On Monday 27 August 2012 15:25:14 Marc Mutz wrote: > If atomic loads and stores on x86 are implemented with a volatile cast, > then the compiler can't reorder stuff around them, either. Not more than > with a std::atomic, at least. QAtomic does that. For load-relaxed, Thiago > thinks that a normal read (non-volatile) is correct and I couldn't prove > him wrong.
I was talking to Julian about this again today, and he pointed me to this writeup: http://software.intel.com/en-us/blogs/2007/11/30/volatile-almost-useless-for-multi-threaded-programming We're looking at how to silence valgrind about Qt atomic ops, but before that, it would actually be good to be sure that what Qt does it correct, on x86.... Where does the claim about "volatile cast means the compiler can't reorder stuff around them" come from? In the Qt source code I see qatomic_gcc.h (which is unused, unfortunately) calling __sync_synchronize() in loadAcquire(). Shouldn't Qt's code call that, when compiled with gcc? This does lead to different assembly, too, on x86. So the claim that x86 doesn't need memory barriers seems wrong? --- testcase_atomic_ops_helgrind.s.orig 2013-01-23 15:04:20.889417624 +0100 +++ testcase_atomic_ops_helgrind.s 2013-01-23 15:07:06.938422071 +0100 @@ -380,6 +380,7 @@ _ZN10QAtomicOpsIiE11loadAcquireERKi: movq -24(%rbp), %rax movl (%rax), %eax movl %eax, -4(%rbp) + mfence movl -4(%rbp), %eax popq %rbp .cfi_def_cfa 7, 8 @@ -403,6 +404,7 @@ _ZN10QAtomicOpsIiE12storeReleaseERii: movq -8(%rbp), %rax movl -12(%rbp), %edx movl %edx, (%rax) + mfence popq %rbp .cfi_def_cfa 7, 8 ret -- David Faure, fa...@kde.org, http://www.davidfaure.fr Working on KDE, in particular KDE Frameworks 5 ------------------------------------------------------------------------------ Master Visual Studio, SharePoint, SQL, ASP.NET, C# 2012, HTML5, CSS, MVC, Windows 8 Apps, JavaScript and much more. Keep your skills current with LearnDevNow - 3,200 step-by-step video tutorials by Microsoft MVPs and experts. ON SALE this month only -- learn more at: http://p.sf.net/sfu/learnnow-d2d _______________________________________________ Valgrind-users mailing list Valgrind-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/valgrind-users
