[GlobalOpt] Dead Eliminate declarations

GlobalOpt is already dead-code-eliminating global definitions. With
this change it also takes care of declarations.
Hopefully this should make it now a strict superset of GlobalDCE.
This is important for LTO/ThinLTO as we don't want the linker to see
"undefined reference" when it processes the input files: it could
prevent proper internalization (or even load an extra file from a
static archive, changing the behavior of the program!).

llvm-svn: 281653

diff --git a/llvm/lib/Transforms/IPO/GlobalOpt.cpp b/llvm/lib/Transforms/IPO/GlobalOpt.cpp
index 52c1cc5..f1686a0 100644 (file)
--- a/llvm/lib/Transforms/IPO/GlobalOpt.cpp
+++ b/llvm/lib/Transforms/IPO/GlobalOpt.cpp
@@ -1653,7 +1653,7 @@ static bool deleteIfDead(GlobalValue &GV,
                          SmallSet<const Comdat *, 8> &NotDiscardableComdats) {
   GV.removeDeadConstantUsers();
 
-  if (!GV.isDiscardableIfUnused())
+  if (!GV.isDiscardableIfUnused() && !GV.isDeclaration())
     return false;
 
   if (const Comdat *C = GV.getComdat())
@@ -1662,7 +1662,7 @@ static bool deleteIfDead(GlobalValue &GV,
 
   bool Dead;
   if (auto *F = dyn_cast<Function>(&GV))
-    Dead = F->isDefTriviallyDead();
+    Dead = (F->isDeclaration() && F->use_empty()) || F->isDefTriviallyDead();
   else
     Dead = GV.use_empty();
   if (!Dead)

diff --git a/llvm/test/Transforms/GlobalOpt/deaddeclaration.ll b/llvm/test/Transforms/GlobalOpt/deaddeclaration.ll
new file mode 100644 (file)
index 0000000..942f2e1
--- /dev/null
+++ b/llvm/test/Transforms/GlobalOpt/deaddeclaration.ll
@@ -0,0 +1,7 @@
+; RUN: opt < %s -globalopt -S | FileCheck %s
+
+; CHECK-NOT: aa
+; CHECK-NOT: bb
+
+declare void @aa()
+@bb = external global i8