lists.openwall.net | lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening PHC | |
Open Source and information security mailing list archives
| ||
|
Date: Fri, 24 Aug 2007 13:19:04 +0100 From: Denys Vlasenko <vda.linux@...glemail.com> To: Linus Torvalds <torvalds@...ux-foundation.org> Cc: Satyam Sharma <satyam@...radead.org>, Christoph Lameter <clameter@....com>, "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>, Herbert Xu <herbert@...dor.apana.org.au>, Nick Piggin <nickpiggin@...oo.com.au>, Paul Mackerras <paulus@...ba.org>, Segher Boessenkool <segher@...nel.crashing.org>, heiko.carstens@...ibm.com, horms@...ge.net.au, linux-kernel@...r.kernel.org, rpjday@...dspring.com, ak@...e.de, netdev@...r.kernel.org, cfriesen@...tel.com, akpm@...ux-foundation.org, jesper.juhl@...il.com, linux-arch@...r.kernel.org, zlynx@....org, schwidefsky@...ibm.com, Chris Snook <csnook@...hat.com>, davem@...emloft.net, wensong@...ux-vs.org, wjiang@...ilience.com Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures 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 majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Powered by blists - more mailing lists