From 91e4ba2f882855310209b95350b9675ccd1a4a06 Mon Sep 17 00:00:00 2001
From: Pete Cooper <peter_cooper@apple.com>
Date: Tue, 8 Jul 2014 17:06:03 +0000
Subject: [PATCH] Revert "GlobalDCE: Delete available_externally initializers
 if it allows removing the value the initializer is referring to."

This reverts commit 5b55a47e94e28fbb56d0cd5d72c3db9105c15b4c.

A test case was found to crash after this was applied.  I'll file a bug to track fixing this with the test case needed.

llvm-svn: 212550
---
 llvm/lib/Transforms/IPO/GlobalDCE.cpp              | 43 ++--------------------
 .../Transforms/GlobalDCE/externally_available.ll   | 35 +-----------------
 2 files changed, 4 insertions(+), 74 deletions(-)

diff --git a/llvm/lib/Transforms/IPO/GlobalDCE.cpp b/llvm/lib/Transforms/IPO/GlobalDCE.cpp
index cdad155..7e7a4c0 100644
--- a/llvm/lib/Transforms/IPO/GlobalDCE.cpp
+++ b/llvm/lib/Transforms/IPO/GlobalDCE.cpp
@@ -44,19 +44,14 @@ namespace {
     bool runOnModule(Module &M) override;
 
   private:
-    SmallPtrSet<Constant *, 32> AliveGlobals;
+    SmallPtrSet<GlobalValue*, 32> AliveGlobals;
     SmallPtrSet<Constant *, 8> SeenConstants;
-    SmallPtrSet<GlobalVariable *, 8> DiscardableGlobalInitializers;
 
     /// 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);
 
-    /// \brief Checks if C is alive or is a ConstantExpr that refers to an alive
-    /// value.
-    bool ContainsUsedGlobal(Constant *C);
-
     bool RemoveUnusedGlobalValue(GlobalValue &GV);
   };
 }
@@ -167,19 +162,6 @@ bool GlobalDCE::runOnModule(Module &M) {
       I->setAliasee(nullptr);
     }
 
-  // Look for available externally constants that we can turn into normal
-  // externals by deleting their initalizers. This allows us to remove other
-  // globals that are referenced by the initializer.
-  if (!DiscardableGlobalInitializers.empty()) {
-    for (GlobalVariable *GV : DiscardableGlobalInitializers) {
-      if (!ContainsUsedGlobal(GV->getInitializer())) {
-        GV->setInitializer(nullptr);
-        GV->setLinkage(GlobalValue::ExternalLinkage);
-        Changed = true;
-      }
-    }
-  }
-
   if (!DeadFunctions.empty()) {
     // Now that all interferences have been dropped, delete the actual objects
     // themselves.
@@ -227,12 +209,8 @@ void GlobalDCE::GlobalIsNeeded(GlobalValue *G) {
   if (GlobalVariable *GV = dyn_cast<GlobalVariable>(G)) {
     // If this is a global variable, we must make sure to add any global values
     // referenced by the initializer to the alive set.
-    if (GV->hasInitializer()) {
-      if (GV->hasAvailableExternallyLinkage())
-        DiscardableGlobalInitializers.insert(GV);
-      else
-        MarkUsedGlobalsAsNeeded(GV->getInitializer());
-    }
+    if (GV->hasInitializer())
+      MarkUsedGlobalsAsNeeded(GV->getInitializer());
   } else if (GlobalAlias *GA = dyn_cast<GlobalAlias>(G)) {
     // The target of a global alias is needed.
     MarkUsedGlobalsAsNeeded(GA->getAliasee());
@@ -270,21 +248,6 @@ void GlobalDCE::MarkUsedGlobalsAsNeeded(Constant *C) {
   }
 }
 
-bool GlobalDCE::ContainsUsedGlobal(Constant *C) {
-  // C contains a used global If C is alive or we visited it while marking
-  // values alive.
-  if (AliveGlobals.count(C) || SeenConstants.count(C))
-    return true;
-
-  // Now check all operands of a ConstantExpr.
-  for (User::op_iterator I = C->op_begin(), E = C->op_end(); I != E; ++I) {
-    Constant *Op = dyn_cast<Constant>(*I);
-    if (Op && ContainsUsedGlobal(Op))
-      return true;
-  }
-  return false;
-}
-
 // RemoveUnusedGlobalValue - Loop over all of the uses of the specified
 // GlobalValue, looking for the constant pointer ref that may be pointing to it.
 // If found, check to see if the constant pointer ref is safe to destroy, and if
diff --git a/llvm/test/Transforms/GlobalDCE/externally_available.ll b/llvm/test/Transforms/GlobalDCE/externally_available.ll
index 3dbada84..cc88cb1 100644
--- a/llvm/test/Transforms/GlobalDCE/externally_available.ll
+++ b/llvm/test/Transforms/GlobalDCE/externally_available.ll
@@ -1,43 +1,10 @@
-; RUN: opt < %s -globaldce -S | FileCheck %s
+; RUN: opt < %s -globaldce -S | not grep test_
 
 ; test_function should not be emitted to the .s file.
-; CHECK-NOT: @test_function
 define available_externally i32 @test_function() {
   ret i32 4
 }
 
 ; test_global should not be emitted to the .s file.
-; CHECK-NOT: @test_global
 @test_global = available_externally global i32 4
 
-; CHECK: @x = external constant void ()*
-@x = available_externally constant void()* @f
-; CHECK: @y = external constant i32
-@y = available_externally constant i32 ptrtoint (void()* @g to i32)
-; @h is still alive, so don't remove the initializer too eagerly.
-; CHECK: @z = available_externally constant i8 ptrtoint (void (i8)* @h to i8)
-@z = available_externally constant i8 ptrtoint (void(i8)* @h to i8)
-
-; CHECK-NOT: @f
-define linkonce_odr void @f() {
-  ret void
-}
-
-; CHECK-NOT: @g
-define linkonce_odr void @g() {
-  ret void
-}
-
-; CHECK: define linkonce_odr void @h
-define linkonce_odr void @h(i8) {
-  ret void
-}
-
-define i32 @main() {
-  %f = load void()** @x
-  call void %f()
-  %g = load i32* @y
-  %h = load i8* @z
-  call void @h(i8 %h)
-  ret i32 %g
-}
-- 
2.7.4