Re: [RFC] LKMM: Add volatile_if()

Alan Stern
2021-06-06T13:45:37Z

On Sun, Jun 06, 2021 at 06:53:36AM -0500, Segher Boessenkool wrote:
> On Sat, Jun 05, 2021 at 09:29:03PM -0400, Alan Stern 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.
>
> And that is a simple fact, since the same assembler code (at the same
> spot in the program) will do the same thing no matter how that ended up
> there.

Sure. But the same assembler code at two different spots in the program
might not do the same thing. (Think of code that stores the current EIP
register's value into a variable.)

So while de-duplicating such code may be allowed, it will give rise to
observable results at execution time.

Alan

> And the compiler always is allowed to duplicate, join, delete, you name
> it, inline assembler code. The only thing that it cares about is
> semantics of the code, just like for any other code.
>
>
> Segher