Allow alias to point to an arbitrary ConstantExpr.

This  patch changes GlobalAlias to point to an arbitrary ConstantExpr and it is
up to MC (or the system assembler) to decide if that expression is valid or not.

This reduces our ability to diagnose invalid uses and how early we can spot
them, but it also lets us do things like

@test5 = alias inttoptr(i32 sub (i32 ptrtoint (i32* @test2 to i32),
                                 i32 ptrtoint (i32* @bar to i32)) to i32*)

An important implication of this patch is that the notion of aliased global
doesn't exist any more. The alias has to encode the information needed to
access it in its metadata (linkage, visibility, type, etc).

Another consequence to notice is that getSection has to return a "const char *".
It could return a NullTerminatedStringRef if there was such a thing, but when
that was proposed the decision was to just uses "const char*" for that.

llvm-svn: 210062

1 files changed, 7 insertions, 35 deletions

diff --git a/llvm/lib/Linker/LinkModules.cpp b/llvm/lib/Linker/LinkModules.cpp
index 45f2d4e03a1..ad8e01dc1be 100644
--- a/llvm/lib/Linker/LinkModules.cpp
+++ b/llvm/lib/Linker/LinkModules.cpp
@@ -389,8 +389,6 @@ namespace {
     /// actually need, but this allows us to reuse the ValueMapper code.
     ValueToValueMapTy ValueMap;
 
-    std::vector<std::pair<GlobalValue *, GlobalAlias *>> ReplaceWithAlias;
-
     struct AppendingVarInfo {
       GlobalVariable *NewGV;  // New aggregate global in dest module.
       Constant *DstInit;      // Old initializer from dest module.
@@ -723,7 +721,7 @@ bool ModuleLinker::linkAppendingVarProto(GlobalVariable *DstGV,
     return emitError(
         "Appending variables with different unnamed_addr need to be linked!");
 
-  if (DstGV->getSection() != SrcGV->getSection())
+  if (StringRef(DstGV->getSection()) != SrcGV->getSection())
     return emitError(
           "Appending variables with different section name need to be linked!");
 
@@ -929,8 +927,11 @@ bool ModuleLinker::linkAliasProto(GlobalAlias *SGA) {
   if (NewVisibility)
     NewDA->setVisibility(*NewVisibility);
 
-  if (DGV)
-    ReplaceWithAlias.push_back(std::make_pair(DGV, NewDA));
+  if (DGV) {
+    // Any uses of DGV need to change to NewDA, with cast.
+    DGV->replaceAllUsesWith(ConstantExpr::getBitCast(NewDA, DGV->getType()));
+    DGV->eraseFromParent();
+  }
 
   ValueMap[SGA] = NewDA;
   return false;
@@ -1016,19 +1017,6 @@ void ModuleLinker::linkFunctionBody(Function *Dst, Function *Src) {
 
 }
 
-static GlobalObject &getGlobalObjectInExpr(Constant &C) {
-  auto *GO = dyn_cast<GlobalObject>(&C);
-  if (GO)
-    return *GO;
-  auto *GA = dyn_cast<GlobalAlias>(&C);
-  if (GA)
-    return *GA->getAliasee();
-  auto &CE = cast<ConstantExpr>(C);
-  assert(CE.getOpcode() == Instruction::BitCast ||
-         CE.getOpcode() == Instruction::AddrSpaceCast);
-  return getGlobalObjectInExpr(*CE.getOperand(0));
-}
-
 /// linkAliasBodies - Insert all of the aliases in Src into the Dest module.
 void ModuleLinker::linkAliasBodies() {
   for (Module::alias_iterator I = SrcM->alias_begin(), E = SrcM->alias_end();
@@ -1039,25 +1027,9 @@ void ModuleLinker::linkAliasBodies() {
       GlobalAlias *DA = cast<GlobalAlias>(ValueMap[I]);
       Constant *Val =
           MapValue(Aliasee, ValueMap, RF_None, &TypeMap, &ValMaterializer);
-      DA->setAliasee(&getGlobalObjectInExpr(*Val));
+      DA->setAliasee(Val);
     }
   }
-
-  // Any uses of DGV need to change to NewDA, with cast.
-  for (auto &Pair : ReplaceWithAlias) {
-    GlobalValue *DGV = Pair.first;
-    GlobalAlias *NewDA = Pair.second;
-
-    for (auto *User : DGV->users()) {
-      if (auto *GA = dyn_cast<GlobalAlias>(User)) {
-        if (GA == NewDA)
-          report_fatal_error("Linking these modules creates an alias cycle.");
-      }
-    }
-
-    DGV->replaceAllUsesWith(ConstantExpr::getBitCast(NewDA, DGV->getType()));
-    DGV->eraseFromParent();
-  }
 }
 
 /// linkNamedMDNodes - Insert all of the named MDNodes in Src into the Dest