Index: llvm/include/llvm/Analysis/TypeMetadataUtils.h
===================================================================
--- llvm/include/llvm/Analysis/TypeMetadataUtils.h
+++ llvm/include/llvm/Analysis/TypeMetadataUtils.h
@@ -56,7 +56,25 @@
     SmallVectorImpl<Instruction *> &Preds, bool &HasNonCallUses,
     const CallInst *CI, DominatorTree &DT);
 
-Constant *getPointerAtOffset(Constant *I, uint64_t Offset, Module &M);
+// Processes a Constant recursively looking into elements of arrays, structs and
+// expressions to find a trivial pointer element that is located at the given
+// offset (relative to the beginning of the whole outer Constant).
+//
+// Used for example from GlobalDCE to find an entry in a C++ vtable that matches
+// a vcall offset.
+//
+// To support Swift vtables, getPointerAtOffset can see through "relative
+// pointers", i.e. (sub-)expressions of the form of:
+//
+//   @symbol = ... {
+//     i32 trunc (i64 sub (
+//       i64 ptrtoint (<type> @target to i64), i64 ptrtoint (... @symbol to i64)
+//     ) to i32)
+//   }
+//
+// For such (sub-)expressions, getPointerAtOffset returns the @target pointer.
+Constant *getPointerAtOffset(Constant *I, uint64_t Offset, Module &M,
+                             Constant *TopLevelGlobal = nullptr);
 }
 
 #endif
Index: llvm/lib/Analysis/TypeMetadataUtils.cpp
===================================================================
--- llvm/lib/Analysis/TypeMetadataUtils.cpp
+++ llvm/lib/Analysis/TypeMetadataUtils.cpp
@@ -126,7 +126,8 @@
                               Offset->getZExtValue(), CI, DT);
 }
 
-Constant *llvm::getPointerAtOffset(Constant *I, uint64_t Offset, Module &M) {
+Constant *llvm::getPointerAtOffset(Constant *I, uint64_t Offset, Module &M,
+                                   Constant *TopLevelGlobal) {
   if (I->getType()->isPointerTy()) {
     if (Offset == 0)
       return I;
@@ -142,7 +143,8 @@
 
     unsigned Op = SL->getElementContainingOffset(Offset);
     return getPointerAtOffset(cast<Constant>(I->getOperand(Op)),
-                              Offset - SL->getElementOffset(Op), M);
+                              Offset - SL->getElementOffset(Op), M,
+                              TopLevelGlobal);
   }
   if (auto *C = dyn_cast<ConstantArray>(I)) {
     ArrayType *VTableTy = C->getType();
@@ -153,7 +155,36 @@
       return nullptr;
 
     return getPointerAtOffset(cast<Constant>(I->getOperand(Op)),
-                              Offset % ElemSize, M);
+                              Offset % ElemSize, M, TopLevelGlobal);
+  }
+
+  // (Swift-specific) relative-pointer support starts here.  
+  if (auto *CI = dyn_cast<ConstantInt>(I)) {
+    if (Offset == 0 && CI->getZExtValue() == 0) {
+      return I;
+    }
+  }
+  if (auto *C = dyn_cast<ConstantExpr>(I)) {
+    switch (C->getOpcode()) {
+    case Instruction::Trunc:
+    case Instruction::PtrToInt:
+      return getPointerAtOffset(cast<Constant>(C->getOperand(0)), Offset, M,
+                                TopLevelGlobal);
+    case Instruction::Sub: {
+      auto *Operand0 = cast<Constant>(C->getOperand(0));
+      auto *Operand1 = cast<Constant>(C->getOperand(1));
+      auto *Operand1TargetGlobal = getPointerAtOffset(Operand1, 0, M);
+
+      // Check that in the "sub (@a, @b)" expression, @b points back to the top
+      // level global that we're processing. Otherwise bail.
+      if (Operand1TargetGlobal != TopLevelGlobal)
+        return nullptr;
+
+      return getPointerAtOffset(Operand0, Offset, M, TopLevelGlobal);
+    }
+    default:
+      return nullptr;
+    }
   }
   return nullptr;
 }
Index: llvm/lib/Transforms/IPO/GlobalDCE.cpp
===================================================================
--- llvm/lib/Transforms/IPO/GlobalDCE.cpp
+++ llvm/lib/Transforms/IPO/GlobalDCE.cpp
@@ -210,7 +210,7 @@
 
     Constant *Ptr =
         getPointerAtOffset(VTable->getInitializer(), VTableOffset + CallOffset,
-                           *Caller->getParent());
+                           *Caller->getParent(), VTable);
     if (!Ptr) {
       LLVM_DEBUG(dbgs() << "can't find pointer in vtable!\n");
       VFESafeVTables.erase(VTable);
Index: llvm/test/Transforms/GlobalDCE/virtual-functions-relative-pointers.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/GlobalDCE/virtual-functions-relative-pointers.ll
@@ -0,0 +1,39 @@
+; RUN: opt < %s -globaldce -S | FileCheck %s
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+
+declare { i8*, i1 } @llvm.type.checked.load(i8*, i32, metadata)
+
+; A vtable with "relative pointers", slots don't contain pointers to implementations, but instead have an i32 offset from the vtable itself to the implementation.
+@vtable = internal unnamed_addr constant { [2 x i32] } { [2 x i32] [
+  i32 trunc (i64 sub (i64 ptrtoint (void ()* @vfunc1_live              to i64), i64 ptrtoint ({ [2 x i32] }* @vtable to i64)) to i32),
+  i32 trunc (i64 sub (i64 ptrtoint (void ()* @vfunc2_dead              to i64), i64 ptrtoint ({ [2 x i32] }* @vtable to i64)) to i32)
+]}, align 8, !type !0, !type !1, !vcall_visibility !{i64 2}
+!0 = !{i64 0, !"vfunc1.type"}
+!1 = !{i64 4, !"vfunc2.type"}
+
+; CHECK:      @vtable = internal unnamed_addr constant { [2 x i32] } { [2 x i32] [
+; CHECK-SAME:   i32 trunc (i64 sub (i64 ptrtoint (void ()* @vfunc1_live              to i64), i64 ptrtoint ({ [2 x i32] }* @vtable to i64)) to i32),
+; CHECK-SAME:   i32 trunc (i64 sub (i64 0,                                                    i64 ptrtoint ({ [2 x i32] }* @vtable to i64)) to i32)
+; CHECK-SAME: ] }, align 8, !type !0, !type !1, !vcall_visibility !2
+
+; (1) vfunc1_live is referenced from @main, stays alive
+define internal void @vfunc1_live() {
+  ; CHECK: define internal void @vfunc1_live(
+  ret void
+}
+
+; (2) vfunc2_dead is never referenced, gets removed and vtable slot is null'd
+define internal void @vfunc2_dead() {
+  ; CHECK-NOT: define internal void @vfunc2_dead(
+  ret void
+}
+
+define void @main() {
+  %1 = ptrtoint { [2 x i32] }* @vtable to i64 ; to keep @vtable alive
+  %2 = tail call { i8*, i1 } @llvm.type.checked.load(i8* null, i32 0, metadata !"vfunc1.type")
+  ret void
+}
+
+!999 = !{i32 1, !"Virtual Function Elim", i32 1}
+!llvm.module.flags = !{!999}