Re: [RFC] LKMM: Add volatile_if()

Alan Stern
2021-06-06T13:48:05Z

On Sun, Jun 06, 2021 at 07:59:55AM -0500, Segher Boessenkool wrote:
> On Sat, Jun 05, 2021 at 08:41:00PM -0700, Linus Torvalds wrote:
> > On Sat, Jun 5, 2021 at 6:29 PM Alan Stern <stern@rowland.harvard.edu> wrote:
> > > Interesting. And changing one of the branches from barrier() to __asm__
> > > __volatile__("nop": : :"memory") also causes a branch to be emitted. So
> > > even though the compiler doesn't "look inside" assembly code, it does
> > > compare two pieces at least textually and apparently assumes if they are
> > > identical then they do the same thing.
> >
> > That's actually a feature in some cases, ie the ability to do CSE on
> > asm statements (ie the "always has the same output" optimization that
> > the docs talk about).
> >
> > So gcc has always looked at the asm string for that reason, afaik.
>
> GCC does not pretend it can understand the asm. But it can see when
> two asm statements are identical.

How similar do two asm strings have to be before they are considered
identical? For instance, do changes to the amount of leading or
trailing whitespace matter?

Or what about including an empty assembly statement in one but not the
other?

Alan