From 1f64d371062ab1343fe2a22e119186e5ca874135 Mon Sep 17 00:00:00 2001 From: Alexey Bataev Date: Thu, 3 Dec 2015 09:34:49 +0000 Subject: [PATCH] PR25575: Make GCC 4.4+ comatible layout for packed bit-fileds of char type, patch by D. Polukhin This CL is for discussion how to better fix bit-filed layout compatibility issue with GCC (see PR25575 for test case and more details). Current clang behavior is compatible with GCC 4.1-4.3 series but it was fixed in 4.4+. Ignoring packed attribute looks very odd and because it was also fixed in GCC 4.4+, it makes sense also fix it in clang. Differential Revision: http://reviews.llvm.org/D14872 git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@254596 91177308-0d34-0410-b5e6-96231b3b80d8 --- include/clang/Basic/DiagnosticSemaKinds.td | 7 ++++--- lib/Sema/SemaDeclAttr.cpp | 15 ++++++-------- test/Sema/struct-packed-align.c | 23 +++++++++++++++++++++- 3 files changed, 32 insertions(+), 13 deletions(-) diff --git a/include/clang/Basic/DiagnosticSemaKinds.td b/include/clang/Basic/DiagnosticSemaKinds.td index 50122ecb8a..e96c233810 100644 --- a/include/clang/Basic/DiagnosticSemaKinds.td +++ b/include/clang/Basic/DiagnosticSemaKinds.td @@ -2784,9 +2784,10 @@ def warn_int_to_void_pointer_cast : Warning< "cast to %1 from smaller integer type %0">, InGroup; -def warn_attribute_ignored_for_field_of_type : Warning< - "%0 attribute ignored for field of type %1">, - InGroup; +def warn_attribute_packed_for_bitfield : Warning< + "'packed' attribute was ignored on bit-fields with single-byte alignment " + "in older versions of GCC and Clang">, + InGroup>; def warn_transparent_union_attribute_field_size_align : Warning< "%select{alignment|size}0 of field %1 (%2 bits) does not match the " "%select{alignment|size}0 of the first field in transparent union; " diff --git a/lib/Sema/SemaDeclAttr.cpp b/lib/Sema/SemaDeclAttr.cpp index 93e9eb15b3..fed22c934c 100644 --- a/lib/Sema/SemaDeclAttr.cpp +++ b/lib/Sema/SemaDeclAttr.cpp @@ -1073,17 +1073,14 @@ static void handlePackedAttr(Sema &S, Decl *D, const AttributeList &Attr) { TD->addAttr(::new (S.Context) PackedAttr(Attr.getRange(), S.Context, Attr.getAttributeSpellingListIndex())); else if (FieldDecl *FD = dyn_cast(D)) { - // If the alignment is less than or equal to 8 bits, the packed attribute - // has no effect. + // Report warning about changed offset in the newer compiler versions. if (!FD->getType()->isDependentType() && - !FD->getType()->isIncompleteType() && + !FD->getType()->isIncompleteType() && FD->isBitField() && S.Context.getTypeAlign(FD->getType()) <= 8) - S.Diag(Attr.getLoc(), diag::warn_attribute_ignored_for_field_of_type) - << Attr.getName() << FD->getType(); - else - FD->addAttr(::new (S.Context) - PackedAttr(Attr.getRange(), S.Context, - Attr.getAttributeSpellingListIndex())); + S.Diag(Attr.getLoc(), diag::warn_attribute_packed_for_bitfield); + + FD->addAttr(::new (S.Context) PackedAttr( + Attr.getRange(), S.Context, Attr.getAttributeSpellingListIndex())); } else S.Diag(Attr.getLoc(), diag::warn_attribute_ignored) << Attr.getName(); } diff --git a/test/Sema/struct-packed-align.c b/test/Sema/struct-packed-align.c index 417c30308c..175866fbe9 100644 --- a/test/Sema/struct-packed-align.c +++ b/test/Sema/struct-packed-align.c @@ -1,5 +1,5 @@ // RUN: %clang_cc1 %s -fsyntax-only -verify -// expected-no-diagnostics +// RUN: %clang_cc1 %s -fsyntax-only -triple=x86_64-windows-coff -verify // Packed structs. struct s { @@ -138,3 +138,24 @@ extern int n2[__alignof(struct nS) == 8 ? 1 : -1]; extern int n1[sizeof(struct nS) == 9 ? 1 : -1]; extern int n2[__alignof(struct nS) == 1 ? 1 : -1]; #endif + +// Packed attribute shouldn't be ignored for bit-field of char types. +// Note from GCC reference manual: The 4.1, 4.2 and 4.3 series of GCC ignore +// the packed attribute on bit-fields of type char. This has been fixed in +// GCC 4.4 but the change can lead to differences in the structure layout. +// See the documentation of -Wpacked-bitfield-compat for more information. +struct packed_chars { + char a:4; + char b:8 __attribute__ ((packed)); + // expected-warning@-1 {{'packed' attribute was ignored on bit-fields with single-byte alignment in older versions of GCC and Clang}} + char c:4; +}; + +#if defined(_WIN32) +// On Windows clang uses MSVC compatible layout in this case. +extern int o1[sizeof(struct packed_chars) == 3 ? 1 : -1]; +extern int o2[__alignof(struct packed_chars) == 1 ? 1 : -1]; +#else +extern int o1[sizeof(struct packed_chars) == 2 ? 1 : -1]; +extern int o2[__alignof(struct packed_chars) == 1 ? 1 : -1]; +#endif -- 2.40.0