From: Alan Stern > Sent: 30 September 2024 19:53 > > On Mon, Sep 30, 2024 at 07:05:06PM +0200, Jonas Oberhauser wrote: > > > > > > Am 9/30/2024 um 6:43 PM schrieb Alan Stern: > > > On Mon, Sep 30, 2024 at 01:26:53PM +0200, Jonas Oberhauser wrote: > > > > > > > > > > > > Am 9/28/2024 um 4:49 PM schrieb Alan Stern: > > > > > > > > I should also point out that it is not enough to prevent the compiler from > > > > using @a instead of @b. > > > > > > > > It must also be prevented from assigning @b=@a, which it is often allowed to > > > > do after finding @a==@b. > > > > > > Wouldn't that be a bug? > > > > That's why I said that it is often allowed to do it. In your case it > > wouldn't, but it is often possible when a and b are non-atomic & > > non-volatile (and haven't escaped, and I believe sometimes even then). > > > > It happens for example here with GCC 14.1.0 -O3: > > > > int fct_hide(void) > > { > > int *a, *b; > > > > do { > > a = READ_ONCE(p); > > asm volatile ("" : : : "memory"); > > b = READ_ONCE(p); > > } while (a != b); > > OPTIMIZER_HIDE_VAR(b); > > return *b; > > } > > > > > > > > ldr r1, [r2] > > ldr r3, [r2] > > cmp r1, r3 > > bne .L6 > > mov r3, r1 // nay... > > A totally unnecessary instruction, which accomplishes nothing other than > to waste time, space, and energy. But nonetheless, allowed -- I agree. > > The people in charge of GCC's optimizer might like to hear about this, > if they're not already aware of it... > > > ldr r0, [r3] // yay! > > bx lr > > One could argue that in this example the compiler _has_ used *a instead > of *b. However, such an argument would have more force if we had > described what we are talking about more precisely. The 'mov r3, r1' has nothing to do with 'a'. It is a more general problem that OPTIMISER_HIDE_VAR() pretty much always ends up allocating a different internal 'register' for the output and then allocating a separate physical rehgister. There doesn't seem to be a later optimisation path to remove 'pointless' register moves. David > > Yes, we do want to prevent compilers from doing this. I'm not sure that > it really needs to be mentioned in the comments or commit description, > though. > > Alan - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)