Re: [RFC] LKMM: Add volatile_if()

[Alan Stern]

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