From: whitequark <whitequark@whitequark.org>
Date: Thu, 27 Jul 2017 19:36:13 +0000 (+0000)
Subject: [MergeFunctions] Remove alias support.
X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=f9f40b539b1dc44b10184684aa3f72e1192c454f;p=llvm

[MergeFunctions] Remove alias support.

The alias support was dead code since 2011. It was last touched
in r124182, where it was reintroduced after being removed
in r110434, and since then it was gated behind a HasGlobalAliases
flag that was permanently stuck as `false`.

It is also broken. I'm not sure if it bitrotted or was just broken
in the first place because it appears to have never been tested,
but the following IR results in a crash:

    define internal i32 @a(i32 %a, i32 %b) unnamed_addr {
      %c = add i32 %a, %b
      %d = xor i32 %a, %c
      ret i32 %c
    }

    define internal i32 @b(i32 %a, i32 %b) unnamed_addr {
      %c = add i32 %a, %b
      %d = xor i32 %a, %c
      ret i32 %c
    }

It seems safe to remove buggy untested code that no one cared about
for seven years.

Differential Revision: https://reviews.llvm.org/D34802

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@309313 91177308-0d34-0410-b5e6-96231b3b80d8
---

diff --git a/lib/Transforms/IPO/MergeFunctions.cpp b/lib/Transforms/IPO/MergeFunctions.cpp
index 0e478ba607b..a43b69f3413 100644
--- a/lib/Transforms/IPO/MergeFunctions.cpp
+++ b/lib/Transforms/IPO/MergeFunctions.cpp
@@ -119,7 +119,6 @@ using namespace llvm;
 
 STATISTIC(NumFunctionsMerged, "Number of functions merged");
 STATISTIC(NumThunksWritten, "Number of thunks generated");
-STATISTIC(NumAliasesWritten, "Number of aliases generated");
 STATISTIC(NumDoubleWeak, "Number of new functions created");
 
 static cl::opt<unsigned> NumFunctionsForSanityCheck(
@@ -179,8 +178,7 @@ class MergeFunctions : public ModulePass {
 public:
   static char ID;
   MergeFunctions()
-    : ModulePass(ID), FnTree(FunctionNodeCmp(&GlobalNumbers)), FNodesInTree(),
-      HasGlobalAliases(false) {
+    : ModulePass(ID), FnTree(FunctionNodeCmp(&GlobalNumbers)), FNodesInTree() {
     initializeMergeFunctionsPass(*PassRegistry::getPassRegistry());
   }
 
@@ -236,9 +234,6 @@ private:
   /// again.
   void mergeTwoFunctions(Function *F, Function *G);
 
-  /// Replace G with a thunk or an alias to F. Deletes G.
-  void writeThunkOrAlias(Function *F, Function *G);
-
   /// Fill PDIUnrelatedWL with instructions from the entry block that are
   /// unrelated to parameter related debug info.
   void filterInstsUnrelatedToPDI(BasicBlock *GEntryBlock,
@@ -256,9 +251,6 @@ private:
   /// delete G.
   void writeThunk(Function *F, Function *G);
 
-  /// Replace G with an alias to F. Deletes G.
-  void writeAlias(Function *F, Function *G);
-
   /// Replace function F with function G in the function tree.
   void replaceFunctionInTree(const FunctionNode &FN, Function *G);
 
@@ -271,9 +263,6 @@ private:
   // dangling iterators into FnTree. The invariant that preserves this is that
   // there is exactly one mapping F -> FN for each FunctionNode FN in FnTree.
   ValueMap<Function*, FnTreeType::iterator> FNodesInTree;
-
-  /// Whether or not the target supports global aliases.
-  bool HasGlobalAliases;
 };
 
 } // end anonymous namespace
@@ -454,19 +443,6 @@ void MergeFunctions::replaceDirectCallers(Function *Old, Function *New) {
   }
 }
 
-// Replace G with an alias to F if possible, or else a thunk to F. Deletes G.
-void MergeFunctions::writeThunkOrAlias(Function *F, Function *G) {
-  if (HasGlobalAliases && G->hasGlobalUnnamedAddr()) {
-    if (G->hasExternalLinkage() || G->hasLocalLinkage() ||
-        G->hasWeakLinkage()) {
-      writeAlias(F, G);
-      return;
-    }
-  }
-
-  writeThunk(F, G);
-}
-
 // Helper for writeThunk,
 // Selects proper bitcast operation,
 // but a bit simpler then CastInst::getCastOpcode.
@@ -734,20 +710,6 @@ void MergeFunctions::writeThunk(Function *F, Function *G) {
   ++NumThunksWritten;
 }
 
-// Replace G with an alias to F and delete G.
-void MergeFunctions::writeAlias(Function *F, Function *G) {
-  auto *GA = GlobalAlias::create(G->getLinkage(), "", F);
-  F->setAlignment(std::max(F->getAlignment(), G->getAlignment()));
-  GA->takeName(G);
-  GA->setVisibility(G->getVisibility());
-  removeUsers(G);
-  G->replaceAllUsesWith(GA);
-  G->eraseFromParent();
-
-  DEBUG(dbgs() << "writeAlias: " << GA->getName() << '\n');
-  ++NumAliasesWritten;
-}
-
 // Merge two equivalent functions. Upon completion, Function G is deleted.
 void MergeFunctions::mergeTwoFunctions(Function *F, Function *G) {
   if (F->isInterposable()) {
@@ -763,19 +725,14 @@ void MergeFunctions::mergeTwoFunctions(Function *F, Function *G) {
 
     unsigned MaxAlignment = std::max(G->getAlignment(), H->getAlignment());
 
-    if (HasGlobalAliases) {
-      writeAlias(F, G);
-      writeAlias(F, H);
-    } else {
-      writeThunk(F, G);
-      writeThunk(F, H);
-    }
+    writeThunk(F, G);
+    writeThunk(F, H);
 
     F->setAlignment(MaxAlignment);
     F->setLinkage(GlobalValue::PrivateLinkage);
     ++NumDoubleWeak;
   } else {
-    writeThunkOrAlias(F, G);
+    writeThunk(F, G);
   }
 
   ++NumFunctionsMerged;