Index: llvm/lib/Transforms/IPO/GlobalDCE.cpp
===================================================================
--- llvm/lib/Transforms/IPO/GlobalDCE.cpp
+++ llvm/lib/Transforms/IPO/GlobalDCE.cpp
@@ -416,7 +416,43 @@
       // virtual function pointers with null, allowing us to remove the
       // function itself.
       ++NumVFuncs;
-      F->replaceNonMetadataUsesWith(ConstantPointerNull::get(F->getType()));
+
+      // 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 *CE = dyn_cast<ConstantExpr>(U)) {
+          if (CE->getOpcode() == Instruction::PtrToInt) {
+            assert(CE->hasOneUser());
+            if (auto *SubExpr = dyn_cast<ConstantExpr>(CE->user_back())) {
+              assert(SubExpr->hasOneUser());
+              if (SubExpr->getOpcode() == Instruction::Sub) {
+                SubExpr->replaceNonMetadataUsesWith(
+                    ConstantInt::get(SubExpr->getType(), 0));
+
+                // Replacing the sub takes care of the entire user U, no need
+                // do U->replaceUsesOfWith.
+                continue;
+              }
+            }
+          }
+
+          if (CE->getOpcode() == Instruction::BitCast) {
+            assert(CE->hasOneUser());
+            CE->replaceNonMetadataUsesWith(ConstantPointerNull::get(F->getType()));
+
+            // Replacing the bitcast takes care of the entire user U, no need
+            // do U->replaceUsesOfWith.
+            continue;
+          }
+        }
+
+        U->replaceUsesOfWith(F, ConstantPointerNull::get(F->getType()));
+      }
     }
     EraseUnusedGlobalValue(F);
   }
Index: llvm/test/Transforms/GlobalDCE/virtual-functions-derived-call.ll
===================================================================
--- llvm/test/Transforms/GlobalDCE/virtual-functions-derived-call.ll
+++ llvm/test/Transforms/GlobalDCE/virtual-functions-derived-call.ll
@@ -25,7 +25,7 @@
 %struct.A = type { i32 (...)** }
 %struct.B = type { %struct.A }
 
-; CHECK: @_ZTV1A = internal unnamed_addr constant { [3 x i8*] } zeroinitializer
+; CHECK: @_ZTV1A = internal unnamed_addr constant { [3 x i8*] } { [3 x i8*] [i8* null, i8* null, i8* null] }
 @_ZTV1A = internal unnamed_addr constant { [3 x i8*] } { [3 x i8*] [i8* null, i8* null, i8* bitcast (i32 (%struct.A*)* @_ZN1A3fooEv to i8*)] }, align 8, !type !0, !type !1, !vcall_visibility !2
 
 ; CHECK: @_ZTV1B = internal unnamed_addr constant { [3 x i8*] } { [3 x i8*] [i8* null, i8* null, i8* bitcast (i32 (%struct.B*)* @_ZN1B3fooEv to i8*)] }
Index: llvm/test/Transforms/GlobalDCE/virtual-functions-derived-pointer-call.ll
===================================================================
--- llvm/test/Transforms/GlobalDCE/virtual-functions-derived-pointer-call.ll
+++ llvm/test/Transforms/GlobalDCE/virtual-functions-derived-pointer-call.ll
@@ -34,7 +34,7 @@
 %struct.A = type { i32 (...)** }
 %struct.B = type { %struct.A }
 
-; CHECK: @_ZTV1A = internal unnamed_addr constant { [4 x i8*] } zeroinitializer
+; CHECK: @_ZTV1A = internal unnamed_addr constant { [4 x i8*] } { [4 x i8*] [i8* null, i8* null, i8* null, i8* null] }
 @_ZTV1A = internal unnamed_addr constant { [4 x i8*] } { [4 x i8*] [i8* null, i8* null, i8* bitcast (i32 (%struct.A*, i32)* @_ZN1A3fooEi to i8*), i8* bitcast (i32 (%struct.A*, float)* @_ZN1A3barEf to i8*)] }, align 8, !type !0, !type !1, !type !2, !vcall_visibility !3
 ; CHECK: @_ZTV1B = internal unnamed_addr constant { [4 x i8*] } { [4 x i8*] [i8* null, i8* null, i8* bitcast (i32 (%struct.B*, i32)* @_ZN1B3fooEi to i8*), i8* null] }
 @_ZTV1B = internal unnamed_addr constant { [4 x i8*] } { [4 x i8*] [i8* null, i8* null, i8* bitcast (i32 (%struct.B*, i32)* @_ZN1B3fooEi to i8*), i8* bitcast (i32 (%struct.B*, float)* @_ZN1B3barEf to i8*)] }, align 8, !type !0, !type !1, !type !2, !type !4, !type !5, !type !6, !vcall_visibility !3
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
@@ -14,11 +14,7 @@
 !0 = !{i64 0, !"vfunc1.type"}
 !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: ] }, align 8, !type !0, !type !1, !vcall_visibility !2
+; CHECK:      @vtable = internal unnamed_addr constant { [3 x i32] } zeroinitializer, align 8, !type !0, !type !1, !vcall_visibility !2
 
 define internal void @vfunc1() { ret void }
 define internal void @vfunc2() { 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