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 "llvm/Support/TimeProfiler.h"
  15 #include <vector>
  16
  17 namespace lld::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   llvm::TimeTraceScope timeScope("Mark live");
  24   ScopedTimer t(ctx.gcTimer);
  25
  26   // We build up a worklist of sections which have been marked as live. We only
  27   // push into the worklist when we discover an unmarked section, and we mark
  28   // as we push, so sections never appear twice in the list.
  29   SmallVector<SectionChunk *, 256> worklist;
  30
  31   // COMDAT section chunks are dead by default. Add non-COMDAT chunks. Do not
  32   // traverse DWARF sections. They are live, but they should not keep other
  33   // sections alive.
  34   for (Chunk *c : ctx.symtab.getChunks())
  35     if (auto *sc = dyn_cast<SectionChunk>(c))
  36       if (sc->live && !sc->isDWARF())
  37         worklist.push_back(sc);
  38
  39   auto enqueue = [&](SectionChunk *c) {
  40     if (c->live)
  41       return;
  42     c->live = true;
  43     worklist.push_back(c);
  44   };
  45
  46   std::function<void(Symbol *)> addSym;
  47
  48   auto addImportFile = [&](ImportFile *file) {
  49     file->live = true;
  50     if (file->impchkThunk && file->impchkThunk->exitThunk)
  51       addSym(file->impchkThunk->exitThunk);
  52   };
  53
  54   addSym = [&](Symbol *b) {
  55     if (auto *sym = dyn_cast<DefinedRegular>(b)) {
  56       enqueue(sym->getChunk());
  57     } else if (auto *sym = dyn_cast<DefinedImportData>(b)) {
  58       addImportFile(sym->file);
  59     } else if (auto *sym = dyn_cast<DefinedImportThunk>(b)) {
  60       addImportFile(sym->wrappedSym->file);
  61       sym->getChunk()->live = true;
  62     }
  63   };
  64
  65   // Add GC root chunks.
  66   for (Symbol *b : ctx.config.gcroot)
  67     addSym(b);
  68
  69   while (!worklist.empty()) {
  70     SectionChunk *sc = worklist.pop_back_val();
  71     assert(sc->live && "We mark as live when pushing onto the worklist!");
  72
  73     // Mark all symbols listed in the relocation table for this section.
  74     for (Symbol *b : sc->symbols())
  75       if (b)
  76         addSym(b);
  77
  78     // Mark associative sections if any.
  79     for (SectionChunk &c : sc->children())
  80       enqueue(&c);
  81
  82     // Mark EC entry thunks.
  83     if (Defined *entryThunk = sc->getEntryThunk())
  84       addSym(entryThunk);
  85   }
  86 }
  87 }