruction &I, ArrayRef<uint32_t> RealWeights,
122 ArrayRef<uint32_t> ExpectedWeights) {
123 // To determine if we emit a diagnostic, we need to compare the branch weights
124 // from the profile to those added by the llvm.expect intrinsic.
125 // So first, we extract the "likely" and "unlikely" weights from
126 // ExpectedWeights And determine the correct weight in the profile to compare
127 // against.
128 uint64_t LikelyBranchWeight = 0,
129 UnlikelyBranchWeight = std::numeric_limits<uint32_t>::max();
130 size_t MaxIndex = 0;
131 for (size_t Idx = 0, End = ExpectedWeights.size(); Idx < End; Idx++) {
132 uint32_t V = ExpectedWeights[Idx];
133 if (LikelyBranchWeight < V) {
134 LikelyBranchWeight = V;
135 MaxIndex = Idx;
136 }
137 if (UnlikelyBranchWeight > V) {
138 UnlikelyBranchWeight = V;
139 }
140 }
141
142 const uint64_t ProfiledWeight = RealWeights[MaxIndex];
143 const uint64_t RealWeightsTotal =
144 std::accumulate(first: RealWeights.begin(), last: RealWeights.end(), init: (uint64_t)0,
145 binary_op: std::plus<uint64_t>());
146 const uint64_t NumUnlikelyTargets = RealWeights.size() - 1;
147
148 uint64_t TotalBranchWeight =
149 LikelyBranchWeight + (UnlikelyBranchWeight * NumUnlikelyTargets);
150
151 // Failing this assert means that we have corrupted metadata.
152 assert((TotalBranchWeight >= LikelyBranchWeight) && (TotalBranchWeight > 0) &&
153 "TotalBranchWeight is less than the Likely branch weight");
154
155 // To determine our threshold value we need to obtain the branch probability
156 // for the weights added by llvm.expect and use that proportion to calculate
157 // our threshold based on the collected profile data.
158 auto LikelyProbablilty = BranchProbability::getBranchProbability(
159 Numerator: LikelyBranchWeight, Denominator: TotalBranchWeight);
160
161 uint64_t ScaledThreshold = LikelyProbablilty.scale(Num: RealWeightsTotal);
162
163 // clamp tolerance range to [0, 100)
164 auto Tolerance = getMisExpectTolerance(Ctx&: I.getContext());
165 Tolerance = std::clamp(val: Tolerance, lo: 0u, hi: 99u);
166
167 // Allow users to relax checking by N% i.e., if they use a 5% tolerance,
168 // then we check against 0.95*ScaledThreshold
169 if (Tolerance > 0)
170 ScaledThreshold *= (1.0 - Tolerance / 100.0);
171
172 // When the profile weight is below the threshold, we emit the diagnostic
173 if (ProfiledWeight < ScaledThreshold)
174 emitMisexpectDiagnostic(I: &I, Ctx&: I.getContext(), ProfCount: ProfiledWeight,
175 TotalCount: RealWeightsTotal);
176}
177
178void checkBackendInstrumentation(Instruction &I,
179 const ArrayRef<uint32_t> RealWeights) {
180 // Backend checking assumes any existing weight comes from an `llvm.expect`
181 // intrinsic. However, SampleProfiling + ThinLTO add branch weights multiple
182 // times, leading to an invalid assumption in our checking. Backend checks
183 // should only operate on branch weights that carry the "!expected" field,
184 // since they are guaranteed to be added by the LowerExpectIntrinsic pass.
185 if (!hasBranchWeightOrigin(I))
186 return;
187 SmallVector<uint32_t> ExpectedWeights;
188 if (!extractBranchWeights(I, Weights&: ExpectedWeights))
189 return;
190 verifyMisExpect(I, RealWeights, ExpectedWeights);
191}
192
193void checkFrontendInstrumentation(Instruction &I,
194 const ArrayRef<uint32_t> ExpectedWeights) {
195 SmallVector<uint32_t> RealWeights;
196 if (!extractBranchWeights(I, Weights&: RealWeights))
197 return;
198 verifyMisExpect(I, RealWeights, ExpectedWeights);
199}
200
201void checkExpectAnnotations(Instruction &I,
202 const ArrayRef<uint32_t> ExistingWeights,
203 bool IsFrontend) {
204 if (IsFrontend) {
205 checkFrontendInstrumentation(I, ExpectedWeights: ExistingWeights);
206 } else {
207 checkBackendInstrumentation(I, RealWeights: ExistingWeights);
208 }
209}
210
211} // namespace misexpect
212} // namespace llvm
213#undef DEBUG_TYPE
214