1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
|
//===--- MisExpect.cpp - Check the use of llvm.expect with PGO data -------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This contains code to emit warnings for potentially incorrect usage of the
// llvm.expect intrinsic. This utility extracts the threshold values from
// metadata associated with the instrumented Branch or Switch instruction. The
// threshold values are then used to determine if a warning should be emmited.
//
// MisExpect metadata is generated when llvm.expect intrinsics are lowered see
// LowerExpectIntrinsic.cpp
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Utils/MisExpect.h"
#include "llvm/ADT/Twine.h"
#include "llvm/Analysis/OptimizationRemarkEmitter.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DiagnosticInfo.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/Support/BranchProbability.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/FormatVariadic.h"
#include <cstdint>
#include <functional>
#include <numeric>
#define DEBUG_TYPE "misexpect"
using namespace llvm;
using namespace misexpect;
namespace llvm {
// Command line option to enable/disable the warning when profile data suggests
// a mismatch with the use of the llvm.expect intrinsic
static cl::opt<bool> PGOWarnMisExpect(
"pgo-warn-misexpect", cl::init(false), cl::Hidden,
cl::desc("Use this option to turn on/off "
"warnings about incorrect usage of llvm.expect intrinsics."));
} // namespace llvm
namespace {
Instruction *getOprndOrInst(Instruction *I) {
assert(I != nullptr && "MisExpect target Instruction cannot be nullptr");
Instruction *Ret = nullptr;
if (auto *B = dyn_cast<BranchInst>(I)) {
Ret = dyn_cast<Instruction>(B->getCondition());
}
// TODO: Find a way to resolve condition location for switches
// Using the condition of the switch seems to often resolve to an earlier
// point in the program, i.e. the calculation of the switch condition, rather
// than the switches location in the source code. Thus, we should use the
// instruction to get source code locations rather than the condition to
// improve diagnostic output, such as the caret. If the same problem exists
// for branch instructions, then we should remove this function and directly
// use the instruction
//
// else if (auto S = dyn_cast<SwitchInst>(I)) {
// Ret = I;
//}
return Ret ? Ret : I;
}
void emitMisexpectDiagnostic(Instruction *I, LLVMContext &Ctx,
uint64_t ProfCount, uint64_t TotalCount) {
double PercentageCorrect = (double)ProfCount / TotalCount;
auto PerString =
formatv("{0:P} ({1} / {2})", PercentageCorrect, ProfCount, TotalCount);
auto RemStr = formatv(
"Potential performance regression from use of the llvm.expect intrinsic: "
"Annotation was correct on {0} of profiled executions.",
PerString);
Twine Msg(PerString);
Instruction *Cond = getOprndOrInst(I);
if (PGOWarnMisExpect)
Ctx.diagnose(DiagnosticInfoMisExpect(Cond, Msg));
OptimizationRemarkEmitter ORE(I->getParent()->getParent());
ORE.emit(OptimizationRemark(DEBUG_TYPE, "misexpect", Cond) << RemStr.str());
}
} // namespace
namespace llvm {
namespace misexpect {
void verifyMisExpect(Instruction *I, const SmallVector<uint32_t, 4> &Weights,
LLVMContext &Ctx) {
if (auto *MisExpectData = I->getMetadata(LLVMContext::MD_misexpect)) {
auto *MisExpectDataName = dyn_cast<MDString>(MisExpectData->getOperand(0));
if (MisExpectDataName &&
MisExpectDataName->getString().equals("misexpect")) {
LLVM_DEBUG(llvm::dbgs() << "------------------\n");
LLVM_DEBUG(llvm::dbgs()
<< "Function: " << I->getFunction()->getName() << "\n");
LLVM_DEBUG(llvm::dbgs() << "Instruction: " << *I << ":\n");
LLVM_DEBUG(for (int Idx = 0, Size = Weights.size(); Idx < Size; ++Idx) {
llvm::dbgs() << "Weights[" << Idx << "] = " << Weights[Idx] << "\n";
});
// extract values from misexpect metadata
const auto *IndexCint =
mdconst::dyn_extract<ConstantInt>(MisExpectData->getOperand(1));
const auto *LikelyCInt =
mdconst::dyn_extract<ConstantInt>(MisExpectData->getOperand(2));
const auto *UnlikelyCInt =
mdconst::dyn_extract<ConstantInt>(MisExpectData->getOperand(3));
if (!IndexCint || !LikelyCInt || !UnlikelyCInt)
return;
const uint64_t Index = IndexCint->getZExtValue();
const uint64_t LikelyBranchWeight = LikelyCInt->getZExtValue();
const uint64_t UnlikelyBranchWeight = UnlikelyCInt->getZExtValue();
const uint64_t ProfileCount = Weights[Index];
const uint64_t CaseTotal = std::accumulate(
Weights.begin(), Weights.end(), (uint64_t)0, std::plus<uint64_t>());
const uint64_t NumUnlikelyTargets = Weights.size() - 1;
const uint64_t TotalBranchWeight =
LikelyBranchWeight + (UnlikelyBranchWeight * NumUnlikelyTargets);
const llvm::BranchProbability LikelyThreshold(LikelyBranchWeight,
TotalBranchWeight);
uint64_t ScaledThreshold = LikelyThreshold.scale(CaseTotal);
LLVM_DEBUG(llvm::dbgs()
<< "Unlikely Targets: " << NumUnlikelyTargets << ":\n");
LLVM_DEBUG(llvm::dbgs() << "Profile Count: " << ProfileCount << ":\n");
LLVM_DEBUG(llvm::dbgs()
<< "Scaled Threshold: " << ScaledThreshold << ":\n");
LLVM_DEBUG(llvm::dbgs() << "------------------\n");
if (ProfileCount < ScaledThreshold)
emitMisexpectDiagnostic(I, Ctx, ProfileCount, CaseTotal);
}
}
}
void checkFrontendInstrumentation(Instruction &I) {
if (auto *MD = I.getMetadata(LLVMContext::MD_prof)) {
unsigned NOps = MD->getNumOperands();
// Only emit misexpect diagnostics if at least 2 branch weights are present.
// Less than 2 branch weights means that the profiling metadata is:
// 1) incorrect/corrupted
// 2) not branch weight metadata
// 3) completely deterministic
// In these cases we should not emit any diagnostic related to misexpect.
if (NOps < 3)
return;
// Operand 0 is a string tag "branch_weights"
if (MDString *Tag = cast<MDString>(MD->getOperand(0))) {
if (Tag->getString().equals("branch_weights")) {
SmallVector<uint32_t, 4> RealWeights(NOps - 1);
for (unsigned i = 1; i < NOps; i++) {
ConstantInt *Value =
mdconst::dyn_extract<ConstantInt>(MD->getOperand(i));
RealWeights[i - 1] = Value->getZExtValue();
}
verifyMisExpect(&I, RealWeights, I.getContext());
}
}
}
}
} // namespace misexpect
} // namespace llvm
#undef DEBUG_TYPE
|
