From 1da5f2dd2ec1b1eea67a25928668f7ea2bd67b96 Mon Sep 17 00:00:00 2001
From: Duncan Sands <baldrick@free.fr>
Date: Mon, 5 Jan 2009 20:37:33 +0000
Subject: [PATCH] Delete unused global aliases with internal linkage. In fact
 this also deletes those with linkonce linkage, however this is currently dead
 because for the moment aliases aren't allowed to have this linkage type.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@61742 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/Transforms/IPO/GlobalDCE.cpp              | 31 ++++++++++++++-----
 .../GlobalDCE/2009-01-05-DeadAliases.ll       |  8 +++++
 2 files changed, 31 insertions(+), 8 deletions(-)
 create mode 100644 test/Transforms/GlobalDCE/2009-01-05-DeadAliases.ll

diff --git a/lib/Transforms/IPO/GlobalDCE.cpp b/lib/Transforms/IPO/GlobalDCE.cpp
index 50bf85ba8ea..37ac94ae47e 100644
--- a/lib/Transforms/IPO/GlobalDCE.cpp
+++ b/lib/Transforms/IPO/GlobalDCE.cpp
@@ -25,6 +25,7 @@
 #include <set>
 using namespace llvm;
 
+STATISTIC(NumAliases, "Number of global aliases removed");
 STATISTIC(NumFunctions, "Number of functions removed");
 STATISTIC(NumVariables, "Number of global variables removed");
 
@@ -32,7 +33,7 @@ namespace {
   struct VISIBILITY_HIDDEN GlobalDCE : public ModulePass {
     static char ID; // Pass identification, replacement for typeid
     GlobalDCE() : ModulePass(&ID) {}
- 
+
     // run - Do the GlobalDCE pass on the specified module, optionally updating
     // the specified callgraph to reflect the changes.
     //
@@ -41,7 +42,7 @@ namespace {
   private:
     std::set<GlobalValue*> AliveGlobals;
 
-    /// GlobalIsNeeded - the specific global value as needed, and
+    /// GlobalIsNeeded - mark the specific global value as needed, and
     /// recursively mark anything that it uses as also needed.
     void GlobalIsNeeded(GlobalValue *GV);
     void MarkUsedGlobalsAsNeeded(Constant *C);
@@ -77,11 +78,12 @@ bool GlobalDCE::runOnModule(Module &M) {
       GlobalIsNeeded(I);
   }
 
-
   for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end();
        I != E; ++I) {
-    // Aliases are always needed even if they are not used.
-    MarkUsedGlobalsAsNeeded(I->getAliasee());
+    Changed |= RemoveUnusedGlobalValue(*I);
+    // Externally visible aliases are needed.
+    if (!I->hasInternalLinkage() && !I->hasLinkOnceLinkage())
+      GlobalIsNeeded(I);
   }
 
   // Now that all globals which are needed are in the AliveGlobals set, we loop
@@ -96,7 +98,6 @@ bool GlobalDCE::runOnModule(Module &M) {
       I->setInitializer(0);
     }
 
-
   // The second pass drops the bodies of functions which are dead...
   std::vector<Function*> DeadFunctions;
   for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
@@ -107,7 +108,7 @@ bool GlobalDCE::runOnModule(Module &M) {
     }
 
   if (!DeadFunctions.empty()) {
-    // Now that all interreferences have been dropped, delete the actual objects
+    // Now that all interferences have been dropped, delete the actual objects
     // themselves.
     for (unsigned i = 0, e = DeadFunctions.size(); i != e; ++i) {
       RemoveUnusedGlobalValue(*DeadFunctions[i]);
@@ -126,6 +127,17 @@ bool GlobalDCE::runOnModule(Module &M) {
     Changed = true;
   }
 
+  // Now delete any dead aliases.
+  for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end(); I != E;) {
+    Module::alias_iterator J = I++;
+    if (!AliveGlobals.count(J)) {
+      RemoveUnusedGlobalValue(*J);
+      M.getAliasList().erase(J);
+      ++NumAliases;
+      Changed = true;
+    }
+  }
+
   // Make sure that all memory is released
   AliveGlobals.clear();
   return Changed;
@@ -147,7 +159,10 @@ void GlobalDCE::GlobalIsNeeded(GlobalValue *G) {
     // referenced by the initializer to the alive set.
     if (GV->hasInitializer())
       MarkUsedGlobalsAsNeeded(GV->getInitializer());
-  } else if (!isa<GlobalAlias>(G)) {
+  } else if (GlobalAlias *GA = dyn_cast<GlobalAlias>(G)) {
+    // The target of a global alias is needed.
+    MarkUsedGlobalsAsNeeded(GA->getAliasee());
+  } else {
     // Otherwise this must be a function object.  We have to scan the body of
     // the function looking for constants and global values which are used as
     // operands.  Any operands of these types must be processed to ensure that
diff --git a/test/Transforms/GlobalDCE/2009-01-05-DeadAliases.ll b/test/Transforms/GlobalDCE/2009-01-05-DeadAliases.ll
new file mode 100644
index 00000000000..264b81dca03
--- /dev/null
+++ b/test/Transforms/GlobalDCE/2009-01-05-DeadAliases.ll
@@ -0,0 +1,8 @@
+; RUN: llvm-as < %s | opt -globaldce | llvm-dis | not grep @D
+; RUN: llvm-as < %s | opt -globaldce | llvm-dis | grep @L | count 3
+
+@A = global i32 0
+@D = alias internal i32* @A
+@L1 = alias i32* @A
+@L2 = alias internal i32* @L1
+@L3 = alias i32* @L2