[LLVMdev] Evaluation of offsetof() macro

[David Majnemer]

On Fri, Jan 2, 2015 at 2:53 AM, Vadim Chugunov <vadimcn at gmail.com> wrote:

>
>
> 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?
>
>
Yes, clang has it's own constant expression evaluator which understands the
rules of C++.

Let's take an example.
The expression (long)&x/(long)&y divides two globals by each other.  This
expression is lowered to the following LLVM IR Constant:
i64 sdiv (i64 ptrtoint (i32* @x to i64), i64 ptrtoint (i32* @y to i64))

This Constant is a ConstantExpr but certainly not indicative of something
that would be 'constexpr' in C++.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150102/7ea2a287/attachment.html>