[cfe-dev] Implementation of the CGRecordLayoutBuilder for Microsoft ABI.

[r4start]

On 26/09/2011 12:00, Eli Friedman wrote:
> On Mon, Sep 26, 2011 at 12:59 AM, r4start<r4start at gmail.com>  wrote:
>> On 26/09/2011 11:48, Eli Friedman wrote:
>>> On Mon, Sep 26, 2011 at 12:34 AM, r4start<r4start at gmail.com>    wrote:
>>>> On 26/09/2011 11:18, Eli Friedman wrote:
>>>>> On Sun, Sep 25, 2011 at 11:55 PM, r4start<r4start at gmail.com>      wrote:
>>>>>> On 23/09/2011 22:44, John McCall wrote:
>>>>>>> On Sep 23, 2011, at 5:30 AM, r4start wrote:
>>>>>>>> I am writing CGRecordLayoutBuilder for Microsoft ABI and I have a
>>>>>>>> problem.
>>>>>>>> I have the following code for testing http://pastebin.com/27U2hcDC .
>>>>>>>>
>>>>>>>> When generated layout for a class C, I have break assert in
>>>>>>>> CGRecordLayoutBuilder.cpp line 968
>>>>>>>> assert(TypeSizeInBits == getTargetData().getTypeAllocSizeInBits(Ty)&&
>>>>>>>>            "Type size mismatch!");.
>>>>>>>> During debugging, I discovered that TypeSizeInBits smaller
>>>>>>>> getTargetData().getTypeAllocSizeInBits(Ty)  4 bytes.
>>>>>>>> After investigation, I discovered that the alignment is added in
>>>>>>>> StructLayout constructor (TargetData.cpp line 73)
>>>>>>>> // Add padding to the end of the struct so that it could be put in an
>>>>>>>> array
>>>>>>>>    // and all array elements would be aligned correctly.
>>>>>>>>     if ((StructSize&        (StructAlignment-1)) != 0)
>>>>>>>>       StructSize = TargetData::RoundUpAlignment(StructSize,
>>>>>>>> StructAlignment);
>>>>>>>>
>>>>>>>> But this alignment is not necessary for class C.
>>>>>>>> MSVC doesn't add to the end of the class alignment, if it has a
>>>>>>>> virtual
>>>>>>>> base classes.
>>>>>>>> I see two solutions to this problem:
>>>>>>>>    1. Rewrite assert like this
>>>>>>>>        (pseudocode)
>>>>>>>>        if ABI == Microsoft&&        Class has virtual basses
>>>>>>>>          assert(TypeSizeInBits ==
>>>>>>>> (getTargetData().getTypeAllocSizeInBits(Ty) - 32)&&
>>>>>>>>                       "Type size mismatch!");
>>>>>>>>         else
>>>>>>>>           old version;
>>>>>>>>     2. Provide to StructLyout information about ABI.
>>>>>>>>
>>>>>>>> I think the first way is more simple.
>>>>>>> No.  A *lot* of downstream code will be broken if LLVM doesn't lay
>>>>>>> out the IR type the way we think it's laid out, and that includes the
>>>>>>> presence or absence of tail padding.
>>>>>>>
>>>>>>> I don't see how this is possible, though.  Can you run through an
>>>>>>> example?  What's the size and layout of class B here?
>>>>>>>
>>>>>>> class A {
>>>>>>>     char c;
>>>>>>> };
>>>>>>>
>>>>>>> class B : public virtual A {
>>>>>>>     void *p;
>>>>>>> };
>>>>>>>
>>>>>>> I would expect that sizeof(B) is 12, and that it's laid out like this:
>>>>>>>     [0-3] virtual base pointer
>>>>>>>     [4-7] B.p
>>>>>>>       [8] A.c
>>>>>>>    [9-11] tail padding
>>>>>> MSVS with pragma pack 8 generates such layout:
>>>>>> 1>        class A    size(1):
>>>>>> 1>            +---
>>>>>> 1>        0  | c
>>>>>> 1>            +---
>>>>>>
>>>>>> 1>        class B    size(9):
>>>>>> 1>            +---
>>>>>> 1>        0  | {vbptr}
>>>>>> 1>        4  | p
>>>>>> 1>            +---
>>>>>> 1>            +--- (virtual base A)
>>>>>> 1>        8  | c
>>>>>> 1>            +---
>>>>>>
>>>>>> My code generates same layout (i32* in B is vbtable pointer):
>>>>>> *** Dumping AST Record Layout
>>>>>>     0 | class A
>>>>>>     0 |   char c
>>>>>>    sizeof=1, dsize=1, align=1
>>>>>>    nvsize=1, nvalign=1
>>>>>>
>>>>>> LLVMType:%class.A = type { i8 }
>>>>>> NonVirtualBaseLLVMType:%class.A = type { i8 }
>>>>>> IsZeroInitializable:1
>>>>>>
>>>>>> *** Dumping AST Record Layout
>>>>>>     0 | class B
>>>>>>     0 |   (B vtable pointer)
>>>>>>     0 |   (B vbtable pointer)
>>>>>>     4 |   void * p
>>>>>>     8 |   class A (virtual base)
>>>>>>     8 |     char c
>>>>>>    sizeof=9, dsize=9, align=4
>>>>>>    nvsize=8, nvalign=4
>>>>> That's can't be what MSVC actually does: the size of B must be a
>>>>> multiple of its alignment.
>>>> I check this layout twice. I get this layout from MSVC compiler output.
>>>> And
>>>> I check it in a memory while debugging.
>>>>>   If you're representation of what MSVC
>>>>> computes is accurate, you must be miscomputing the alignment.
>>>> Can you clarify?
>>> What is the value of (long)(((B*)0)+1)?  From what you are saying, it
>>> should be 9;
>> Yes, that right.
>>>   in that case, the "align=4" from your dump is not
>>> correct.
>>>
>> How I can modify align from command line options?
> Err, what?  The align=4 is getting computed by the RecordLayoutBuilder.
>
> -Eli
Oh sorry, it`s really stupid question :)

  - Dmitry.