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 {
  17 namespace coff {
  18
  19 // Set live bit on for each reachable chunk. Unmarked (unreachable)
  20 // COMDAT chunks will be ignored by Writer, so they will be excluded
  21 // from the final output.
  22 void markLive(COFFLinkerContext &ctx) {
  23   ScopedTimer t(ctx.gcTimer);
  24
  25   // We build up a worklist of sections which have been marked as live. We only
  26   // push into the worklist when we discover an unmarked section, and we mark
  27   // as we push, so sections never appear twice in the list.
  28   SmallVector<SectionChunk *, 256> worklist;
  29
  30   // COMDAT section chunks are dead by default. Add non-COMDAT chunks. Do not
  31   // traverse DWARF sections. They are live, but they should not keep other
  32   // sections alive.
  33   for (Chunk *c : ctx.symtab.getChunks())
  34     if (auto *sc = dyn_cast<SectionChunk>(c))
  35       if (sc->live && !sc->isDWARF())
  36         worklist.push_back(sc);
  37
  38   auto enqueue = [&](SectionChunk *c) {
  39     if (c->live)
  40       return;
  41     c->live = true;
  42     worklist.push_back(c);
  43   };
  44
  45   auto addSym = [&](Symbol *b) {
  46     if (auto *sym = dyn_cast<DefinedRegular>(b))
  47       enqueue(sym->getChunk());
  48     else if (auto *sym = dyn_cast<DefinedImportData>(b))
  49       sym->file->live = true;
  50     else if (auto *sym = dyn_cast<DefinedImportThunk>(b))
  51       sym->wrappedSym->file->live = sym->wrappedSym->file->thunkLive = true;
  52   };
  53
  54   // Add GC root chunks.
  55   for (Symbol *b : config->gcroot)
  56     addSym(b);
  57
  58   while (!worklist.empty()) {
  59     SectionChunk *sc = worklist.pop_back_val();
  60     assert(sc->live && "We mark as live when pushing onto the worklist!");
  61
  62     // Mark all symbols listed in the relocation table for this section.
  63     for (Symbol *b : sc->symbols())
  64       if (b)
  65         addSym(b);
  66
  67     // Mark associative sections if any.
  68     for (SectionChunk &c : sc->children())
  69       enqueue(&c);
  70   }
  71 }
  72 }
  73 }