[llvm][IR] Add dso_local_equivalent Constant

The `dso_local_equivalent` constant is a wrapper for functions that represents a
value which is functionally equivalent to the global passed to this. That is, if
this accepts a function, calling this constant should have the same effects as
calling the function directly. This could be a direct reference to the function,
the `@plt` modifier on X86/AArch64, a thunk, or anything that's equivalent to the
resolved function as a call target.

When lowered, the returned address must have a constant offset at link time from
some other symbol defined within the same binary. The address of this value is
also insignificant. The name is leveraged from `dso_local` where use of a function
or variable is resolved to a symbol in the same linkage unit.

In this patch:
- Addition of `dso_local_equivalent` and handling it
- Update Constant::needsRelocation() to strip constant inbound GEPs and take
  advantage of `dso_local_equivalent` for relative references

This is useful for the [Relative VTables C++ ABI](https://reviews.llvm.org/D72959)
which makes vtables readonly. This works by replacing the dynamic relocations for
function pointers in them with static relocations that represent the offset between
the vtable and virtual functions. If a function is externally defined,
`dso_local_equivalent` can be used as a generic wrapper for the function to still
allow for this static offset calculation to be done.

See [RFC](http://lists.llvm.org/pipermail/llvm-dev/2020-August/144469.html) for more details.

Differential Revision: https://reviews.llvm.org/D77248

1 files changed, 60 insertions, 2 deletions

diff --git a/llvm/lib/IR/Constants.cpp b/llvm/lib/IR/Constants.cpp
index 3d8555b..732d6e6 100644
--- a/llvm/lib/IR/Constants.cpp
+++ b/llvm/lib/IR/Constants.cpp
@@ -509,6 +509,9 @@ void llvm::deleteConstant(Constant *C) {
   case Constant::BlockAddressVal:
     delete static_cast<BlockAddress *>(C);
     break;
+  case Constant::DSOLocalEquivalentVal:
+    delete static_cast<DSOLocalEquivalent *>(C);
+    break;
   case Constant::UndefValueVal:
     delete static_cast<UndefValue *>(C);
     break;
@@ -654,10 +657,17 @@ bool Constant::needsRelocation() const {
           return false;
 
         // Relative pointers do not need to be dynamically relocated.
-        if (auto *LHSGV = dyn_cast<GlobalValue>(LHSOp0->stripPointerCasts()))
-          if (auto *RHSGV = dyn_cast<GlobalValue>(RHSOp0->stripPointerCasts()))
+        if (auto *RHSGV =
+                dyn_cast<GlobalValue>(RHSOp0->stripInBoundsConstantOffsets())) {
+          auto *LHS = LHSOp0->stripInBoundsConstantOffsets();
+          if (auto *LHSGV = dyn_cast<GlobalValue>(LHS)) {
             if (LHSGV->isDSOLocal() && RHSGV->isDSOLocal())
               return false;
+          } else if (isa<DSOLocalEquivalent>(LHS)) {
+            if (RHSGV->isDSOLocal())
+              return false;
+          }
+        }
       }
     }
   }
@@ -1763,6 +1773,54 @@ Value *BlockAddress::handleOperandChangeImpl(Value *From, Value *To) {
   return nullptr;
 }
 
+DSOLocalEquivalent *DSOLocalEquivalent::get(GlobalValue *GV) {
+  DSOLocalEquivalent *&Equiv = GV->getContext().pImpl->DSOLocalEquivalents[GV];
+  if (!Equiv)
+    Equiv = new DSOLocalEquivalent(GV);
+
+  assert(Equiv->getGlobalValue() == GV &&
+         "DSOLocalFunction does not match the expected global value");
+  return Equiv;
+}
+
+DSOLocalEquivalent::DSOLocalEquivalent(GlobalValue *GV)
+    : Constant(GV->getType(), Value::DSOLocalEquivalentVal, &Op<0>(), 1) {
+  setOperand(0, GV);
+}
+
+/// Remove the constant from the constant table.
+void DSOLocalEquivalent::destroyConstantImpl() {
+  const GlobalValue *GV = getGlobalValue();
+  GV->getContext().pImpl->DSOLocalEquivalents.erase(GV);
+}
+
+Value *DSOLocalEquivalent::handleOperandChangeImpl(Value *From, Value *To) {
+  assert(From == getGlobalValue() && "Changing value does not match operand.");
+  assert(To->getType() == getType() && "Mismatched types");
+  assert(isa<Constant>(To) && "Can only replace the operands with a constant");
+
+  // The replacement is with another global value.
+  if (const auto *ToObj = dyn_cast<GlobalValue>(To)) {
+    DSOLocalEquivalent *&NewEquiv =
+        getContext().pImpl->DSOLocalEquivalents[ToObj];
+    if (NewEquiv)
+      return NewEquiv;
+  }
+
+  // The replacement could be a bitcast or an alias to another function. We can
+  // replace it with a bitcast to the dso_local_equivalent of that function.
+  auto *Func = cast<Function>(To->stripPointerCastsAndAliases());
+  DSOLocalEquivalent *&NewEquiv = getContext().pImpl->DSOLocalEquivalents[Func];
+  if (NewEquiv)
+    return llvm::ConstantExpr::getBitCast(NewEquiv, getType());
+
+  // Replace this with the new one.
+  getContext().pImpl->DSOLocalEquivalents.erase(getGlobalValue());
+  NewEquiv = this;
+  setOperand(0, Func);
+  return nullptr;
+}
+
 //---- ConstantExpr::get() implementations.
 //