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 "WebAssembly.h"
19#include "WebAssemblyMachineFunctionInfo.h"
20#include "WebAssemblySubtarget.h"
21#include "WebAssemblyUtilities.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"
31using namespace llvm;
32
33#define DEBUG_TYPE "wasm-debug-fixup"
34
35namespace {
36class 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
46public:
47  static char ID; // Pass identification, replacement for typeid
48  WebAssemblyDebugFixup() : MachineFunctionPass(ID) {}
49};
50} // end anonymous namespace
51
52char WebAssemblyDebugFixup::ID = 0;
53INITIALIZE_PASS(
54    WebAssemblyDebugFixup, DEBUG_TYPE,
55    "Ensures debug_value's that have been stackified become stack relative",
56    false, false)
57
58FunctionPass *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".
68static 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
80bool 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