[LLVMdev] question about the DIBuilder::createStructType
Hui Zhang
wayne.huizhang at gmail.com
Fri Dec 19 12:09:24 PST 2014
Hello, Richard
Thanks ! My case is similar, there is a AST that I traverse. Could you tell
me more about that layout algorithm ?
On Fri, Dec 19, 2014 at 1:55 PM, RICHARD STUCKEY <richard.stuckey at virgin.net
> wrote:
>
> 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
>>
>
>
--
Best regards
Hui Zhang
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