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(Range&: MBB)) {
71 if (MI.isDebugValue() && MI.getDebugOperand(Index: 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(i: 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(VReg: 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(C&: 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(Idx: WebAssembly::TI_OPERAND_STACK, Offset: 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(C: MI.explicit_uses())) {
127 if (MO.isReg() && MFI.isVRegStackified(VReg: 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(BB&: *Prev.DebugValue->getParent(), I: std::next(x: MII),
140 DL: Prev.DebugValue->getDebugLoc(),
141 MCID: TII->get(Opcode: WebAssembly::DBG_VALUE), IsIndirect: false, Reg: Register(),
142 Variable: Prev.DebugValue->getOperand(i: 2).getMetadata(),
143 Expr: Prev.DebugValue->getOperand(i: 3).getMetadata());
144 }
145 }
146 }
147 for (MachineOperand &MO : MI.defs()) {
148 if (MO.isReg() && MFI.isVRegStackified(VReg: MO.getReg())) {
149 Stack.push_back(x: {.Reg: MO.getReg(), .DebugValue: 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