From c90a97f9be9ffef858b0e450de5006f61a12fafd Mon Sep 17 00:00:00 2001
From: Luuk de Gram <luuk@degram.dev>
Date: Mon, 23 May 2022 18:24:03 +0200
Subject: [PATCH] codegen: Order error union fields per alignment

Based on the size of the payload the native backends will lower
the error union with its fields (errorset & payload) in the correct order.
e.g. ErrorA!u8 will first lower the error set's value and then the payload.
In the event of ErrorA!u32 will lower the payload first.
---
 src/codegen.zig | 78 +++++++++++++++++++++++++++++++++++--------------
 1 file changed, 56 insertions(+), 22 deletions(-)

diff --git a/src/codegen.zig b/src/codegen.zig
index bd556baa5e1d67b0adae173f5bc72fa2a578ce4a..81b303ab82534bcbc343fee15301f6de00d1627a 100644
--- a/src/codegen.zig
+++ b/src/codegen.zig
@@ -702,33 +702,45 @@ pub fn generateSymbol(
         .ErrorUnion => {
             const error_ty = typed_value.ty.errorUnionSet();
             const payload_ty = typed_value.ty.errorUnionPayload();
+
+            if (error_ty.errorSetCardinality() == .zero) {
+                const payload_val = typed_value.val.castTag(.eu_payload).?.data;
+                return generateSymbol(bin_file, src_loc, .{
+                    .ty = payload_ty,
+                    .val = payload_val,
+                }, code, debug_output, reloc_info);
+            }
+
             const is_payload = typed_value.val.errorUnionIsPayload();
 
+            if (!payload_ty.hasRuntimeBitsIgnoreComptime()) {
+                const err_val = if (!is_payload) typed_value.val else Value.initTag(.zero);
+                return generateSymbol(bin_file, src_loc, .{
+                    .ty = error_ty,
+                    .val = err_val,
+                }, code, debug_output, reloc_info);
+            }
+
+            const payload_align = payload_ty.abiAlignment(target);
+            const error_align = Type.anyerror.abiAlignment(target);
             const abi_align = typed_value.ty.abiAlignment(target);
 
+            // error value first when its type is larger than the error union's payload
+            if (error_align > payload_align) {
+                switch (try generateSymbol(bin_file, src_loc, .{
+                    .ty = error_ty,
+                    .val = if (is_payload) Value.initTag(.zero) else typed_value.val,
+                }, code, debug_output, reloc_info)) {
+                    .appended => {},
+                    .externally_managed => |external_slice| {
+                        code.appendSliceAssumeCapacity(external_slice);
+                    },
+                    .fail => |em| return Result{ .fail = em },
+                }
+            }
+
+            // emit payload part of the error union
             {
-                const error_val = if (!is_payload) typed_value.val else Value.initTag(.zero);
-                const begin = code.items.len;
-                switch (try generateSymbol(bin_file, src_loc, .{
-                    .ty = error_ty,
-                    .val = error_val,
-                }, code, debug_output, reloc_info)) {
-                    .appended => {},
-                    .externally_managed => |external_slice| {
-                        code.appendSliceAssumeCapacity(external_slice);
-                    },
-                    .fail => |em| return Result{ .fail = em },
-                }
-                const unpadded_end = code.items.len - begin;
-                const padded_end = mem.alignForwardGeneric(u64, unpadded_end, abi_align);
-                const padding = try math.cast(usize, padded_end - unpadded_end);
-
-                if (padding > 0) {
-                    try code.writer().writeByteNTimes(0, padding);
-                }
-            }
-
-            if (payload_ty.hasRuntimeBits()) {
                 const begin = code.items.len;
                 const payload_val = if (typed_value.val.castTag(.eu_payload)) |val| val.data else Value.initTag(.undef);
                 switch (try generateSymbol(bin_file, src_loc, .{
@@ -750,6 +762,28 @@ pub fn generateSymbol(
                 }
             }
 
+            // Payload size is larger than error set, so emit our error set last
+            if (error_align < payload_align) {
+                const begin = code.items.len;
+                switch (try generateSymbol(bin_file, src_loc, .{
+                    .ty = error_ty,
+                    .val = if (is_payload) Value.initTag(.zero) else typed_value.val,
+                }, code, debug_output, reloc_info)) {
+                    .appended => {},
+                    .externally_managed => |external_slice| {
+                        code.appendSliceAssumeCapacity(external_slice);
+                    },
+                    .fail => |em| return Result{ .fail = em },
+                }
+                const unpadded_end = code.items.len - begin;
+                const padded_end = mem.alignForwardGeneric(u64, unpadded_end, abi_align);
+                const padding = try math.cast(usize, padded_end - unpadded_end);
+
+                if (padding > 0) {
+                    try code.writer().writeByteNTimes(0, padding);
+                }
+            }
+
             return Result{ .appended = {} };
         },
         .ErrorSet => {
-- 
2.54.0