[clang] da3efc7 - Fix packed being ignored on ms_struct bitfields (#182792)

[via cfe-commits]

Author: Fangrui Song
Date: 2026-03-11T18:17:43-07:00
New Revision: da3efc79f0cc6d41a4c5627a3dc9fce6f5c701b4

URL: https://github.com/llvm/llvm-project/commit/da3efc79f0cc6d41a4c5627a3dc9fce6f5c701b4
DIFF: https://github.com/llvm/llvm-project/commit/da3efc79f0cc6d41a4c5627a3dc9fce6f5c701b4.diff

LOG: Fix packed being ignored on ms_struct bitfields (#182792)

For ms_struct structs on Itanium layout targets, the packed attribute is
ignored on bit-fields (2014 commit
76e1818a2b1248579557de2927c135c322577c82), mismatching the GCC behavior.
Remove the `!IsMsStruct` guard to fix it.

Added: 
    clang/test/CodeGen/ms_struct-packed.c

Modified: 
    clang/lib/AST/RecordLayoutBuilder.cpp
    clang/test/CodeGen/ms_struct-bitfield.c

Removed: 
    


################################################################################
diff  --git a/clang/lib/AST/RecordLayoutBuilder.cpp b/clang/lib/AST/RecordLayoutBuilder.cpp
index 0b1bf813efd10..f417588accd78 100644
--- a/clang/lib/AST/RecordLayoutBuilder.cpp
+++ b/clang/lib/AST/RecordLayoutBuilder.cpp
@@ -1676,7 +1676,7 @@ void ItaniumRecordLayoutBuilder::LayoutBitField(const FieldDecl *D) {
   unsigned UnpackedFieldAlign = FieldAlign;
 
   // Ignore the field alignment if the field is packed unless it has zero-size.
-  if (!IsMsStruct && FieldPacked && FieldSize != 0)
+  if (FieldPacked && FieldSize != 0)
     FieldAlign = 1;
 
   // But, if there's an 'aligned' attribute on the field, honor that.

diff  --git a/clang/test/CodeGen/ms_struct-bitfield.c b/clang/test/CodeGen/ms_struct-bitfield.c
index 37b0245e57c45..4598b880dd975 100644
--- a/clang/test/CodeGen/ms_struct-bitfield.c
+++ b/clang/test/CodeGen/ms_struct-bitfield.c
@@ -172,5 +172,5 @@ struct {
   __attribute__((packed)) unsigned short c : 6;
 } ATTR t13;
 int s13 = sizeof(t13);
-// CHECK: @s13 ={{.*}} global i32 4
-// CHECK-ARM: @s13 ={{.*}} global i32 4
+// CHECK: @s13 ={{.*}} global i32 3
+// CHECK-ARM: @s13 ={{.*}} global i32 3

diff  --git a/clang/test/CodeGen/ms_struct-packed.c b/clang/test/CodeGen/ms_struct-packed.c
new file mode 100644
index 0000000000000..0cbf61a759b81
--- /dev/null
+++ b/clang/test/CodeGen/ms_struct-packed.c
@@ -0,0 +1,18 @@
+// RUN: %clang_cc1 -emit-llvm-only -triple x86_64 %s
+
+/// Packed struct: 
diff erent storage unit types should pack without alignment
+/// padding between storage units.
+struct __attribute__((packed,ms_struct)) P1 {
+  short a : 8;
+  int b : 30;
+};
+
+static int p1[(sizeof(struct P1) == 6) ? 1 : -1];
+
+// Packed struct with char and int fields.
+struct [[gnu::ms_struct,gnu::packed]] P2 {
+  char a : 4;
+  int b : 20;
+};
+
+static int p2[(sizeof(struct P2) == 5) ? 1 : -1];