On Saturday 18 August 2007 05:13, Linus Torvalds wrote: > On Sat, 18 Aug 2007, Satyam Sharma wrote: > > No code does (or would do, or should do): > > > > x.counter++; > > > > on an "atomic_t x;" anyway. > > That's just an example of a general problem. > > No, you don't use "x.counter++". But you *do* use > > if (atomic_read(&x) <= 1) > > and loading into a register is stupid and pointless, when you could just > do it as a regular memory-operand to the cmp instruction.
It doesn't mean that (volatile int*) cast is bad, it means that current gcc is bad (or "not good enough"). IOW: instead of avoiding volatile cast, it's better to fix the compiler. > And as far as the compiler is concerned, the problem is the 100% same: > combining operations with the volatile memop. > > The fact is, a compiler that thinks that > > movl mem,reg > cmpl $val,reg > > is any better than > > cmpl $val,mem > > is just not a very good compiler. Linus, in all honesty gcc has many more cases of suboptimal code, case of "volatile" is just one of many. Off the top of my head: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28417 unsigned v; void f(unsigned A) { v = ((unsigned long long)A) * 365384439 >> (27+32); } gcc-4.1.1 -S -Os -fomit-frame-pointer t.c f: movl $365384439, %eax mull 4(%esp) movl %edx, %eax <===== ? shrl $27, %eax movl %eax, v ret Why is it moving %edx to %eax? gcc-4.2.1 -S -Os -fomit-frame-pointer t.c f: movl $365384439, %eax mull 4(%esp) movl %edx, %eax <===== ? xorl %edx, %edx <===== ??! shrl $27, %eax movl %eax, v ret Progress... Now we also zero out %edx afterwards for no apparent reason. -- vda - To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
