1 //===- XRayInstrumentation.cpp - Adds XRay instrumentation to functions. --===//
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 implements a MachineFunctionPass that inserts the appropriate
11 // XRay instrumentation instructions. We look for XRay-specific attributes
12 // on the function to determine whether we should insert the replacement
13 // operations.
14 //
15 //===---------------------------------------------------------------------===//
16 
17 #include "llvm/ADT/SmallVector.h"
18 #include "llvm/ADT/STLExtras.h"
19 #include "llvm/ADT/Triple.h"
20 #include "llvm/CodeGen/MachineBasicBlock.h"
21 #include "llvm/CodeGen/MachineDominators.h"
22 #include "llvm/CodeGen/MachineFunction.h"
23 #include "llvm/CodeGen/MachineFunctionPass.h"
24 #include "llvm/CodeGen/MachineInstrBuilder.h"
25 #include "llvm/CodeGen/MachineLoopInfo.h"
26 #include "llvm/IR/Attributes.h"
27 #include "llvm/IR/Function.h"
28 #include "llvm/Pass.h"
29 #include "llvm/Target/TargetInstrInfo.h"
30 #include "llvm/Target/TargetMachine.h"
31 #include "llvm/Target/TargetSubtargetInfo.h"
32 
33 using namespace llvm;
34 
35 namespace {
36 
37 struct XRayInstrumentation : public MachineFunctionPass {
38   static char ID;
39 
40   XRayInstrumentation() : MachineFunctionPass(ID) {
41     initializeXRayInstrumentationPass(*PassRegistry::getPassRegistry());
42   }
43 
44   void getAnalysisUsage(AnalysisUsage &AU) const override {
45     AU.setPreservesCFG();
46     AU.addRequired<MachineLoopInfo>();
47     AU.addPreserved<MachineLoopInfo>();
48     AU.addPreserved<MachineDominatorTree>();
49     MachineFunctionPass::getAnalysisUsage(AU);
50   }
51 
52   bool runOnMachineFunction(MachineFunction &MF) override;
53 
54 private:
55   // Replace the original RET instruction with the exit sled code ("patchable
56   //   ret" pseudo-instruction), so that at runtime XRay can replace the sled
57   //   with a code jumping to XRay trampoline, which calls the tracing handler
58   //   and, in the end, issues the RET instruction.
59   // This is the approach to go on CPUs which have a single RET instruction,
60   //   like x86/x86_64.
61   void replaceRetWithPatchableRet(MachineFunction &MF,
62                                   const TargetInstrInfo *TII);
63 
64   // Prepend the original return instruction with the exit sled code ("patchable
65   //   function exit" pseudo-instruction), preserving the original return
66   //   instruction just after the exit sled code.
67   // This is the approach to go on CPUs which have multiple options for the
68   //   return instruction, like ARM. For such CPUs we can't just jump into the
69   //   XRay trampoline and issue a single return instruction there. We rather
70   //   have to call the trampoline and return from it to the original return
71   //   instruction of the function being instrumented.
72   void prependRetWithPatchableExit(MachineFunction &MF,
73                                    const TargetInstrInfo *TII);
74 };
75 
76 } // end anonymous namespace
77 
78 void XRayInstrumentation::replaceRetWithPatchableRet(
79     MachineFunction &MF, const TargetInstrInfo *TII) {
80   // We look for *all* terminators and returns, then replace those with
81   // PATCHABLE_RET instructions.
82   SmallVector<MachineInstr *, 4> Terminators;
83   for (auto &MBB : MF) {
84     for (auto &T : MBB.terminators()) {
85       unsigned Opc = 0;
86       if (T.isReturn() && T.getOpcode() == TII->getReturnOpcode()) {
87         // Replace return instructions with:
88         //   PATCHABLE_RET <Opcode>, <Operand>...
89         Opc = TargetOpcode::PATCHABLE_RET;
90       }
91       if (TII->isTailCall(T)) {
92         // Treat the tail call as a return instruction, which has a
93         // different-looking sled than the normal return case.
94         Opc = TargetOpcode::PATCHABLE_TAIL_CALL;
95       }
96       if (Opc != 0) {
97         auto MIB = BuildMI(MBB, T, T.getDebugLoc(), TII->get(Opc))
98                        .addImm(T.getOpcode());
99         for (auto &MO : T.operands())
100           MIB.add(MO);
101         Terminators.push_back(&T);
102       }
103     }
104   }
105 
106   for (auto &I : Terminators)
107     I->eraseFromParent();
108 }
109 
110 void XRayInstrumentation::prependRetWithPatchableExit(
111     MachineFunction &MF, const TargetInstrInfo *TII) {
112   for (auto &MBB : MF) {
113     for (auto &T : MBB.terminators()) {
114       unsigned Opc = 0;
115       if (T.isReturn()) {
116         Opc = TargetOpcode::PATCHABLE_FUNCTION_EXIT;
117       }
118       if (TII->isTailCall(T)) {
119         Opc = TargetOpcode::PATCHABLE_TAIL_CALL;
120       }
121       if (Opc != 0) {
122         // Prepend the return instruction with PATCHABLE_FUNCTION_EXIT or
123         //   PATCHABLE_TAIL_CALL .
124         BuildMI(MBB, T, T.getDebugLoc(), TII->get(Opc));
125       }
126     }
127   }
128 }
129 
130 bool XRayInstrumentation::runOnMachineFunction(MachineFunction &MF) {
131   auto &F = *MF.getFunction();
132   auto InstrAttr = F.getFnAttribute("function-instrument");
133   bool AlwaysInstrument = !InstrAttr.hasAttribute(Attribute::None) &&
134                           InstrAttr.isStringAttribute() &&
135                           InstrAttr.getValueAsString() == "xray-always";
136   Attribute Attr = F.getFnAttribute("xray-instruction-threshold");
137   unsigned XRayThreshold = 0;
138   if (!AlwaysInstrument) {
139     if (Attr.hasAttribute(Attribute::None) || !Attr.isStringAttribute())
140       return false; // XRay threshold attribute not found.
141     if (Attr.getValueAsString().getAsInteger(10, XRayThreshold))
142       return false; // Invalid value for threshold.
143 
144     // Count the number of MachineInstr`s in MachineFunction
145     int64_t MICount = 0;
146     for (const auto& MBB : MF)
147       MICount += MBB.size();
148 
149     // Check if we have a loop.
150     // FIXME: Maybe make this smarter, and see whether the loops are dependent
151     // on inputs or side-effects?
152     MachineLoopInfo &MLI = getAnalysis<MachineLoopInfo>();
153     if (MLI.empty() && MICount < XRayThreshold)
154       return false; // Function is too small and has no loops.
155   }
156 
157   // We look for the first non-empty MachineBasicBlock, so that we can insert
158   // the function instrumentation in the appropriate place.
159   auto MBI = llvm::find_if(
160       MF, [&](const MachineBasicBlock &MBB) { return !MBB.empty(); });
161   if (MBI == MF.end())
162     return false; // The function is empty.
163 
164   auto *TII = MF.getSubtarget().getInstrInfo();
165   auto &FirstMBB = *MBI;
166   auto &FirstMI = *FirstMBB.begin();
167 
168   if (!MF.getSubtarget().isXRaySupported()) {
169     FirstMI.emitError("An attempt to perform XRay instrumentation for an"
170                       " unsupported target.");
171     return false;
172   }
173 
174   // First, insert an PATCHABLE_FUNCTION_ENTER as the first instruction of the
175   // MachineFunction.
176   BuildMI(FirstMBB, FirstMI, FirstMI.getDebugLoc(),
177           TII->get(TargetOpcode::PATCHABLE_FUNCTION_ENTER));
178 
179   switch (MF.getTarget().getTargetTriple().getArch()) {
180   case Triple::ArchType::arm:
181   case Triple::ArchType::thumb:
182   case Triple::ArchType::aarch64:
183   case Triple::ArchType::ppc64le:
184   case Triple::ArchType::mips:
185   case Triple::ArchType::mipsel:
186   case Triple::ArchType::mips64:
187   case Triple::ArchType::mips64el:
188     // For the architectures which don't have a single return instruction
189     prependRetWithPatchableExit(MF, TII);
190     break;
191   default:
192     // For the architectures that have a single return instruction (such as
193     //   RETQ on x86_64).
194     replaceRetWithPatchableRet(MF, TII);
195     break;
196   }
197   return true;
198 }
199 
200 char XRayInstrumentation::ID = 0;
201 char &llvm::XRayInstrumentationID = XRayInstrumentation::ID;
202 INITIALIZE_PASS_BEGIN(XRayInstrumentation, "xray-instrumentation",
203                       "Insert XRay ops", false, false)
204 INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
205 INITIALIZE_PASS_END(XRayInstrumentation, "xray-instrumentation",
206                     "Insert XRay ops", false, false)
207