Index: llvm/lib/Transforms/IPO/GlobalDCE.cpp
===================================================================
--- llvm/lib/Transforms/IPO/GlobalDCE.cpp
+++ llvm/lib/Transforms/IPO/GlobalDCE.cpp
@@ -416,6 +416,29 @@
       // virtual function pointers with null, allowing us to remove the
       // function itself.
       ++NumVFuncs;
+
+      // Detect vfuncs that are referenced as "relative pointers" which are used
+      // in Swift vtables, i.e. entries in the form of:
+      //
+      //   i32 trunc (i64 sub (i64 ptrtoint @f, i64 ptrtoint ...)) to i32)
+      //
+      // In this case, replace the whole "sub" expression with constant 0 to
+      // avoid leaving a weird sub(0, symbol) expression behind.
+      for (auto *U : F->users()) {
+        if (auto *PtrExpr = dyn_cast<ConstantExpr>(U)) {
+          if (PtrExpr->getOpcode() == Instruction::PtrToInt) {
+            assert(PtrExpr->hasOneUser());
+            if (auto *SubExpr = dyn_cast<ConstantExpr>(PtrExpr->user_back())) {
+              assert(SubExpr->hasOneUser());
+              if (SubExpr->getOpcode() == Instruction::Sub) {
+                SubExpr->replaceNonMetadataUsesWith(
+                    ConstantInt::get(SubExpr->getType(), 0));
+              }
+            }
+          }
+        }
+      }
+
       F->replaceNonMetadataUsesWith(ConstantPointerNull::get(F->getType()));
     }
     EraseUnusedGlobalValue(F);
Index: llvm/test/Transforms/GlobalDCE/virtual-functions-relative-pointers-bad.ll
===================================================================
--- llvm/test/Transforms/GlobalDCE/virtual-functions-relative-pointers-bad.ll
+++ llvm/test/Transforms/GlobalDCE/virtual-functions-relative-pointers-bad.ll
@@ -15,9 +15,9 @@
 !1 = !{i64 4, !"vfunc2.type"}
 
 ; CHECK:      @vtable = internal unnamed_addr constant { [3 x i32] } { [3 x i32] [
-; CHECK-SAME:   i32 trunc (i64 sub (i64 0, i64 ptrtoint ({ [3 x i32] }* @vtable to i64)) to i32),
-; CHECK-SAME:   i32 trunc (i64 sub (i64 0, i64 ptrtoint ({ [3 x i32] }* @vtable to i64)) to i32),
-; CHECK-SAME:   i32 trunc (i64 sub (i64 0, i64 ptrtoint (void ()* @weird_ref_2 to i64)) to i32)
+; CHECK-SAME:   i32 0,
+; CHECK-SAME:   i32 0,
+; CHECK-SAME:   i32 0
 ; CHECK-SAME: ] }, align 8, !type !0, !type !1, !vcall_visibility !2
 
 define internal void @vfunc1() { ret void }
Index: llvm/test/Transforms/GlobalDCE/virtual-functions-relative-pointers-gep.ll
===================================================================
--- llvm/test/Transforms/GlobalDCE/virtual-functions-relative-pointers-gep.ll
+++ llvm/test/Transforms/GlobalDCE/virtual-functions-relative-pointers-gep.ll
@@ -16,7 +16,7 @@
 
 ; CHECK:      @vtable = internal unnamed_addr constant { [4 x i32] } { [4 x i32] [
 ; CHECK-SAME:   i32 trunc (i64 sub (i64 ptrtoint (void ()* @vfunc1_live              to i64), i64 ptrtoint (i32* getelementptr inbounds ({ [4 x i32] }, { [4 x i32] }* @vtable, i32 0, i32 0, i32 2) to i64)) to i32),
-; CHECK-SAME:   i32 trunc (i64 sub (i64 0,                                                    i64 ptrtoint (i32* getelementptr inbounds ({ [4 x i32] }, { [4 x i32] }* @vtable, i32 0, i32 0, i32 2) to i64)) to i32)
+; CHECK-SAME:   i32 0
 ; CHECK-SAME: ] }, align 8, !type !0, !type !1, !vcall_visibility !2
 
 ; (1) vfunc1_live is referenced from @main, stays alive
Index: llvm/test/Transforms/GlobalDCE/virtual-functions-relative-pointers.ll
===================================================================
--- llvm/test/Transforms/GlobalDCE/virtual-functions-relative-pointers.ll
+++ llvm/test/Transforms/GlobalDCE/virtual-functions-relative-pointers.ll
@@ -14,7 +14,7 @@
 
 ; 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:   i32 0
 ; CHECK-SAME: ] }, align 8, !type !0, !type !1, !vcall_visibility !2
 
 ; (1) vfunc1_live is referenced from @main, stays alive