[cfe-dev] -mms_bitfields question

[Eli Friedman]

On Thu, Oct 4, 2012 at 1:51 PM, Jeremiah Zanin
<Jeremiah.Zanin at volition-inc.com> wrote:
> Interesting. There is currently one place where it checks the compiler option (inside RecordLayoutBuilder::LayoutField) as well as IsMsStruct. There are several places just checking the MsStructAttr attribute. Would those need to be changed as well?

Yes... the -mms-bitfields is essentially a default which can be
overridden by ms_struct/gcc_struct.

-Eli

> Jeremiah
>
> -----Original Message-----
> From: Eli Friedman [mailto:eli.friedman at gmail.com]
> Sent: Thursday, October 04, 2012 3:26 PM
> To: Jeremiah Zanin
> Cc: cfe-dev at cs.uiuc.edu
> Subject: Re: [cfe-dev] -mms_bitfields question
>
> On Thu, Oct 4, 2012 at 1:14 PM, Jeremiah Zanin <Jeremiah.Zanin at volition-inc.com> wrote:
>> Hello,
>>
>> We are using Clang for generating reflection data and ran into an issue with how Microsoft lays out bitfields. Here's the structure we were having a problem with:
>>
>>         struct reflected woof
>>         {
>>            bool m_b1 : 1 reflected;     // Clang bit offset: 0, MS bit offset: 0
>>            bool m_b2 : 1 reflected;     // Clang bit offset: 1, MS bit offset: 1
>>            bool m_b3 : 1 reflected;     // Clang bit offset: 2, MS bit offset: 2
>>            bool m_b4 : 1 reflected;     // Clang bit offset: 3, MS bit offset: 3
>>            uint m_i1 reflected;                 // Clang bit offset: 32, MS bit offset: 32
>>            uint m_i2 : 5 reflected;     // Clang bit offset: 64, MS bit offset: 64
>>            uint m_i3 : 8 reflected;     // Clang bit offset: 69, MS bit offset: 69
>>            uint16 m_i5 : 2 reflected;   // Clang bit offset: 77, MS bit offset: 96
>>            uint8 m_i4 : 2 reflected;
>>            uint8 m_ix1 : 2;
>>            uint16 m_i6 : 6 reflected;
>>         };
>>
>> When compiling with Clang the bit offset of "m_i5" is 77, while when compiling with VS 2010 the bit offset is 96. We tried compiling with the "-mms-bitfields" command-line parameter but it didn't change the results. We stepped through Clang code and discovered that the "RecordLayoutBuilder::LayoutFields" function is looking for the "IsMsStruct" attribute. So we tried adding __attribute__((ms_struct)) to the structure and that fixed the problem. Adding "#pragma ms_struct on" also worked. We were wondering if the code inside "RecordLayoutBuilder::LayoutFields" should be checking "Context.getLangOpts().MSBitfields" along with "IsMsStruct":
>>
>>         for (RecordDecl::field_iterator Field = D->field_begin(),
>>                FieldEnd = D->field_end(); Field != FieldEnd; ++Field) {
>>             if (IsMsStruct) {                   // <-- Should this check Context.getLangOpts().MSBitfields?
>>               FieldDecl *FD = *Field;
>>               if (Context.ZeroBitfieldFollowsBitfield(FD, LastFD))
>>                 ZeroLengthBitfield = FD;
>>
>>         ...
>>
>>           if (IsMsStruct && RemainingInAlignment &&     // <-- Should this check Context.getLangOpts().MSBitfields?
>>               LastFD && LastFD->isBitField() &&
>> LastFD->getBitWidthValue(Context)) {
>
> Looking at the gcc docs, the computation of IsMsStruct (whether to use ms_struct layout) should depend on getLangOpts().MSBitfields, and we shouldn't check getLangOpts().MSBitfields directly anywhere else.
>
> -Eli
>
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