[LLVMdev] Evaluation of offsetof() macro

[Vadim Chugunov]

On Fri, Jan 2, 2015 at 2:20 AM, David Majnemer <david.majnemer at gmail.com>
wrote:

>
>
> On Fri, Jan 2, 2015 at 1:58 AM, Vadim Chugunov <vadimcn at gmail.com> wrote:
>
>> Hi!
>> LLVM has a class, ConstantExpr, that is very handy for compile-time
>> evaluation of const expressions.   Unfortunately I cannot find any methods
>> in it that would be helpful in evaluation of expressions similar to this:
>> (uintptr_t)(&(*(TYPE*)0).FIELD), which is basically the implementation of
>> the offsetof(TYPE, FIELD) macro.
>>
>
> I think the closest entity for this sort of thing is LLVM's
> ConstantExpr::getOffsetOf.
>

Certainly, but I think this requires either making `offsetof` a first-class
operation in the source language, or handling the folding of <pointer
deref>-<field select>-<get-address-of> combo in the front-end.


> Specifically, there seem to be no provisions for dereferencing a pointer.
>>
>> Does LLVM have any facilities (that I missed), that would help language
>> front-ends in dealing with this sort of expressions?
>> Obviously, clang does it somehow, but so far I was not able to locate the
>> relevant bit of code.  Any pointers would be appreciated!
>>
>
> Clang handles record types in a very abstract way, it doesn't rely on LLVM
> IR at any level. An ASTRecordLayout is built for a record type and the
> ASTRecordLayout::getFieldIndex method is used to determine the offset for a
> particular field.
>

 So it handles all const expression evaluation in the front-end?
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