1 //===- AddDiscriminators.cpp - Insert DWARF path discriminators -----------===//
2 //
3 // The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // This file adds DWARF discriminators to the IR. Path discriminators are
11 // used to decide what CFG path was taken inside sub-graphs whose instructions
12 // share the same line and column number information.
13 //
14 // The main user of this is the sample profiler. Instruction samples are
15 // mapped to line number information. Since a single line may be spread
16 // out over several basic blocks, discriminators add more precise location
17 // for the samples.
18 //
19 // For example,
20 //
21 // 1 #define ASSERT(P)
22 // 2 if (!(P))
23 // 3 abort()
24 // ...
25 // 100 while (true) {
26 // 101 ASSERT (sum < 0);
27 // 102 ...
28 // 130 }
29 //
30 // when converted to IR, this snippet looks something like:
31 //
32 // while.body: ; preds = %entry, %if.end
33 // %0 = load i32* %sum, align 4, !dbg !15
34 // %cmp = icmp slt i32 %0, 0, !dbg !15
35 // br i1 %cmp, label %if.end, label %if.then, !dbg !15
36 //
37 // if.then: ; preds = %while.body
38 // call void @abort(), !dbg !15
39 // br label %if.end, !dbg !15
40 //
41 // Notice that all the instructions in blocks 'while.body' and 'if.then'
42 // have exactly the same debug information. When this program is sampled
43 // at runtime, the profiler will assume that all these instructions are
44 // equally frequent. This, in turn, will consider the edge while.body->if.then
45 // to be frequently taken (which is incorrect).
46 //
47 // By adding a discriminator value to the instructions in block 'if.then',
48 // we can distinguish instructions at line 101 with discriminator 0 from
49 // the instructions at line 101 with discriminator 1.
50 //
51 // For more details about DWARF discriminators, please visit
52 // http://wiki.dwarfstd.org/index.php?title=Path_Discriminators
53 //===----------------------------------------------------------------------===//
54
55 #include "llvm/Transforms/Scalar.h"
56 #include "llvm/IR/BasicBlock.h"
57 #include "llvm/IR/Constants.h"
58 #include "llvm/IR/DIBuilder.h"
59 #include "llvm/IR/DebugInfo.h"
60 #include "llvm/IR/Instructions.h"
61 #include "llvm/IR/LLVMContext.h"
62 #include "llvm/IR/Module.h"
63 #include "llvm/Pass.h"
64 #include "llvm/Support/CommandLine.h"
65 #include "llvm/Support/Debug.h"
66 #include "llvm/Support/raw_ostream.h"
67
68 using namespace llvm;
69
70 #define DEBUG_TYPE "add-discriminators"
71
72 namespace {
73 struct AddDiscriminators : public FunctionPass {
74 static char ID; // Pass identification, replacement for typeid
AddDiscriminators__anonb00c4c840111::AddDiscriminators75 AddDiscriminators() : FunctionPass(ID) {
76 initializeAddDiscriminatorsPass(*PassRegistry::getPassRegistry());
77 }
78
79 bool runOnFunction(Function &F) override;
80 };
81 }
82
83 char AddDiscriminators::ID = 0;
84 INITIALIZE_PASS_BEGIN(AddDiscriminators, "add-discriminators",
85 "Add DWARF path discriminators", false, false)
86 INITIALIZE_PASS_END(AddDiscriminators, "add-discriminators",
87 "Add DWARF path discriminators", false, false)
88
89 // Command line option to disable discriminator generation even in the
90 // presence of debug information. This is only needed when debugging
91 // debug info generation issues.
92 static cl::opt<bool>
93 NoDiscriminators("no-discriminators", cl::init(false),
94 cl::desc("Disable generation of discriminator information."));
95
createAddDiscriminatorsPass()96 FunctionPass *llvm::createAddDiscriminatorsPass() {
97 return new AddDiscriminators();
98 }
99
hasDebugInfo(const Function & F)100 static bool hasDebugInfo(const Function &F) {
101 NamedMDNode *CUNodes = F.getParent()->getNamedMetadata("llvm.dbg.cu");
102 return CUNodes != nullptr;
103 }
104
105 /// \brief Assign DWARF discriminators.
106 ///
107 /// To assign discriminators, we examine the boundaries of every
108 /// basic block and its successors. Suppose there is a basic block B1
109 /// with successor B2. The last instruction I1 in B1 and the first
110 /// instruction I2 in B2 are located at the same file and line number.
111 /// This situation is illustrated in the following code snippet:
112 ///
113 /// if (i < 10) x = i;
114 ///
115 /// entry:
116 /// br i1 %cmp, label %if.then, label %if.end, !dbg !10
117 /// if.then:
118 /// %1 = load i32* %i.addr, align 4, !dbg !10
119 /// store i32 %1, i32* %x, align 4, !dbg !10
120 /// br label %if.end, !dbg !10
121 /// if.end:
122 /// ret void, !dbg !12
123 ///
124 /// Notice how the branch instruction in block 'entry' and all the
125 /// instructions in block 'if.then' have the exact same debug location
126 /// information (!dbg !10).
127 ///
128 /// To distinguish instructions in block 'entry' from instructions in
129 /// block 'if.then', we generate a new lexical block for all the
130 /// instruction in block 'if.then' that share the same file and line
131 /// location with the last instruction of block 'entry'.
132 ///
133 /// This new lexical block will have the same location information as
134 /// the previous one, but with a new DWARF discriminator value.
135 ///
136 /// One of the main uses of this discriminator value is in runtime
137 /// sample profilers. It allows the profiler to distinguish instructions
138 /// at location !dbg !10 that execute on different basic blocks. This is
139 /// important because while the predicate 'if (x < 10)' may have been
140 /// executed millions of times, the assignment 'x = i' may have only
141 /// executed a handful of times (meaning that the entry->if.then edge is
142 /// seldom taken).
143 ///
144 /// If we did not have discriminator information, the profiler would
145 /// assign the same weight to both blocks 'entry' and 'if.then', which
146 /// in turn will make it conclude that the entry->if.then edge is very
147 /// hot.
148 ///
149 /// To decide where to create new discriminator values, this function
150 /// traverses the CFG and examines instruction at basic block boundaries.
151 /// If the last instruction I1 of a block B1 is at the same file and line
152 /// location as instruction I2 of successor B2, then it creates a new
153 /// lexical block for I2 and all the instruction in B2 that share the same
154 /// file and line location as I2. This new lexical block will have a
155 /// different discriminator number than I1.
runOnFunction(Function & F)156 bool AddDiscriminators::runOnFunction(Function &F) {
157 // If the function has debug information, but the user has disabled
158 // discriminators, do nothing.
159 // Simlarly, if the function has no debug info, do nothing.
160 // Finally, if this module is built with dwarf versions earlier than 4,
161 // do nothing (discriminator support is a DWARF 4 feature).
162 if (NoDiscriminators ||
163 !hasDebugInfo(F) ||
164 F.getParent()->getDwarfVersion() < 4)
165 return false;
166
167 bool Changed = false;
168 Module *M = F.getParent();
169 LLVMContext &Ctx = M->getContext();
170 DIBuilder Builder(*M, /*AllowUnresolved*/ false);
171
172 // Traverse all the blocks looking for instructions in different
173 // blocks that are at the same file:line location.
174 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) {
175 BasicBlock *B = I;
176 TerminatorInst *Last = B->getTerminator();
177 DILocation LastDIL = Last->getDebugLoc().get();
178 if (!LastDIL)
179 continue;
180
181 for (unsigned I = 0; I < Last->getNumSuccessors(); ++I) {
182 BasicBlock *Succ = Last->getSuccessor(I);
183 Instruction *First = Succ->getFirstNonPHIOrDbgOrLifetime();
184 DILocation FirstDIL = First->getDebugLoc().get();
185 if (!FirstDIL)
186 continue;
187
188 // If the first instruction (First) of Succ is at the same file
189 // location as B's last instruction (Last), add a new
190 // discriminator for First's location and all the instructions
191 // in Succ that share the same location with First.
192 if (!FirstDIL->canDiscriminate(*LastDIL)) {
193 // Create a new lexical scope and compute a new discriminator
194 // number for it.
195 StringRef Filename = FirstDIL->getFilename();
196 auto *Scope = FirstDIL->getScope();
197 DIFile File = Builder.createFile(Filename, Scope->getDirectory());
198
199 // FIXME: Calculate the discriminator here, based on local information,
200 // and delete MDLocation::computeNewDiscriminator(). The current
201 // solution gives different results depending on other modules in the
202 // same context. All we really need is to discriminate between
203 // FirstDIL and LastDIL -- a local map would suffice.
204 unsigned Discriminator = FirstDIL->computeNewDiscriminator();
205 DILexicalBlockFile NewScope =
206 Builder.createLexicalBlockFile(Scope, File, Discriminator);
207 auto *NewDIL =
208 MDLocation::get(Ctx, FirstDIL->getLine(), FirstDIL->getColumn(),
209 NewScope, FirstDIL->getInlinedAt());
210 DebugLoc newDebugLoc = NewDIL;
211
212 // Attach this new debug location to First and every
213 // instruction following First that shares the same location.
214 for (BasicBlock::iterator I1(*First), E1 = Succ->end(); I1 != E1;
215 ++I1) {
216 if (I1->getDebugLoc().get() != FirstDIL)
217 break;
218 I1->setDebugLoc(newDebugLoc);
219 DEBUG(dbgs() << NewDIL->getFilename() << ":" << NewDIL->getLine()
220 << ":" << NewDIL->getColumn() << ":"
221 << NewDIL->getDiscriminator() << *I1 << "\n");
222 }
223 DEBUG(dbgs() << "\n");
224 Changed = true;
225 }
226 }
227 }
228 return Changed;
229 }
230