[llvm-dev] [cfe-dev] [RFC] Introducing a byte type to LLVM

[Madhur Amilkanthwar via llvm-dev]

HI George,

I don't think this is scalable model to add a new type just to benefit
an analysis and draw specific conclusions from it. I can argue that
if b8 is proposed then why not b16 for half? Why not b32 for some
other reason? This won't stop just there and one can go beyond
and introduce types to benefit domain specific languages.

Given the problem, *I'd say we should think about a way to annotate*
* types with attribute or metadata or flags which optimizations can use*
*to do a better job.* The attribute/metadata could carry the semantic
meaning for the type. Frontends can generate this "type attribute/metadata"
and optimizations can choose to use this extra information to do the
better job. It would be a hint though and not a mandate for optimizations.
This approach is very similar to attributes in LLVM IR and just like an IR
function can have attributes, a type can also posses attributes/metadata.
(Whether it should be an attribute or metadata is a choice but
that would not deviate from the purpose).

This approach is far more adoptable and convincing than introducing
a whole new type which would be massive complexity for the type system.



On Sun, Jun 6, 2021 at 2:32 PM James Courtier-Dutton via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Also, the comment below is wrong. At this point, arr3 is equivalent to
> arr2, which is q.
>
>  // Now arr3 is equivalent to arr1, which is p.
>   int *r;
>   memcpy(&r, (unsigned char *)arr3, sizeof(r));
>   // Now r is p.
>   *p = 1;
>   *r = 10;
>
>
>
> On Sun, 6 Jun 2021 at 08:54, James Courtier-Dutton
> <james.dutton at gmail.com> wrote:
> >
> > Hi,
> >
> > I would also oppose adding a byte type, but mainly because the bug
> > report mentioned (https://bugs.llvm.org/show_bug.cgi?id=37469) is not
> > a bug at all.
> > The example in the bug report is just badly written C code.
> > Specifically:
> >
> > int main() {
> >   int A[4], B[4];
> >   printf("%p %p\n", A, &B[4]);
> >   if ((uintptr_t)A == (uintptr_t)&B[4]) {
> >     store_10_to_p(A, &B[4]);
> >     printf("%d\n", A[0]);
> >   }
> >   return 0;
> > }
> >
> > "int B[4];" allows values between 0 and 3 only, and referring to 4 in
> > &B[4] is undef, so in my view, it is correctly optimised out which is
> > why it disappears in -O3.
> >
> > Kind Regards
> >
> > James
> >
> >
> > On Sun, 6 Jun 2021 at 05:26, Chris Lattner via cfe-dev
> > <cfe-dev at lists.llvm.org> wrote:
> > >
> > > On Jun 4, 2021, at 11:25 AM, John McCall via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:On 4 Jun 2021, at 11:24, George Mitenkov
> wrote:
> > >
> > > Hi all,
> > >
> > > Together with Nuno Lopes and Juneyoung Lee we propose to add a new byte
> > > type to LLVM to fix miscompilations due to load type punning. Please
> see
> > > the proposal below. It would be great to hear the
> > > feedback/comments/suggestions!
> > >
> > >
> > > Motivation
> > > ==========
> > >
> > > char and unsigned char are considered to be universal holders in C.
> They
> > > can access raw memory and are used to implement memcpy. i8 is the
> LLVM’s
> > > counterpart but it does not have such semantics, which is also not
> > > desirable as it would disable many optimizations.
> > >
> > > I don’t believe this is correct. LLVM does not have an innate
> > > concept of typed memory. The type of a global or local allocation
> > > is just a roundabout way of giving it a size and default alignment,
> > > and similarly the type of a load or store just determines the width
> > > and default alignment of the access. There are no restrictions on
> > > what types can be used to load or store from certain objects.
> > >
> > > C-style type aliasing restrictions are imposed using tbaa
> > > metadata, which are unrelated to the IR type of the access.
> > >
> > > I completely agree with John.  “i8” in LLVM doesn’t carry any
> implications about aliasing (in fact, LLVM pointers are going towards being
> typeless).  Any such thing occurs at the accesses, and are part of TBAA.
> > >
> > > I’m opposed to adding a byte type to LLVM, as such semantic carrying
> types are entirely unprecedented, and would add tremendous complexity to
> the entire system.
> > >
> > > -Chris
> > >
> > > _______________________________________________
> > > cfe-dev mailing list
> > > cfe-dev at lists.llvm.org
> > > https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
> _______________________________________________
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>


-- 
*Disclaimer: Views, concerns, thoughts, questions, ideas expressed in this
mail are of my own and my employer has no take in it. *
Thank You.
Madhur D. Amilkanthwar
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