[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, 19 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp b/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
index 6f9268f..a6c5a26 100644
--- a/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
+++ b/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
@@ -105,6 +105,11 @@ static void GetObjCImageInfo(Module &M, unsigned &Version, unsigned &Flags,
 //                                  ELF
 //===----------------------------------------------------------------------===//
 
+TargetLoweringObjectFileELF::TargetLoweringObjectFileELF()
+    : TargetLoweringObjectFile() {
+  SupportDSOLocalEquivalentLowering = true;
+}
+
 void TargetLoweringObjectFileELF::Initialize(MCContext &Ctx,
                                              const TargetMachine &TgtM) {
   TargetLoweringObjectFile::Initialize(Ctx, TgtM);
@@ -1007,6 +1012,20 @@ const MCExpr *TargetLoweringObjectFileELF::lowerRelativeReference(
       MCSymbolRefExpr::create(TM.getSymbol(RHS), getContext()), getContext());
 }
 
+const MCExpr *TargetLoweringObjectFileELF::lowerDSOLocalEquivalent(
+    const DSOLocalEquivalent *Equiv, const TargetMachine &TM) const {
+  assert(supportDSOLocalEquivalentLowering());
+
+  const auto *GV = Equiv->getGlobalValue();
+
+  // A PLT entry is not needed for dso_local globals.
+  if (GV->isDSOLocal() || GV->isImplicitDSOLocal())
+    return MCSymbolRefExpr::create(TM.getSymbol(GV), getContext());
+
+  return MCSymbolRefExpr::create(TM.getSymbol(GV), PLTRelativeVariantKind,
+                                 getContext());
+}
+
 MCSection *TargetLoweringObjectFileELF::getSectionForCommandLines() const {
   // Use ".GCC.command.line" since this feature is to support clang's
   // -frecord-gcc-switches which in turn attempts to mimic GCC's switch of the