From e0bc5f65b98d154b4318027d56f780b55605e33c Mon Sep 17 00:00:00 2001
From: Jacob Young <jacobly0@users.noreply.github.com>
Date: Sun, 18 Dec 2022 01:25:19 -0500
Subject: [PATCH] llvm: fix lowering pointer to final zero-width field of a
 comptime value

 * Handle a `null` return from `llvmFieldIndex`.
 * Add a behavior test to test this code path.
 * Reword this test name, which incorrectly described how pointers to
   zero-bit fields behave, and instead describe the actual test.
---
 src/codegen/llvm.zig     | 22 +++++++++++++++-------
 test/behavior/struct.zig | 36 +++++++++++++++++++++++-------------
 2 files changed, 38 insertions(+), 20 deletions(-)

diff --git a/src/codegen/llvm.zig b/src/codegen/llvm.zig
index cd9ef98e44..68d929c5a4 100644
--- a/src/codegen/llvm.zig
+++ b/src/codegen/llvm.zig
@@ -4033,16 +4033,24 @@ pub const DeclGen = struct {
                             const final_llvm_ty = (try dg.lowerType(ptr_child_ty)).pointerType(0);
                             break :blk field_addr.constIntToPtr(final_llvm_ty);
                         }
-                        bitcast_needed = !field_ty.eql(ptr_child_ty, dg.module);
 
                         var ty_buf: Type.Payload.Pointer = undefined;
-                        const llvm_field_index = llvmFieldIndex(parent_ty, field_index, target, &ty_buf).?;
-                        const indices: [2]*llvm.Value = .{
-                            llvm_u32.constInt(0, .False),
-                            llvm_u32.constInt(llvm_field_index, .False),
-                        };
+
                         const parent_llvm_ty = try dg.lowerType(parent_ty);
-                        break :blk parent_llvm_ty.constInBoundsGEP(parent_llvm_ptr, &indices, indices.len);
+                        if (llvmFieldIndex(parent_ty, field_index, target, &ty_buf)) |llvm_field_index| {
+                            bitcast_needed = !field_ty.eql(ptr_child_ty, dg.module);
+                            const indices: [2]*llvm.Value = .{
+                                llvm_u32.constInt(0, .False),
+                                llvm_u32.constInt(llvm_field_index, .False),
+                            };
+                            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),
+                            };
+                            break :blk parent_llvm_ty.constInBoundsGEP(parent_llvm_ptr, &indices, indices.len);
+                        }
                     },
                     .Pointer => {
                         assert(parent_ty.isSlice());
diff --git a/test/behavior/struct.zig b/test/behavior/struct.zig
index db7092ab82..0984f7d1e4 100644
--- a/test/behavior/struct.zig
+++ b/test/behavior/struct.zig
@@ -1359,23 +1359,33 @@ test "under-aligned struct field" {
     try expect(result == 1234);
 }
 
-test "address of zero-bit field is equal to address of only field" {
+test "fieldParentPtr of a zero-bit 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
 
-    {
-        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);
-    }
+    const S = struct {
+        fn testOneType(comptime A: type) !void {
+            {
+                const a = A{ .u = 0 };
+                const b_ptr = &a.b;
+                const a_ptr = @fieldParentPtr(A, "b", b_ptr);
+                try std.testing.expectEqual(&a, a_ptr);
+            }
+            {
+                var a = A{ .u = 0 };
+                const b_ptr = &a.b;
+                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 S.doTheTest();
+    comptime try S.doTheTest();
 }
 
 test "struct field has a pointer to an aligned version of itself" {