Re: [PATCH 01/12] extarray: define helpers for arrays defined in linker scripts

From: Peter Zijlstra
Date: Wed Oct 19 2016 - 12:11:23 EST

On Wed, Oct 19, 2016 at 10:18:43AM +0200, Richard Biener wrote:

> The commit implements a long-standing failure to optimize trivial pointer
> comparisons that arise for example from libstdc++. PR65686 contains
> a simple C example:
> mytype f(struct S *e)
> {
> mytype x;
> if(&x != e->pu)
> __builtin_memcpy(&x, e->pu, sizeof(unsigned));
> return x;
> }
> where GCC before the commit could not optimize the &x != e->pu test
> as trivial false.

Which is fine; x is stack based and could not possibly have been handed
as the argument to this same function.

This is also an entirely different class of optimizations than the whole
pointer arithmetic is only valid inside an object thing.

The kernel very much relies on unbounded pointer arithmetic, including
overflow. Sure, C language says its UB, but we know our memory layout,
and it would be very helpful if we could define it.

Can't we get a knob extending -fno-strict-aliasing to define pointer
arithmetic outside of objects and overflow? I mean, we already use that,
we also use -fno-strict-overflow and a whole bunch of others.

At the very least, it would be nice to get a -W flag for when this alias
analysis stuff kills something so we can at least know when GCC goes and
defeats us.