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

[Mehdi AMINI via llvm-dev]

On Sun, Jun 6, 2021 at 9:25 PM Juneyoung Lee via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> According to C17 it is legal:
>
> 6.5.9.p6
>
> Two pointers compare equal if and only if both are null pointers, both are
> pointers to the same object (including a pointer to an object and a
> subobject at its beginning) or function, both are pointers to one past the
> last element of the same array object, or *one is a pointer to one past
> the end of one array object and the other is a pointer to the start of a
> different array object that happens to immediately follow the first array
> object in the address space*.
>

This is interesting, I'm curious (we're a bit off-topic though) how is the
part about "that happens to immediately follow the first array object in
the address space" defined?


>
>
>
> On Mon, Jun 7, 2021 at 6:25 AM Joerg Sonnenberger via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> On Sun, Jun 06, 2021 at 11:52:13AM -0700, Marshall Clow via llvm-dev
>> wrote:
>> > On Jun 6, 2021, at 12:54 AM, James Courtier-Dutton via cfe-dev <
>> cfe-dev at lists.llvm.org> 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.
>> >
>> > Taking the address of the “one-past-the end” element of an array is
>> perfectly legal in both C and C++.
>> > *Dereferencing* that pointer is UB.
>>
>> Not just dereferencing, but also comparing it to a random other point?
>>
>> Joerg
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>>
>
>
> --
>
> Juneyoung Lee
> Software Foundation Lab, Seoul National University
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