[llvm-dev] RFC: Allowing @llvm.objectsize to be more conservative with null.

[George Burgess IV via llvm-dev]

Thanks for the comments!

> Have you considered changing our existing behavior to match GCC's
> builtin_object_size instead of adding a new parameter

Yup! My issue with turning `i1 %min` into `i8 %flags` is that
__builtin_object_size would get lowered like:

__builtin_object_size(p, 0) -> @llvm.objectsize(i8* %p, i8 2)
__builtin_object_size(p, 1) -> @llvm.objectsize(i8* %p, i8 2)
__builtin_object_size(p, 2) -> @llvm.objectsize(i8* %p, i8 3)
(__builtin_object_size(p, 3) doesn't actually get lowered)

...Which might be a bit surprising to some people. If we think that's a
non-issue, I'm happy to take the simpler approach and use an i8.

> We'll have to have some <min>-awareness either in clang (to decide if
> the <null-is-unknown> bit needs to be set) or in the middle end.  What
> is your plan here?

My plan was just to always set the `null-is-unknown` bit when lowering a
call to __builtin_object_size in clang. If `min` is true, we treat unknown
and null values identically in @llvm.objectsize, so whether
`null-is-unknown` is set in that case shouldn't matter.

> However, since Malloc can return null:

I think I was unclear: this behavior would only exist if @llvm.objectsize
was actually handed null. I wasn't planning on changing how we'd handle
memory allocation functions that may return null (GCC gives back 2 for
__builtin_object_size(malloc(2), 0)). In other words, this
`null-is-unknown` bit only makes the objectsize evaluator see `T* null` as
though it was `call T* @opaque_user_defined_function()`, nothing else.

This is also consistent with how @llvm.objectsize already acts: if the
pointer it's given is the result of a call to `malloc(N)`, it'll return N
regardless of the value of `min`.

But yeah, I agree that some of the features of __builtin_object_size aren't
entirely intuitive. :)

On Mon, Jan 2, 2017 at 12:01 AM, Sanjoy Das <sanjoy at playingwithpointers.com>
wrote:

> Hi George,
>
> Have you considered changing our existing behavior to match GCC's
> builtin_object_size instead of adding a new parameter?  That may be
> simpler overall.  There's also a clear upgrade strategy -- fold every
> old style call to "<min> ? 0 : 1".
>
> You probably already know this, but GCC folds
> builtin_object_size(0, 0) to -1 and builtin_object_size(0, 2) to 0.
> We'll have to have some <min>-awareness either in clang (to decide if
> the <null-is-unknown> bit needs to be set) or in the middle end.  What
> is your plan here?
>
> I also found gcc's choice of folding builtin_object_size(0, 2) to 0 and
> builtin_object_size(0, 0) to -1 somewhat odd; I'd have expected the
> inverse.  This follows from the following "intuitive" rules
>
> ObjSizeMax(X) = UMAX(ObjSizeMax(A), ObjSizeMax(B))
> ObjSizeMin(X) = UMIN(ObjSizeMin(A), ObjSizeMin(B))
>
> (where X is a value that can either be A or B at runtime)
>
> and that we want to fold
>
> ObjSizeMax(Malloc(N)) = ObjSizeMin(Malloc(N)) = N
>
> However, since Malloc can return null:
>
> ObjSizeMax(Malloc(N)) = UMAX(N, ObjSizeMax(NULL)) = N
> ObjSizeMin(Malloc(N)) = UMIN(N, ObjSizeMin(NULL)) = N
>
> and for that to be true ObjSizeMax(NULL) =
> builtin_object_size(NULL, 0) needs to be 0 and ObjSizeMin(NULL) =
> builtin_object_size(NULL, 2) needs to be (unsigned)-1.
>
> However, I'm not sure if it is up to us to change that; given the very
> motivation of this thread is GCC compatibility.
>
> -- Sanjoy
>
> On Sun, Jan 1, 2017 at 10:03 PM, George Burgess IV via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> > Happy New Year ping. :)
> >
> > Will ping again on Wednesday. If I don't get comments by EOD Thursday,
> I'll
> > assume everyone's OK with this and put together a patch.
> >
> > On Wed, Dec 21, 2016 at 11:44 AM, George Burgess IV
> > <george.burgess.iv at gmail.com> wrote:
> >>
> >> tl;dr: We'd like to add a bit to @llvm.objectsize to make it optionally
> be
> >> conservative when it's handed a null pointer.
> >>
> >> Happy Wednesday!
> >>
> >> We're trying to fix PR23277, which is a bug about how clang+LLVM treat
> >> __builtin_object_size when it's given a null pointer. For compatibility
> with
> >> GCC, clang would like to be able to hand back a conservative result
> (e.g.
> >> (size_t)-1), but the LLVM intrinsic that clang lowers
> >> __builtin_object_size(p, N) to, @llvm.objectsize, only hands back 0 when
> >> given a null pointer. Because it's often assumed that
> __builtin_object_size
> >> always folds to a constant, handling this only in clang may be tricky:
> if we
> >> emit the IR equivalent to ((p) ? __builtin_object_size(p, N) : -1ULL)
> and
> >> LLVM can't fold away the null check, we've failed to emit a constant.
> >>
> >> So, the best path forward that I can see is to add a "null is unknown
> >> size" bit to @llvm.objectsize, much like the "min" bit it already has.
> If
> >> this bit is true, null would be treated like an opaque pointer.
> Otherwise,
> >> @llvm.objectsize would act no differently than it does today.
> >>
> >> If we like this approach, I'm assuming it would be best to have this bit
> >> as a third argument to @llvm.objectsize, rather than making the second
> >> argument an i8 and using it as a bag of bits.
> >>
> >> All thoughts/comments/alternative approaches/etc. highly appreciated. :)
> >>
> >> Thanks (and Happy Holidays)!
> >> George
> >
> >
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >
>
>
>
> --
> Sanjoy Das
> http://playingwithpointers.com
>
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