[asan] Fix dead stripping of globals on Linux.

Use a combination of !associated, comdat, @llvm.compiler.used and
custom sections to allow dead stripping of globals and their asan
metadata. Sometimes.

Currently this works on LLD, which supports SHF_LINK_ORDER with
sh_link pointing to the associated section.

This also works on BFD, which seems to treat comdats as
all-or-nothing with respect to linker GC. There is a weird quirk
where the "first" global in each link is never GC-ed because of the
section symbols.

At this moment it does not work on Gold (as in the globals are never
stripped).

This is a second re-land of r298158. This time, this feature is
limited to -fdata-sections builds.

llvm-svn: 301587

1 files changed, 2 insertions, 33 deletions

diff --git a/llvm/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp b/llvm/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp
index 9801a0a6141..d3a3c24ce7b 100644
--- a/llvm/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp
+++ b/llvm/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp
@@ -30,42 +30,11 @@
 #include "llvm/Transforms/IPO.h"
 #include "llvm/Transforms/IPO/FunctionAttrs.h"
 #include "llvm/Transforms/Utils/Cloning.h"
+#include "llvm/Transforms/Utils/ModuleUtils.h"
 using namespace llvm;
 
 namespace {
 
-// Produce a unique identifier for this module by taking the MD5 sum of the
-// names of the module's strong external symbols. This identifier is
-// normally guaranteed to be unique, or the program would fail to link due to
-// multiply defined symbols.
-//
-// If the module has no strong external symbols (such a module may still have a
-// semantic effect if it performs global initialization), we cannot produce a
-// unique identifier for this module, so we return the empty string, which
-// causes the entire module to be written as a regular LTO module.
-std::string getModuleId(Module *M) {
-  MD5 Md5;
-  bool ExportsSymbols = false;
-  for (auto &GV : M->global_values()) {
-    if (GV.isDeclaration() || GV.getName().startswith("llvm.") ||
-        !GV.hasExternalLinkage())
-      continue;
-    ExportsSymbols = true;
-    Md5.update(GV.getName());
-    Md5.update(ArrayRef<uint8_t>{0});
-  }
-
-  if (!ExportsSymbols)
-    return "";
-
-  MD5::MD5Result R;
-  Md5.final(R);
-
-  SmallString<32> Str;
-  MD5::stringifyResult(R, Str);
-  return ("$" + Str).str();
-}
-
 // Promote each local-linkage entity defined by ExportM and used by ImportM by
 // changing visibility and appending the given ModuleId.
 void promoteInternals(Module &ExportM, Module &ImportM, StringRef ModuleId) {
@@ -251,7 +220,7 @@ void forEachVirtualFunction(Constant *C, function_ref<void(Function *)> Fn) {
 void splitAndWriteThinLTOBitcode(
     raw_ostream &OS, raw_ostream *ThinLinkOS,
     function_ref<AAResults &(Function &)> AARGetter, Module &M) {
-  std::string ModuleId = getModuleId(&M);
+  std::string ModuleId = getUniqueModuleId(&M);
   if (ModuleId.empty()) {
     // We couldn't generate a module ID for this module, just write it out as a
     // regular LTO module.