[PATCH] D47894: [clang]: Add support for "-fno-delete-null-pointer-checks"

[James Y Knight via Phabricator via cfe-commits]

jyknight added a comment.

In https://reviews.llvm.org/D47894#1125961, @srhines wrote:

> In https://reviews.llvm.org/D47894#1125728, @jyknight wrote:
>
> > In https://reviews.llvm.org/D47894#1125653, @efriedma wrote:
> >
> > > The problem would come from propagating nonnull-ness from something which isn't inherently nonnull.  For example, strlen has a nonnull argument, so `strlen(NULL)` is UB, therefore given `int z = strlen(x); if (x) {...}`, we can remove the null check.  (Not sure we actually do this transform at the moment, but it's something we could do in the future.)
> >
> >
> > I think the question there is actually whether we need to, in addition to supporting null pointer dereference, also cause all `__attribute__((nonnull))` annotations at the C/C++ level to be ignored.
> >
> > And, yes, I believe we should.
>
>
> Is that what the kernel community actually wants though? There are certainly others who want a way to strip `__attribute__((nonnull))` completely, which seems a bit orthogonal to the removal of null checks in general. I think it would be good to support such a flag, but the presence of `nonnull` inside kernel sources leads me to believe they want those cases to be treated specially.


In GCC, the nonnull attribute still has an effect on warnings but not on codegen if you enable -fno-delete-null-pointer-checks: https://godbolt.org/g/7SSi2V

That seems a pretty sensible behavior, IMO.


Repository:
  rC Clang

https://reviews.llvm.org/D47894