1 //===- MarkLive.cpp -------------------------------------------------------===//
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 #include "COFFLinkerContext.h"
10 #include "Chunks.h"
11 #include "Symbols.h"
12 #include "lld/Common/Timer.h"
13 #include "llvm/ADT/STLExtras.h"
14 #include <vector>
15 
16 namespace lld::coff {
17 
18 // Set live bit on for each reachable chunk. Unmarked (unreachable)
19 // COMDAT chunks will be ignored by Writer, so they will be excluded
20 // from the final output.
21 void markLive(COFFLinkerContext &ctx) {
22   ScopedTimer t(ctx.gcTimer);
23 
24   // We build up a worklist of sections which have been marked as live. We only
25   // push into the worklist when we discover an unmarked section, and we mark
26   // as we push, so sections never appear twice in the list.
27   SmallVector<SectionChunk *, 256> worklist;
28 
29   // COMDAT section chunks are dead by default. Add non-COMDAT chunks. Do not
30   // traverse DWARF sections. They are live, but they should not keep other
31   // sections alive.
32   for (Chunk *c : ctx.symtab.getChunks())
33     if (auto *sc = dyn_cast<SectionChunk>(c))
34       if (sc->live && !sc->isDWARF())
35         worklist.push_back(sc);
36 
37   auto enqueue = [&](SectionChunk *c) {
38     if (c->live)
39       return;
40     c->live = true;
41     worklist.push_back(c);
42   };
43 
44   auto addSym = [&](Symbol *b) {
45     if (auto *sym = dyn_cast<DefinedRegular>(b))
46       enqueue(sym->getChunk());
47     else if (auto *sym = dyn_cast<DefinedImportData>(b))
48       sym->file->live = true;
49     else if (auto *sym = dyn_cast<DefinedImportThunk>(b))
50       sym->wrappedSym->file->live = sym->wrappedSym->file->thunkLive = true;
51   };
52 
53   // Add GC root chunks.
54   for (Symbol *b : ctx.config.gcroot)
55     addSym(b);
56 
57   while (!worklist.empty()) {
58     SectionChunk *sc = worklist.pop_back_val();
59     assert(sc->live && "We mark as live when pushing onto the worklist!");
60 
61     // Mark all symbols listed in the relocation table for this section.
62     for (Symbol *b : sc->symbols())
63       if (b)
64         addSym(b);
65 
66     // Mark associative sections if any.
67     for (SectionChunk &c : sc->children())
68       enqueue(&c);
69   }
70 }
71 }
72