[ThinLTO] Prevent importing of "llvm.used" values

Summary:
This patch prevents importing from (and therefore exporting from) any
module with a "llvm.used" local value. Local values need to be promoted
and renamed when importing, and their presense on the llvm.used variable
indicates that there are opaque uses that won't see the rename. One such
example is a use in inline assembly.

See also the discussion at:
http://lists.llvm.org/pipermail/llvm-dev/2016-April/098047.html

As part of this, move collectUsedGlobalVariables out of Transforms/Utils
and into IR/Module so that it can be used more widely. There are several
other places in LLVM that used copies of this code that can be cleaned
up as a follow on NFC patch.

Reviewers: joker.eph

Subscribers: pcc, llvm-commits, joker.eph

Differential Revision: http://reviews.llvm.org/D18986

llvm-svn: 266877

2 files changed, 20 insertions, 17 deletions

diff --git a/llvm/lib/Transforms/Utils/FunctionImportUtils.cpp b/llvm/lib/Transforms/Utils/FunctionImportUtils.cpp
index c0628b3bd49..f1d12bce06c 100644
--- a/llvm/lib/Transforms/Utils/FunctionImportUtils.cpp
+++ b/llvm/lib/Transforms/Utils/FunctionImportUtils.cpp
@@ -206,6 +206,26 @@ void FunctionImportGlobalProcessing::processGlobalForThinLTO(GlobalValue &GV) {
 }
 
 void FunctionImportGlobalProcessing::processGlobalsForThinLTO() {
+  // We cannot currently promote or rename anything that is in llvm.used,
+  // since any such value may have a use that won't see the new name.
+  // Specifically, any uses within inline assembly are not visible to the
+  // compiler. Prevent changing any such values on the exporting side,
+  // since we would already have guarded against an import from this module by
+  // suppressing its index generation. See comments on what is required
+  // in order to implement a finer grained solution in
+  // ModuleSummaryIndexBuilder::ModuleSummaryIndexBuilder().
+  SmallPtrSet<GlobalValue *, 8> Used;
+  collectUsedGlobalVariables(M, Used, /*CompilerUsed*/ false);
+  for (GlobalValue *V : Used) {
+    if (!V->hasLocalLinkage())
+      continue;
+    // We would have blocked importing from this module by suppressing index
+    // generation.
+    assert(!isPerformingImport() &&
+           "Should have blocked importing from module with local used");
+    return;
+  }
+
   for (GlobalVariable &GV : M.globals())
     processGlobalForThinLTO(GV);
   for (Function &SF : M)
diff --git a/llvm/lib/Transforms/Utils/ModuleUtils.cpp b/llvm/lib/Transforms/Utils/ModuleUtils.cpp
index 4d73a6bf037..eb918851862 100644
--- a/llvm/lib/Transforms/Utils/ModuleUtils.cpp
+++ b/llvm/lib/Transforms/Utils/ModuleUtils.cpp
@@ -12,7 +12,6 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/Transforms/Utils/ModuleUtils.h"
-#include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/IR/DerivedTypes.h"
 #include "llvm/IR/Function.h"
 #include "llvm/IR/IRBuilder.h"
@@ -90,22 +89,6 @@ void llvm::appendToGlobalDtors(Module &M, Function *F, int Priority, Constant *D
   appendToGlobalArray("llvm.global_dtors", M, F, Priority, Data);
 }
 
-GlobalVariable *
-llvm::collectUsedGlobalVariables(Module &M, SmallPtrSetImpl<GlobalValue *> &Set,
-                                 bool CompilerUsed) {
-  const char *Name = CompilerUsed ? "llvm.compiler.used" : "llvm.used";
-  GlobalVariable *GV = M.getGlobalVariable(Name);
-  if (!GV || !GV->hasInitializer())
-    return GV;
-
-  const ConstantArray *Init = cast<ConstantArray>(GV->getInitializer());
-  for (Value *Op : Init->operands()) {
-    GlobalValue *G = cast<GlobalValue>(Op->stripPointerCastsNoFollowAliases());
-    Set.insert(G);
-  }
-  return GV;
-}
-
 Function *llvm::checkSanitizerInterfaceFunction(Constant *FuncOrBitcast) {
   if (isa<Function>(FuncOrBitcast))
     return cast<Function>(FuncOrBitcast);