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Fix packed being ignored on ms_struct bitfields (#182792)

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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.
This commit is contained in:
Fangrui Song 2026-03-11 18:17:43 -07:00 committed by GitHub
parent c56410fdc7
commit da3efc79f0
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3 changed files with 21 additions and 3 deletions
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2
clang/lib/AST/RecordLayoutBuilder.cpp
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@ -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.

4
clang/test/CodeGen/ms_struct-bitfield.c
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@ -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

18
clang/test/CodeGen/ms_struct-packed.c Normal file
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@ -0,0 +1,18 @@
// RUN: %clang_cc1 -emit-llvm-only -triple x86_64 %s
/// Packed struct: different 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];