codegen: fix taking the address of a zero-bit field in a zero-bit struct

Normally when we want a pointer to the end of a struct we just add 1 to
the struct pointer.  However, when it is a zero-bit struct, the pointer
type being used during lowering is often a dummy pointer type that
actually points to a non-zero-bit type, so we actually want to add 0
instead, since a zero-bit struct begins and ends at the same address.

3 files changed, 30 insertions, 9 deletions

diff --git a/src/codegen/c.zig b/src/codegen/c.zig
index a7a2b2cf2a..9488fa420b 100644
--- a/src/codegen/c.zig
+++ b/src/codegen/c.zig
@@ -5131,7 +5131,9 @@ fn structFieldPtr(f: *Function, inst: Air.Inst.Index, struct_ptr_ty: Type, struc
         .begin, .end => {
             try writer.writeByte('(');
             try f.writeCValue(writer, struct_ptr, .Other);
-            try writer.print(")[{}]", .{@boolToInt(field_loc == .end)});
+            try writer.print(")[{}]", .{
+                @boolToInt(field_loc == .end and struct_ty.hasRuntimeBitsIgnoreComptime()),
+            });
         },
         .field => |field| if (extra_name != .none) {
             try f.writeCValueDerefMember(writer, struct_ptr, extra_name);
diff --git a/src/codegen/llvm.zig b/src/codegen/llvm.zig
index 43ffcb0aba..3b180b4c50 100644
--- a/src/codegen/llvm.zig
+++ b/src/codegen/llvm.zig
@@ -4046,9 +4046,8 @@ pub const DeclGen = struct {
                             break :blk parent_llvm_ty.constInBoundsGEP(parent_llvm_ptr, &indices, indices.len);
                         } else {
                             bitcast_needed = !parent_ty.eql(ptr_child_ty, dg.module);
-                            const indices: [1]*llvm.Value = .{
-                                llvm_u32.constInt(1, .False),
-                            };
+                            const llvm_index = llvm_u32.constInt(@boolToInt(parent_ty.hasRuntimeBitsIgnoreComptime()), .False);
+                            const indices: [1]*llvm.Value = .{llvm_index};
                             break :blk parent_llvm_ty.constInBoundsGEP(parent_llvm_ptr, &indices, indices.len);
                         }
                     },
@@ -9774,8 +9773,8 @@ pub const FuncGen = struct {
                         // end of the struct. Treat our struct pointer as an array of two and get
                         // the index to the element at index `1` to get a pointer to the end of
                         // the struct.
-                        const llvm_usize = try self.dg.lowerType(Type.usize);
-                        const llvm_index = llvm_usize.constInt(1, .False);
+                        const llvm_u32 = self.dg.context.intType(32);
+                        const llvm_index = llvm_u32.constInt(@boolToInt(struct_ty.hasRuntimeBitsIgnoreComptime()), .False);
                         const indices: [1]*llvm.Value = .{llvm_index};
                         return self.builder.buildInBoundsGEP(struct_llvm_ty, struct_ptr, &indices, indices.len, "");
                     }
diff --git a/test/behavior/struct.zig b/test/behavior/struct.zig
index 0984f7d1e4..b6567b14c3 100644
--- a/test/behavior/struct.zig
+++ b/test/behavior/struct.zig
@@ -1365,7 +1365,7 @@ test "fieldParentPtr of a zero-bit field" {
     if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest; // TODO
 
     const S = struct {
-        fn testOneType(comptime A: type) !void {
+        fn testStruct(comptime A: type) !void {
             {
                 const a = A{ .u = 0 };
                 const b_ptr = &a.b;
@@ -1379,9 +1379,29 @@ test "fieldParentPtr of a zero-bit field" {
                 try std.testing.expectEqual(&a, a_ptr);
             }
         }
+        fn testNestedStruct(comptime A: type) !void {
+            {
+                const a = A{ .u = 0 };
+                const c_ptr = &a.b.c;
+                const b_ptr = @fieldParentPtr(@TypeOf(a.b), "c", c_ptr);
+                try std.testing.expectEqual(&a.b, b_ptr);
+                const a_ptr = @fieldParentPtr(A, "b", b_ptr);
+                try std.testing.expectEqual(&a, a_ptr);
+            }
+            {
+                var a = A{ .u = 0 };
+                const c_ptr = &a.b.c;
+                const b_ptr = @fieldParentPtr(@TypeOf(a.b), "c", c_ptr);
+                try std.testing.expectEqual(&a.b, b_ptr);
+                const a_ptr = @fieldParentPtr(A, "b", b_ptr);
+                try std.testing.expectEqual(&a, a_ptr);
+            }
+        }
         fn doTheTest() !void {
-            try testOneType(struct { b: void = {}, u: u8 });
-            try testOneType(struct { u: u8, b: void = {} });
+            try testStruct(struct { b: void = {}, u: u8 });
+            try testStruct(struct { u: u8, b: void = {} });
+            try testNestedStruct(struct { b: struct { c: void = {} } = .{}, u: u8 });
+            try testNestedStruct(struct { u: u8, b: struct { c: void = {} } = .{} });
         }
     };
     try S.doTheTest();