[LLVMdev] Evaluation of offsetof() macro

[David Majnemer]

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.


> 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.


>
>
> Vadim
>
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