From cec9b217071ea01563f7bd924f8bf9e010ba3868 Mon Sep 17 00:00:00 2001
From: Christoffer Lerno <christoffer@aegik.com>
Date: Thu, 13 Feb 2025 21:15:27 +0100
Subject: [PATCH] Missing end padding when including a packed struct #1966.

---
 releasenotes.md                              |  1 +
 src/compiler/sema_decls.c                    | 13 +++++-
 test/test_suite/struct/struct_pack_error.c3t | 48 ++++++++++++++++++++
 3 files changed, 61 insertions(+), 1 deletion(-)
 create mode 100644 test/test_suite/struct/struct_pack_error.c3t

diff --git a/releasenotes.md b/releasenotes.md
index 1a5018cfd..d6bbaaba8 100644
--- a/releasenotes.md
+++ b/releasenotes.md
@@ -66,6 +66,7 @@
 - Fixes to ByteBuffer allocation/free.
 - Fix issue where compiling both for asm and object file would corrupt the obj file output.
 - Fix `poll` and `POLL_FOREVER`.
+- Missing end padding when including a packed struct #1966.
 
 ### Stdlib changes
 - Added '%h' and '%H' for printing out binary data in hexadecimal using the formatter.
diff --git a/src/compiler/sema_decls.c b/src/compiler/sema_decls.c
index 0301cfdfe..192043adc 100755
--- a/src/compiler/sema_decls.c
+++ b/src/compiler/sema_decls.c
@@ -486,7 +486,7 @@ static bool sema_analyse_struct_members(SemaContext *context, Decl *decl)
 	Decl **struct_members = decl->strukt.members;
 	unsigned member_count = vec_size(struct_members);
 	ASSERT(member_count > 0 && "This analysis should only be called on member_count > 0");
-
+	bool is_naturally_aligned = !is_packed;
 	for (unsigned i = 0; i < member_count; i++)
 	{
 	AGAIN:;
@@ -579,6 +579,7 @@ static bool sema_analyse_struct_members(SemaContext *context, Decl *decl)
 		// If the natural align is different from the aligned offset we have two cases:
 		if (natural_align_offset != align_offset)
 		{
+			is_naturally_aligned = false;
 			// If the natural alignment is greater, in this case the struct is unaligned.
 			if (member_natural_alignment > member_alignment)
 			{
@@ -604,6 +605,7 @@ static bool sema_analyse_struct_members(SemaContext *context, Decl *decl)
 			{
 				RETURN_SEMA_ERROR(member, "%d bytes of padding would be added to align this member which is not allowed with `@nopadding` and `@compact`.", align_offset - offset);
 			}
+			member->padding = align_offset - offset;
 		}
 
 		if (!sema_check_struct_holes(context, decl, member)) return false;
@@ -665,6 +667,15 @@ static bool sema_analyse_struct_members(SemaContext *context, Decl *decl)
 	}
 
 	decl->is_packed = is_unaligned;
+	// Strip padding if we are aligned.
+	if (!decl->is_packed && is_naturally_aligned)
+	{
+		for (unsigned i = 0; i < member_count; i++)
+		{
+			Decl *member = struct_members[i];
+			member->padding = 0;
+		}
+	}
 	decl->strukt.size = size;
 	return true;
 }
diff --git a/test/test_suite/struct/struct_pack_error.c3t b/test/test_suite/struct/struct_pack_error.c3t
new file mode 100644
index 000000000..cb8022635
--- /dev/null
+++ b/test/test_suite/struct/struct_pack_error.c3t
@@ -0,0 +1,48 @@
+// #target: macos-x64
+module test;
+import std;
+
+// issue 1966
+
+struct MsgHeader @packed
+{
+    ulong enqueued_by_router_us;
+}
+
+enum ClientState
+{
+    NOT_INITIALIZED,
+    WAITING_FOR_LOGIN,
+    READY,
+    WAITING_FOR_CLOSE,
+}
+
+struct Client
+{
+    ClientState state;
+    MsgHeader header;
+    ushort service_id;
+    ElasticArray(<ulong, 1>) subscribers;
+}
+
+Client[10] clients;
+
+fn void main()
+{
+	void* a = ((void*)(&clients[0])) + Client.sizeof;
+	void* b =  &clients[1];
+}
+
+/* #expect: test.ll
+
+%Client = type <{ i32, %MsgHeader, i16, [2 x i8], %ElasticArray }>
+%MsgHeader = type { i64 }
+%ElasticArray = type { i64, [1 x i64] }
+define void @test.main() #0 {
+entry:
+  %a = alloca ptr, align 8
+  %b = alloca ptr, align 8
+  store ptr getelementptr (i8, ptr @test.clients, i64 32), ptr %a, align 8
+  store ptr getelementptr inbounds (i8, ptr @test.clients, i64 32), ptr %b, align 8
+  ret void
+}
\ No newline at end of file