Revert "[lld][WebAssembly] Retain data segments referenced via __start/__stop"

[kateinoigakukun]

Summary

To avoid making incur unnecessary size costs to metadata sections by
default.

This reverts commit dcf3368e33c3a01bd21b692d3be5dc1ecee587f4.

lld/test/wasm/gc-sections-startstop.s

-# RUN: llvm-mc -triple=wasm32-unknown-unknown -filetype=obj -o %t.o %s
-# RUN: wasm-ld %t.o -o %t.wasm
-# RUN: llvm-objdump -d --no-show-raw-insn %t.wasm | FileCheck %s
-
-# FOO_MD symbol is not used directly, but is referenced through __start/__stop_foo_md
-foo_md_size:
-  .functype foo_md_size () -> (i32)
-  i32.const __stop_foo_md
-  i32.const __start_foo_md
-  i32.sub
-  end_function
-
-# CHECK: <foo_md_size>:
-# CHECK-EMPTY:
-# CHECK-NEXT: i32.const [[#STOP_ADDR:]]
-# CHECK-NEXT: i32.const [[#STOP_ADDR - 4]]
-# CHECK-NEXT: i32.sub
-
-# All segments in concat_section section are marked as live.
-concat_section_size:
-  .functype concat_section_size () -> (i32)
-  i32.const __stop_concat_section
-  i32.const __start_concat_section
-  i32.sub
-  end_function
-
-# CHECK: <concat_section_size>:
-# CHECK-EMPTY:
-# CHECK-NEXT: i32.const [[#STOP_ADDR:]]
-# CHECK-NEXT: i32.const [[#STOP_ADDR - 8]]
-# CHECK-NEXT: i32.sub
-
-
-# __start/__stop symbols don't retain invalid C name sections
-invalid_name_section_size:
-  .functype invalid_name_section_size () -> (i32)
-  i32.const __stop_invalid.dot.name
-  i32.const __start_invalid.dot.name
-  i32.sub
-  end_function
-
-# CHECK: <invalid_name_section_size>:
-# CHECK-EMPTY:
-# CHECK-NEXT: i32.const 0
-# CHECK-NEXT: i32.const 0
-# CHECK-NEXT: i32.sub
-
-
-  .globl  _start
-_start:
-  .functype _start () -> ()
-  call  foo_md_size
-  drop
-  call  concat_section_size
-  drop
-  call  invalid_name_section_size
-  drop
-  end_function
-
-
-  .section  foo_md,"",@
-FOO_MD:
-  .asciz  "bar"
-  .size FOO_MD, 4
-
-  .size __start_foo_md, 4
-  .size __stop_foo_md, 4
-
-
-  .section  concat_section,"",@
-concat_segment_1:
-  .asciz  "xxx"
-  .size concat_segment_1, 4
-
-concat_segment_2:
-  .asciz  "yyy"
-  .size concat_segment_2, 4
-
-  .size __start_concat_section, 4
-  .size __stop_concat_section, 4
-
-
-  .section  invalid.dot.name,"",@
-invalid_name_section:
-  .asciz  "fizz"
-  .size invalid_name_section, 5
-
-  .weak __start_invalid.dot.name
-  .weak __stop_invalid.dot.name

lld/wasm/MarkLive.cpp

 //===----------------------------------------------------------------------===//
 
 #include "MarkLive.h"
 #include "Config.h"
 #include "InputChunks.h"
 #include "InputElement.h"
 #include "SymbolTable.h"
 #include "Symbols.h"
-#include "lld/Common/Strings.h"
 
 #define DEBUG_TYPE "lld"
 
 using namespace llvm;
 using namespace llvm::wasm;
 
 namespace lld {
 namespace wasm {
 
 namespace {
 
 class MarkLive {
 public:
   void run();
 
 private:
   void enqueue(Symbol *sym);
-  void enqueue(InputChunk *chunk);
   void enqueueInitFunctions(const ObjFile *sym);
   void mark();
   bool isCallCtorsLive();
 
   // A list of chunks to visit.
   SmallVector<InputChunk *, 256> queue;
 };
 
void MarkLive::enqueueInitFunctions(const ObjFile *obj) {
   const WasmLinkingData &l = obj->getWasmObj()->linkingData();
   for (const WasmInitFunc &f : l.InitFunctions) {
     auto *initSym = obj->getFunctionSymbol(f.Symbol);
     if (!initSym->isDiscarded())
       enqueue(initSym);
   }
 }
 
-void MarkLive::enqueue(InputChunk *chunk) {
-  LLVM_DEBUG(dbgs() << "markLive: " << chunk->getName() << "\n");
-  chunk->live = true;
-  queue.push_back(chunk);
-}
-
 void MarkLive::run() {
   // Add GC root symbols.
   if (!config->entry.empty())
     enqueue(symtab->find(config->entry));
 
   // We need to preserve any no-strip or exported symbol
   for (Symbol *sym : symtab->getSymbols())
     if (sym->isNoStrip() || sym->isExported())
       enqueue(sym);
 
   if (WasmSym::callDtors)
     enqueue(WasmSym::callDtors);
 
-  for (const ObjFile *obj : symtab->objectFiles) {
-    if (!obj->isLive()) {
-      continue;
-    }
-    // Enqueue constructors in objects explicitly live from the command-line.
+  for (const ObjFile *obj : symtab->objectFiles)
+    if (obj->isLive())
       enqueueInitFunctions(obj);
 
-    // Enqueue data segments referenced through __start/__stop symbols.
-    for (InputChunk *segment : obj->segments) {
-      auto name = segment->name;
-      if (!isValidCIdentifier(name))
-        continue;
-      if (symtab->find(("__start_" + name).str()) ||
-          symtab->find(("__stop_" + name).str())) {
-        enqueue(segment);
-      }
-    }
-  }
-
   mark();
 
   // If we have any non-discarded init functions, mark `__wasm_call_ctors` as
   // live so that we assign it an index and call it.
   if (isCallCtorsLive())
     WasmSym::callCtors->markLive();
 }