Re: [RFC] LKMM: Add volatile_if()

On Fri, Jun 04, 2021 at 06:37:22PM +0200, Peter Zijlstra wrote:

On Fri, Jun 04, 2021 at 09:30:01AM -0700, Linus Torvalds wrote:

quoted

Why is "volatile_if()" not just

       #define barier_true() ({ barrier(); 1; })

       #define volatile_if(x) if ((x) && barrier_true())

because that should essentially cause the same thing - the compiler
should be *forced* to create one conditional branch (because "barrier"
is an asm that can't be done on the false side, so it can't do it with
arithmetic or other games), and after that we're done.

No need for per-architecture "asm goto" games. No new memory barriers.
No actual new code generation (except for the empty asm volatile that
is a barrier).

Because we weren't sure compilers weren't still allowed to optimize the
branch away.

barrier_true is a volatile asm, so it should be executed on the real
machine exactly as often as on the abstract machine (and in order with
other side effects).  And the && short-circuits, so you will always have
the same effect as a branch.  But there of course is nothing that forces
there to be a branch (as a silly example, the compiler could convert
some control flow to go via computed return addresses).


Segher