llvm: correct calculation of index of zero-bit field

If the field comes before any non-zero-bit field then the index of
the next field should be returned.

Closes #13363

Author: Vexu

src/codegen/llvm.zig

@@ -10216,7 +10216,7 @@ fn llvmFieldIndex(
                 llvm_field_index += 1;
             }
 
-            if (field_index == i) {
+            if (field_index <= i) {
                 ptr_pl_buf.* = .{
                     .data = .{
                         .pointee_type = field_ty,
@@ -10249,7 +10249,7 @@ fn llvmFieldIndex(
             llvm_field_index += 1;
         }
 
-        if (field_index == i) {
+        if (field_index <= i) {
             ptr_pl_buf.* = .{
                 .data = .{
                     .pointee_type = field.ty,

test/behavior/struct.zig

@@ -1398,3 +1398,23 @@ test "under-aligned struct field" {
     const result = std.mem.readIntNative(u64, array[4..12]);
     try expect(result == 1234);
 }
+
+test "address of zero-bit field is equal to address of only field" {
+    if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_c) return error.SkipZigTest; // TODO
+
+    {
+        const A = struct { b: void = {}, u: u8 };
+        var a = A{ .u = 0 };
+        const a_ptr = @fieldParentPtr(A, "b", &a.b);
+        try std.testing.expectEqual(&a, a_ptr);
+    }
+    {
+        const A = struct { u: u8, b: void = {} };
+        var a = A{ .u = 0 };
+        const a_ptr = @fieldParentPtr(A, "b", &a.b);
+        try std.testing.expectEqual(&a, a_ptr);
+    }
+}