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

[Joerg Sonnenberger via cfe-dev]

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