IR: Add COMDATs to the IR

This new IR facility allows us to represent the object-file semantic of
a COMDAT group.

COMDATs allow us to tie together sections and make the inclusion of one
dependent on another. This is required to implement features like MS
ABI VFTables and optimizing away certain kinds of initialization in C++.

This functionality is only representable in COFF and ELF, Mach-O has no
similar mechanism.

Differential Revision: http://reviews.llvm.org/D4178

llvm-svn: 211920

1 files changed, 14 insertions, 4 deletions

diff --git a/llvm/lib/Transforms/IPO/GlobalOpt.cpp b/llvm/lib/Transforms/IPO/GlobalOpt.cpp
index 36c04f092bc..dc9b2a8105e 100644
--- a/llvm/lib/Transforms/IPO/GlobalOpt.cpp
+++ b/llvm/lib/Transforms/IPO/GlobalOpt.cpp
@@ -17,6 +17,7 @@
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/SmallPtrSet.h"
+#include "llvm/ADT/SmallSet.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/Analysis/ConstantFolding.h"
@@ -1699,9 +1700,6 @@ static bool TryToShrinkGlobalToBoolean(GlobalVariable *GV, Constant *OtherVal) {
 /// possible.  If we make a change, return true.
 bool GlobalOpt::ProcessGlobal(GlobalVariable *GV,
                               Module::global_iterator &GVI) {
-  if (!GV->isDiscardableIfUnused())
-    return false;
-
   // Do more involved optimizations if the global is internal.
   GV->removeDeadConstantUsers();
 
@@ -1944,6 +1942,13 @@ bool GlobalOpt::OptimizeFunctions(Module &M) {
 
 bool GlobalOpt::OptimizeGlobalVars(Module &M) {
   bool Changed = false;
+
+  SmallSet<const Comdat *, 8> NotDiscardableComdats;
+  for (const GlobalVariable &GV : M.globals())
+    if (const Comdat *C = GV.getComdat())
+      if (!GV.isDiscardableIfUnused())
+        NotDiscardableComdats.insert(C);
+
   for (Module::global_iterator GVI = M.global_begin(), E = M.global_end();
        GVI != E; ) {
     GlobalVariable *GV = GVI++;
@@ -1958,7 +1963,12 @@ bool GlobalOpt::OptimizeGlobalVars(Module &M) {
           GV->setInitializer(New);
       }
 
-    Changed |= ProcessGlobal(GV, GVI);
+    if (GV->isDiscardableIfUnused()) {
+      if (const Comdat *C = GV->getComdat())
+        if (NotDiscardableComdats.count(C))
+          continue;
+      Changed |= ProcessGlobal(GV, GVI);
+    }
   }
   return Changed;
 }