[LLVMdev] question about the DIBuilder::createStructType
RICHARD STUCKEY
richard.stuckey at virgin.net
Fri Dec 19 10:55:22 PST 2014
Hi, Hui Zang,
Here's a helpful article:
http://xathrya.web.id/blog/2013/12/17/data-structure-alignment-in-c-on-x86-and-x64-machine/
I am not familiar with Chapel, and don't know what a Chapel::Type is. If
the APIs you use to manipulate instances of that type support traversal
(e.g. given a Chapel::Type that represents a structure, there is an
operation that gives you a list of Chapel::Type's that represent the
members of that structure) you might be able to get the information from
that.
In my case, I work with an abstract syntax tree that has been generated by
a parser; traversing it gets me to nodes in the tree which represent
structure types, and the necessary size/offset/alignment information for
the structure members is generated by a layout algorithm.
Regards,
Richard
On 19 December 2014 at 17:55, Hui Zhang <wayne.huizhang at gmail.com> wrote:
> Hello, Richard
>
> Thanks a lot !
>
> On Fri, Dec 19, 2014 at 10:04 AM, RICHARD STUCKEY <
> richard.stuckey at virgin.net> wrote:
>>
>> Hi, Hui Zang,
>>
>> It helps to remember that at runtime a variable of a structure type has
>> to be allocated store, and to think about how that variable is laid out in
>> store.
>>
>> You must use createMemberType to create a DIType for *each* field (aka
>> member) in the structure; you then store those DIType's as the elements of
>> a DIArray which you pass as the 'Elements' parameter to createStructType.
>>
>> When creating a structure member, you have to specify this information :
>>
>> SizeinBits - the actual size of the member
>> OffsetInBits - the number of bits between the start of the
>> member in the layout of the structure and the start of the structure
>> AlignInBits - the address boundary (if any) to which the
>> element has to be aligned.
>>
>> Remember that on some processor architectures (e.g. Sun),
>>
>> basic data items (e.g shorts, ints, floats) have to be allocated to store
>> at addresses which are multiples of their size; e.g. shorts must be
>> allocated at addresses
>> which are multiples of two bytes, ints allocated at addresses which are
>> multiples of 4 bytes, etc. If this rule is not followed, e.g. a 4-byte
>> integer is allocated at an odd address, then any attempt to load or store
>> that variable will result in a bus error. On architectures which do not
>> have this constraint, it is sufficient for objects to be byte-aligned.
>>
>> So in your example, you might have
>>
>> age : size 32, offset 0, align 32
>> name: size 32, offset 32, align 32
>>
>> or
>>
>> age : size 32, offset 0, align 8
>> name: size 32, offset 32, align 8
>>
>> depending on your target processor.
>>
>>
>> The sizeInBits and AliginInBits parameters to createStructType describe
>> the whole structure (e.g. Actor: sizeInBits:64 aligInBits: 32). Note
>> that the size of the structure may be more than the sum of the size of the
>> individual members - there may be gaps in the structure layout, depending
>> on the sizes and alignments of its members. The alignment of a structure
>> is generally the largest alignment of any of its members; the structure
>> type must be laid out so that if a structure variable starts at an address
>> which is a multiple of the structure's alignment then each of its members
>> starts at an address which is a multiple of its alignment.
>>
>> E.g. if you have a structure
>>
>> struct S
>> {
>> int I1;
>> short S;
>> int I2;
>> } V;
>>
>> and an int is 4 bytes and a short 2 bytes, then either you must leave a
>> 2-byte gap between S and I2 in the layout or you must re-order the fields
>> (if the language you are implementing allows that) so that S comes after I1
>> and I2 in the layout; otherwise either I1 or I2 will be misaligned in store
>> regardless of the address at which V is allocated.
>>
>
> Is this just the example for Sun processors that basic data items (e.g
> shorts, ints, floats) have to be allocated to store at addresses which are
> multiples of their size ? what if intel processor like your example 2(align
> 8)?
>
>
>
>>
>> The 'Ty' parameter to createMemberType specifies the type of the member.
>> In your example, you will need to create DIType objects for the types 'int'
>> and 'const char*'; use the first one in the call to createMemberType for
>> "age" and the second one in the call for "name".
>>
>> As a general principle, when creating the DIType object for a given type,
>> you must work from the bottom up (or recursively, from the top down)
>> through the hierarchy of types which you are compiling : you must create
>> the DIype for a type *before* you create the DItype for any other type
>> which uses that type (e.g. as a member of a structure, or an element of an
>> array, or the target type of a pointer type, etc.). Note that you can
>> have cycles of type dependencies in your hierarchy; e.g. a struct S could
>> have a field P which is of type S* - so before you try to create the DIType
>> for a given type you must check whether you have already created it,
>> otherwise you could recurse endlessly around the cycles! (until your
>> stack blows up...)
>>
>
> I have a situation that in my language(Chapel), I don't have the
> llvm::Type, only Chapel::Type, so what do you suggest that how I'm gonna
> get the Member aligh and Member size ？
> Thanks ！
>
>>
>> Hope this helps,
>>
>> Richard
>>
>>
>>
>>
>>
>> On 18 December 2014 at 20:44, Hui Zhang <wayne.huizhang at gmail.com> wrote:
>>
>>> Hello,
>>>
>>> I'm using DIBuilder to create debugging information, I'm not clear about
>>> two things:
>>>
>>> [1] Could you help explain the meaning and the difference
>>> between"alignment" and "offset" of a type ?
>>>
>>> e.g class Actor{
>>> int age;
>>> const char* name;
>>> }
>>>
>>> Besides, I found the denotation of createMemberType and createStructType
>>> both have "Member Alignment" and "Member Size" fields, are they the same
>>> thing ? If they are, then for the "Actor", which member is it referring to ?
>>>
>>> [2] In createMemberType, the last arg “Ty” is parent type, does it mean
>>> "Actor" in the example ?
>>>
>>> thanks
>>>
>>> --
>>> Best regards
>>>
>>>
>>> Hui Zhang
>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu
>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>>
>>>
>>
>
> --
> Best regards
>
>
> Hui Zhang
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20141219/afdfd61f/attachment.html>
More information about the llvm-dev
mailing list