CodeGen: Use PLT relocations for relative references to unnamed_addr functions.

The relative vtable ABI (PR26723) needs PLT relocations to refer to virtual
functions defined in other DSOs. The unnamed_addr attribute means that the
function's address is not significant, so we're allowed to substitute it
with the address of a PLT entry.

Also includes a bonus feature: addends for COFF image-relative references.

Differential Revision: http://reviews.llvm.org/D17938

llvm-svn: 267211

2 files changed, 46 insertions, 5 deletions

diff --git a/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp b/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
index bff22844992..9ddf20bc3c9 100644
--- a/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
@@ -1763,10 +1763,6 @@ const MCExpr *AsmPrinter::lowerConstant(const Constant *CV) {
     llvm_unreachable("Unknown constant value to lower!");
   }
 
-  if (const MCExpr *RelocExpr
-      = getObjFileLowering().getExecutableRelativeSymbol(CE, *Mang, TM))
-    return RelocExpr;
-
   switch (CE->getOpcode()) {
   default:
     // If the code isn't optimized, there may be outstanding folding
@@ -1842,10 +1838,34 @@ const MCExpr *AsmPrinter::lowerConstant(const Constant *CV) {
     return MCBinaryExpr::createAnd(OpExpr, MaskExpr, Ctx);
   }
 
+  case Instruction::Sub: {
+    GlobalValue *LHSGV;
+    APInt LHSOffset;
+    if (IsConstantOffsetFromGlobal(CE->getOperand(0), LHSGV, LHSOffset,
+                                   getDataLayout())) {
+      GlobalValue *RHSGV;
+      APInt RHSOffset;
+      if (IsConstantOffsetFromGlobal(CE->getOperand(1), RHSGV, RHSOffset,
+                                     getDataLayout())) {
+        const MCExpr *RelocExpr = getObjFileLowering().lowerRelativeReference(
+            LHSGV, RHSGV, *Mang, TM);
+        if (!RelocExpr)
+          RelocExpr = MCBinaryExpr::createSub(
+              MCSymbolRefExpr::create(getSymbol(LHSGV), Ctx),
+              MCSymbolRefExpr::create(getSymbol(RHSGV), Ctx), Ctx);
+        int64_t Addend = (LHSOffset - RHSOffset).getSExtValue();
+        if (Addend != 0)
+          RelocExpr = MCBinaryExpr::createAdd(
+              RelocExpr, MCConstantExpr::create(Addend, Ctx), Ctx);
+        return RelocExpr;
+      }
+    }
+  }
+  // else fallthrough
+
   // The MC library also has a right-shift operator, but it isn't consistently
   // signed or unsigned between different targets.
   case Instruction::Add:
-  case Instruction::Sub:
   case Instruction::Mul:
   case Instruction::SDiv:
   case Instruction::SRem:
diff --git a/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp b/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
index 4229741cf5a..a4626556878 100644
--- a/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
+++ b/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
@@ -438,6 +438,27 @@ MCSection *TargetLoweringObjectFileELF::getStaticDtorSection(
                                   KeySym);
 }
 
+const MCExpr *TargetLoweringObjectFileELF::lowerRelativeReference(
+    const GlobalValue *LHS, const GlobalValue *RHS, Mangler &Mang,
+    const TargetMachine &TM) const {
+  // We may only use a PLT-relative relocation to refer to unnamed_addr
+  // functions.
+  if (!LHS->hasUnnamedAddr() || !LHS->getValueType()->isFunctionTy())
+    return nullptr;
+
+  // Basic sanity checks.
+  if (LHS->getType()->getPointerAddressSpace() != 0 ||
+      RHS->getType()->getPointerAddressSpace() != 0 || LHS->isThreadLocal() ||
+      RHS->isThreadLocal())
+    return nullptr;
+
+  return MCBinaryExpr::createSub(
+      MCSymbolRefExpr::create(TM.getSymbol(LHS, Mang), PLTRelativeVariantKind,
+                              getContext()),
+      MCSymbolRefExpr::create(TM.getSymbol(RHS, Mang), getContext()),
+      getContext());
+}
+
 void
 TargetLoweringObjectFileELF::InitializeELF(bool UseInitArray_) {
   UseInitArray = UseInitArray_;