[LLVMdev] question about the DIBuilder::createStructType

[Hui Zhang]

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
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