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/CodeGen/Analysis.h"
18 #include "llvm/CodeGen/MachineFunction.h"
19 #include "llvm/CodeGen/MachineFunctionPass.h"
20 #include "llvm/CodeGen/MachineInstrBuilder.h"
21 #include "llvm/CodeGen/Passes.h"
22 #include "llvm/Support/TargetRegistry.h"
23 #include "llvm/Target/TargetInstrInfo.h"
24 #include "llvm/Target/TargetSubtargetInfo.h"
25 
26 using namespace llvm;
27 
28 namespace {
29 struct XRayInstrumentation : public MachineFunctionPass {
30   static char ID;
31 
32   XRayInstrumentation() : MachineFunctionPass(ID) {
33     initializeXRayInstrumentationPass(*PassRegistry::getPassRegistry());
34   }
35 
36   bool runOnMachineFunction(MachineFunction &MF) override;
37 
38 private:
39   // Replace the original RET instruction with the exit sled code ("patchable
40   //   ret" pseudo-instruction), so that at runtime XRay can replace the sled
41   //   with a code jumping to XRay trampoline, which calls the tracing handler
42   //   and, in the end, issues the RET instruction.
43   // This is the approach to go on CPUs which have a single RET instruction,
44   //   like x86/x86_64.
45   void replaceRetWithPatchableRet(MachineFunction &MF,
46     const TargetInstrInfo *TII);
47   // Prepend the original return instruction with the exit sled code ("patchable
48   //   function exit" pseudo-instruction), preserving the original return
49   //   instruction just after the exit sled code.
50   // This is the approach to go on CPUs which have multiple options for the
51   //   return instruction, like ARM. For such CPUs we can't just jump into the
52   //   XRay trampoline and issue a single return instruction there. We rather
53   //   have to call the trampoline and return from it to the original return
54   //   instruction of the function being instrumented.
55   void prependRetWithPatchableExit(MachineFunction &MF,
56     const TargetInstrInfo *TII);
57 };
58 } // anonymous namespace
59 
60 void XRayInstrumentation::replaceRetWithPatchableRet(MachineFunction &MF,
61   const TargetInstrInfo *TII)
62 {
63   // We look for *all* terminators and returns, then replace those with
64   // PATCHABLE_RET instructions.
65   SmallVector<MachineInstr *, 4> Terminators;
66   for (auto &MBB : MF) {
67     for (auto &T : MBB.terminators()) {
68       unsigned Opc = 0;
69       if (T.isReturn() && T.getOpcode() == TII->getReturnOpcode()) {
70         // Replace return instructions with:
71         //   PATCHABLE_RET <Opcode>, <Operand>...
72         Opc = TargetOpcode::PATCHABLE_RET;
73       }
74       if (TII->isTailCall(T)) {
75         // Treat the tail call as a return instruction, which has a
76         // different-looking sled than the normal return case.
77         Opc = TargetOpcode::PATCHABLE_TAIL_CALL;
78       }
79       if (Opc != 0) {
80         auto MIB = BuildMI(MBB, T, T.getDebugLoc(), TII->get(Opc))
81                        .addImm(T.getOpcode());
82         for (auto &MO : T.operands())
83           MIB.addOperand(MO);
84         Terminators.push_back(&T);
85       }
86     }
87   }
88 
89   for (auto &I : Terminators)
90     I->eraseFromParent();
91 }
92 
93 void XRayInstrumentation::prependRetWithPatchableExit(MachineFunction &MF,
94   const TargetInstrInfo *TII)
95 {
96   for (auto &MBB : MF) {
97     for (auto &T : MBB.terminators()) {
98       if (T.isReturn()) {
99         // Prepend the return instruction with PATCHABLE_FUNCTION_EXIT
100         BuildMI(MBB, T, T.getDebugLoc(),
101                 TII->get(TargetOpcode::PATCHABLE_FUNCTION_EXIT));
102       }
103     }
104   }
105 }
106 
107 bool XRayInstrumentation::runOnMachineFunction(MachineFunction &MF) {
108   auto &F = *MF.getFunction();
109   auto InstrAttr = F.getFnAttribute("function-instrument");
110   bool AlwaysInstrument = !InstrAttr.hasAttribute(Attribute::None) &&
111                           InstrAttr.isStringAttribute() &&
112                           InstrAttr.getValueAsString() == "xray-always";
113   Attribute Attr = F.getFnAttribute("xray-instruction-threshold");
114   unsigned XRayThreshold = 0;
115   if (!AlwaysInstrument) {
116     if (Attr.hasAttribute(Attribute::None) || !Attr.isStringAttribute())
117       return false; // XRay threshold attribute not found.
118     if (Attr.getValueAsString().getAsInteger(10, XRayThreshold))
119       return false; // Invalid value for threshold.
120     if (F.size() < XRayThreshold)
121       return false; // Function is too small.
122   }
123 
124   auto &FirstMBB = *MF.begin();
125   auto &FirstMI = *FirstMBB.begin();
126 
127   if (!MF.getSubtarget().isXRaySupported()) {
128     FirstMI.emitError("An attempt to perform XRay instrumentation for an"
129       " unsupported target.");
130     return false;
131   }
132 
133   // FIXME: Do the loop triviality analysis here or in an earlier pass.
134 
135   // First, insert an PATCHABLE_FUNCTION_ENTER as the first instruction of the
136   // MachineFunction.
137   auto *TII = MF.getSubtarget().getInstrInfo();
138   BuildMI(FirstMBB, FirstMI, FirstMI.getDebugLoc(),
139           TII->get(TargetOpcode::PATCHABLE_FUNCTION_ENTER));
140 
141   switch (MF.getTarget().getTargetTriple().getArch()) {
142   case Triple::ArchType::arm:
143   case Triple::ArchType::thumb:
144     // For the architectures which don't have a single return instruction
145     prependRetWithPatchableExit(MF, TII);
146     break;
147   default:
148     // For the architectures that have a single return instruction (such as
149     //   RETQ on x86_64).
150     replaceRetWithPatchableRet(MF, TII);
151     break;
152   }
153   return true;
154 }
155 
156 char XRayInstrumentation::ID = 0;
157 char &llvm::XRayInstrumentationID = XRayInstrumentation::ID;
158 INITIALIZE_PASS(XRayInstrumentation, "xray-instrumentation", "Insert XRay ops",
159                 false, false)
160