1 //===-- WebAssemblyDebugFixup.cpp - Debug Fixup ------------------===//
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 /// \file
10 /// Several prior passes may "stackify" registers, here we ensure any references
11 /// in such registers in debug_value instructions become stack relative also.
12 /// This is done in a separate pass such that not all previous passes need to
13 /// track stack depth when values get stackified.
14 ///
15 //===----------------------------------------------------------------------===//
16 
17 #include "MCTargetDesc/WebAssemblyMCTargetDesc.h"
18 #include "Utils/WebAssemblyUtilities.h"
19 #include "WebAssembly.h"
20 #include "WebAssemblyMachineFunctionInfo.h"
21 #include "WebAssemblySubtarget.h"
22 #include "llvm/ADT/SCCIterator.h"
23 #include "llvm/CodeGen/MachineFrameInfo.h"
24 #include "llvm/CodeGen/MachineFunction.h"
25 #include "llvm/CodeGen/MachineInstrBuilder.h"
26 #include "llvm/CodeGen/MachineLoopInfo.h"
27 #include "llvm/CodeGen/MachineRegisterInfo.h"
28 #include "llvm/CodeGen/Passes.h"
29 #include "llvm/Support/Debug.h"
30 #include "llvm/Support/raw_ostream.h"
31 using namespace llvm;
32 
33 #define DEBUG_TYPE "wasm-debug-fixup"
34 
35 namespace {
36 class WebAssemblyDebugFixup final : public MachineFunctionPass {
37   StringRef getPassName() const override { return "WebAssembly Debug Fixup"; }
38 
39   void getAnalysisUsage(AnalysisUsage &AU) const override {
40     AU.setPreservesCFG();
41     MachineFunctionPass::getAnalysisUsage(AU);
42   }
43 
44   bool runOnMachineFunction(MachineFunction &MF) override;
45 
46 public:
47   static char ID; // Pass identification, replacement for typeid
48   WebAssemblyDebugFixup() : MachineFunctionPass(ID) {}
49 };
50 } // end anonymous namespace
51 
52 char WebAssemblyDebugFixup::ID = 0;
53 INITIALIZE_PASS(
54     WebAssemblyDebugFixup, DEBUG_TYPE,
55     "Ensures debug_value's that have been stackified become stack relative",
56     false, false)
57 
58 FunctionPass *llvm::createWebAssemblyDebugFixup() {
59   return new WebAssemblyDebugFixup();
60 }
61 
62 // At this very end of the compilation pipeline, if any DBG_VALUEs with
63 // registers remain, it means they are dangling info which we failed to update
64 // when their corresponding def instruction was transformed/moved/splitted etc.
65 // Because Wasm cannot access values in LLVM virtual registers in the debugger,
66 // these dangling DBG_VALUEs in effect kill the effect of any previous DBG_VALUE
67 // associated with the variable, which will appear as "optimized out".
68 static void setDanglingDebugValuesUndef(MachineBasicBlock &MBB,
69                                         const TargetInstrInfo *TII) {
70   for (auto &MI : llvm::make_early_inc_range(MBB)) {
71     if (MI.isDebugValue() && MI.getDebugOperand(0).isReg() &&
72         !MI.isUndefDebugValue()) {
73       LLVM_DEBUG(dbgs() << "Warning: dangling DBG_VALUE set to undef: " << MI
74                         << "\n");
75       MI.setDebugValueUndef();
76     }
77   }
78 }
79 
80 bool WebAssemblyDebugFixup::runOnMachineFunction(MachineFunction &MF) {
81   LLVM_DEBUG(dbgs() << "********** Debug Fixup **********\n"
82                        "********** Function: "
83                     << MF.getName() << '\n');
84 
85   WebAssemblyFunctionInfo &MFI = *MF.getInfo<WebAssemblyFunctionInfo>();
86   const auto *TII = MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
87 
88   struct StackElem {
89     unsigned Reg;
90     MachineInstr *DebugValue;
91   };
92   std::vector<StackElem> Stack;
93   for (MachineBasicBlock &MBB : MF) {
94     // We may insert into this list.
95     for (auto MII = MBB.begin(); MII != MBB.end(); ++MII) {
96       MachineInstr &MI = *MII;
97       if (MI.isDebugValue()) {
98         auto &MO = MI.getOperand(0);
99         // Also check if not a $noreg: likely a DBG_VALUE we just inserted.
100         if (MO.isReg() && MO.getReg().isValid() &&
101             MFI.isVRegStackified(MO.getReg())) {
102           // Found a DBG_VALUE with a stackified register we will
103           // change into a stack operand.
104           // Search for register rather than assume it is on top (which it
105           // typically is if it appears right after the def), since
106           // DBG_VALUE's may shift under some circumstances.
107           for (auto &Elem : reverse(Stack)) {
108             if (MO.getReg() == Elem.Reg) {
109               auto Depth = static_cast<unsigned>(&Elem - &Stack[0]);
110               LLVM_DEBUG(dbgs() << "Debug Value VReg " << MO.getReg()
111                                 << " -> Stack Relative " << Depth << "\n");
112               MO.ChangeToTargetIndex(WebAssembly::TI_OPERAND_STACK, Depth);
113               // Save the DBG_VALUE instruction that defined this stackified
114               // variable since later we need it to construct another one on
115               // pop.
116               Elem.DebugValue = &MI;
117               break;
118             }
119           }
120           // If the Reg was not found, we have a DBG_VALUE outside of its
121           // def-use range, and we leave it unmodified as reg, which means
122           // it will be culled later.
123         }
124       } else {
125         // Track stack depth.
126         for (MachineOperand &MO : reverse(MI.explicit_uses())) {
127           if (MO.isReg() && MFI.isVRegStackified(MO.getReg())) {
128             auto Prev = Stack.back();
129             Stack.pop_back();
130             assert(Prev.Reg == MO.getReg() &&
131                    "WebAssemblyDebugFixup: Pop: Register not matched!");
132             // We should not put a DBG_VALUE after a terminator; debug ranges
133             // are terminated at the end of a BB anyway.
134             if (Prev.DebugValue && !MI.isTerminator()) {
135               // This stackified reg is a variable that started life at
136               // Prev.DebugValue, so now that we're popping it we must insert
137               // a $noreg DBG_VALUE for the variable to end it, right after
138               // the current instruction.
139               BuildMI(*Prev.DebugValue->getParent(), std::next(MII),
140                       Prev.DebugValue->getDebugLoc(),
141                       TII->get(WebAssembly::DBG_VALUE), false, Register(),
142                       Prev.DebugValue->getOperand(2).getMetadata(),
143                       Prev.DebugValue->getOperand(3).getMetadata());
144             }
145           }
146         }
147         for (MachineOperand &MO : MI.defs()) {
148           if (MO.isReg() && MFI.isVRegStackified(MO.getReg())) {
149             Stack.push_back({MO.getReg(), nullptr});
150           }
151         }
152       }
153     }
154     assert(Stack.empty() &&
155            "WebAssemblyDebugFixup: Stack not empty at end of basic block!");
156 
157     setDanglingDebugValuesUndef(MBB, TII);
158   }
159 
160   return true;
161 }
162