[llvm-dev] RFC: Absolute or "fixed address" symbols as immediate operands

[Peter Collingbourne via llvm-dev]

On Wed, Oct 26, 2016 at 9:48 PM, Chris Lattner <clattner at apple.com> wrote:

> On Oct 26, 2016, at 1:34 AM, Peter Collingbourne <peter at pcc.me.uk> wrote:
>
> On Tue, Oct 25, 2016 at 10:48 PM, Chris Lattner <clattner at apple.com>
> wrote:
>
>> Responding to both of your emails in one, sorry for the delay:
>>
>> On Oct 25, 2016, at 11:20 AM, Peter Collingbourne <peter at pcc.me.uk>
>> wrote:
>> I think there are a couple of additional considerations we should make
>> here:
>>
>>    - What are we trying to model? To me it's clear that GlobalConstant
>>    is for modelling integers, not pointers. That alone may not necessarily be
>>    enough to motivate a representational change, but…
>>
>> I understand where you’re coming from, but I think we’re modeling three
>> different things, and disagreeing about how to clump them together.  The
>> three things I see in flight are:
>>
>> 1) typical globals that are laid out in some unknown way in the address
>> space.
>> 2) globals that may be tied to a specific knowable address range due to a
>> limited compilation model (e.g. a deeply embedded core) that fits into an
>> immedaite range (e.g. 0…255, 0…65536, etc).
>> 3) Immediates that are treated as symbolic for CFI’s perspective (so they
>> can’t just be used as a literal immediate) that are resolved at link time,
>> but are known to have limited range.
>>
>> There is also "4) immediates with an obvious known value”, but those are
>> obviously ConstantInt’s and not interesting to discuss here.
>>
>> The design I’m arguing for is to clump #2 and #3 into the same group.
>>
>
> I am not sure if this is sound if we want the no-alias assumption (see
> also below) to hold for #2 but not for #3.
>
>
>>   This can be done one of two different ways, but both ways use the same
>> “declaration side” reference, which has a !range metadata attached to it.
>> The three approaches I see are:
>>
>> a) Introduce a new GlobalConstant definition, whose value is the concrete
>> address that the linker should resolve.
>> b) Use an alias as the definition, whose body is a ptrtoint constant of
>> the same value.
>> c) Use a zero size globalvariable with a range metadata specifying the
>> exact address decided.
>>
>> I’m not very knowledgable about why approach b won’t work, but if it
>> could, it seems preferable because it fits in with our current model.
>>
>
> b would work in that it would give us the right bits in the object file,
> but it would be a little odd to use a different type for declarations as
> for definitions. That said, I don't have a strong objection to it.
>
>
> I can understand what you’re saying here, but this is already the case for
> aliases.  You can never have a “declaration side” for an alias that is an
> alias (you have to use an external global variable or a function with no
> body).
>
> From the discussion over the last day it sounds to me that “b” is the best
> approach, except for the (significant) annoyance that these things can be
> possibly aliased.  However, I don’t understand how this works in practice
> today for aliases.  By their very name, they are *all about* introducing
> aliases, so how is AA allowed to assume that two external global variable
> references are unaliased anyway?  One may be resolved as an alias to the
> other afterall, completely independent of your proposal.
>

I suppose that one way to think about it is that by using aliases you are
stepping outside of the bounds of the language, i.e. no valid C/C++
declaration can be used to take the address of an alias without using
reserved names or language extensions (clang uses aliases to implement some
standard language features but they all have reserved names as far as I'm
aware).

Maybe this hasn't come up simply because language implementations (and
users of language extensions) happen to never use aliases in a way that
could expose the AA assumption.

Thanks,
-- 
-- 
Peter
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