On Wed, Mar 28, 2018 at 10:30:51AM -0500, Josh Poimboeuf wrote:
> On Wed, Mar 28, 2018 at 07:47:53AM -0700, Manoj Gupta wrote:
> > Please note that there is nothing wrong in the generated code, just
> > that it confuses objtool.
> > Clang has simply omitted the statement where NULL is returned since
> > the pointer was always dereferenced post inlining.
>
> ... but returning NULL would be far more sane than falling through to
> the next function.
Or, as the case may be, oopsing at the point of failure.
> > Note that GCC will also remove the NULL pointers if it knows that the
> > pointer is dereferenced.
> > Here is an example.
> >
> > void null_check(int *P) {
> > int deref = *P;
> > if (P == 0) // GCC won't check the condition.
> > return;
> > *P = 4;
> > }
> >
> > Compiling with gcc -O2 gives:
> > movl $4, (%rdi)
> > ret
>
> This is why we use -fno-delete-null-pointer-checks.
--
Josh
