[ThinLTO] Convert dead alias to declarations

Summary:
This complements the fixes in r323633 and r324075 which drop the
definitions of dead functions and variables, respectively.

Fixes PR36208.

Reviewers: grimar, rafael

Subscribers: mehdi_amini, llvm-commits, inglorion

Differential Revision: https://reviews.llvm.org/D42856

llvm-svn: 324242

include/llvm/Transforms/IPO/FunctionImport.h

@@ -122,8 +122,9 @@ void computeDeadSymbols(
     const DenseSet<GlobalValue::GUID> &GUIDPreservedSymbols,
     function_ref<PrevailingType(GlobalValue::GUID)> isPrevailing);
 
-/// Converts value \p GV to declaration.
+/// Converts value \p GV to declaration, or replaces with a declaration if
-void convertToDeclaration(GlobalValue &GV);
+/// it is an alias. Returns true if converted, false if replaced.
+bool convertToDeclaration(GlobalValue &GV);
 
 /// Compute the set of summaries needed for a ThinLTO backend compilation of
 /// \p ModulePath.

lib/LTO/LTOBackend.cpp

@@ -401,18 +401,14 @@ Error lto::backend(Config &C, AddStreamFn AddStream,
 
 static void dropDeadSymbols(Module &Mod, const GVSummaryMapTy &DefinedGlobals,
                             const ModuleSummaryIndex &Index) {
-  auto MaybeDrop = [&](GlobalValue &GV) {
+  std::vector<GlobalValue *> ReplacedGlobals;
+  for (auto &GV : Mod.global_values())
     if (GlobalValueSummary *GVS = DefinedGlobals.lookup(GV.getGUID()))
-      if (!Index.isGlobalValueLive(GVS))
+      if (!Index.isGlobalValueLive(GVS) && !convertToDeclaration(GV))
-        convertToDeclaration(GV);
+        ReplacedGlobals.push_back(&GV);
-  };
 
-  // Process functions and global now.
+  for (GlobalValue *GV : ReplacedGlobals)
-  // FIXME: add support for aliases (needs support in convertToDeclaration).
+    GV->eraseFromParent();
-  for (auto &GV : Mod)
-    MaybeDrop(GV);
-  for (auto &GV : Mod.globals())
-    MaybeDrop(GV);
 }
 
 Error lto::thinBackend(Config &Conf, unsigned Task, AddStreamFn AddStream,

lib/Transforms/IPO/FunctionImport.cpp

@@ -613,7 +613,7 @@ llvm::EmitImportsFiles(StringRef ModulePath, StringRef OutputFilename,
   return std::error_code();
 }
 
-void llvm::convertToDeclaration(GlobalValue &GV) {
+bool llvm::convertToDeclaration(GlobalValue &GV) {
   DEBUG(dbgs() << "Converting to a declaration: `" << GV.getName() << "\n");
   if (Function *F = dyn_cast<Function>(&GV)) {
     F->deleteBody();
@@ -624,14 +624,24 @@ void llvm::convertToDeclaration(GlobalValue &GV) {
     V->setLinkage(GlobalValue::ExternalLinkage);
     V->clearMetadata();
     V->setComdat(nullptr);
-  } else
+  } else {
-    // For now we don't resolve or drop aliases. Once we do we'll
+    GlobalValue *NewGV;
-    // need to add support here for creating either a function or
+    if (GV.getValueType()->isFunctionTy())
-    // variable declaration, and return the new GlobalValue* for
+      NewGV =
-    // the caller to use.
+          Function::Create(cast<FunctionType>(GV.getValueType()),
-    // Support of dropping aliases is required for correct dead code
+                           GlobalValue::ExternalLinkage, "", GV.getParent());
-    // elimination performed in thin LTO backends (see 'dropDeadSymbols').
+    else
-    llvm_unreachable("Expected function or variable");
+      NewGV =
+          new GlobalVariable(*GV.getParent(), GV.getValueType(),
+                             /*isConstant*/ false, GlobalValue::ExternalLinkage,
+                             /*init*/ nullptr, "",
+                             /*insertbefore*/ nullptr, GV.getThreadLocalMode(),
+                             GV.getType()->getAddressSpace());
+    NewGV->takeName(&GV);
+    GV.replaceAllUsesWith(NewGV);
+    return false;
+  }
+  return true;
 }
 
 /// Fixup WeakForLinker linkages in \p TheModule based on summary analysis.
@@ -666,9 +676,13 @@ void llvm::thinLTOResolveWeakForLinkerModule(
     // interposable property and possibly get inlined. Simply drop
     // the definition in that case.
     if (GlobalValue::isAvailableExternallyLinkage(NewLinkage) &&
-        GlobalValue::isInterposableLinkage(GV.getLinkage()))
+        GlobalValue::isInterposableLinkage(GV.getLinkage())) {
-      convertToDeclaration(GV);
+      if (!convertToDeclaration(GV))
-    else {
+        // FIXME: Change this to collect replaced GVs and later erase
+        // them from the parent module once thinLTOResolveWeakForLinkerGUID is
+        // changed to enable this for aliases.
+        llvm_unreachable("Expected GV to be converted");
+    } else {
       DEBUG(dbgs() << "ODR fixing up linkage for `" << GV.getName() << "` from "
                    << GV.getLinkage() << " to " << NewLinkage << "\n");
       GV.setLinkage(NewLinkage);

test/LTO/Resolution/X86/not-prevailing-alias.ll

@@ -0,0 +1,43 @@
+; Test to ensure that dead alias are dropped by converting to a declaration
+; RUN: opt -module-summary %s -o %t1.bc
+; RUN: llvm-lto2 run %t1.bc -r %t1.bc,barAlias,x \
+; RUN:   -r %t1.bc,bar,x -r %t1.bc,zed,px \
+; RUN:   -r %t1.bc,var,x -r %t1.bc,varAlias,x \
+; RUN:   -o %t2.o -save-temps
+
+; Check that bar and barAlias were dropped to declarations
+; RUN: llvm-dis %t2.o.1.1.promote.bc -o - | FileCheck %s --check-prefix=DROP
+; DROP-DAG: declare void @bar()
+; DROP-DAG: declare void @barAlias()
+; DROP-DAG: @var = external global i32
+; DROP-DAG: @varAlias = external global i32
+
+; Check that 'barAlias' and 'varAlias' were not inlined.
+; RUN: llvm-objdump -d %t2.o.1 | FileCheck %s
+; CHECK:      zed:
+; CHECK-NEXT:  {{.*}}  pushq
+; CHECK-NEXT:  {{.*}}  callq   0
+; CHECK-NEXT:   movq  (%rip), %rax
+
+; Check that 'barAlias' and 'varAlias' produced as undefined.
+; RUN: llvm-readelf --symbols %t2.o.1 | FileCheck %s --check-prefix=SYMBOLS
+; SYMBOLS: NOTYPE  GLOBAL DEFAULT  UND barAlias
+; SYMBOLS: NOTYPE  GLOBAL DEFAULT  UND varAlias
+; SYMBOLS: FUNC    GLOBAL DEFAULT    2 zed
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+@barAlias = alias void(), void()* @bar
+define void @bar() {
+  ret void
+}
+
+@var = global i32 99
+@varAlias = alias i32, i32* @var
+
+define i32 @zed() {
+  call void @barAlias()
+  %1 = load i32, i32* @varAlias, align 4
+  ret i32 %1
+}