lld/COFF/MarkLive.cpp

   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 }