Re: Linux and aliasing?

[Joe Buck <jbuck@Synopsys.COM>]

> AFAICT, in a cast to `(some_type_t *volatile)', the `volatile' doesn't
> have any actual effect on the generated code, because the pointer has
> already been evaluated.  Couldn't we implement an extension by which
> this `volatile' would kind of have the opposite meaning of `restrict'?
> It would mean that the resulting pointer may be aliased to anything
> else, so the compiler shouldn't move it around nor optimize it ``too
> much''.

It's not a matter of not moving it around or optimizing the pointer
itself.  Rather, after someone writes through one of these anti-restrict
pointers, the compiler has to assume the worst, and re-read everything
from memory that it might have cached away in registers, because we're
saying that the pointer could point to *anything*.  Even a read through
such a pointer can impede optimization, as we'd have to flush everything
out to memory before the read, since the pointer might be reading any
object (the old value of which might be in a register).  This kills
performance of loops over arrays.

For this reason, the effect of proposals like this might be similar in
performance to just saying, if a function contains a cast of a pointer
that is later dereferenced, apply -fno-strict-aliasing to the entire
function, or at least a significant chunk of it.