Messages in this thread | | | Date | Sun, 9 Jul 2006 23:42:50 -0400 | From | Theodore Tso <> | Subject | Re: [OT] 'volatile' in userspace |
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On Sun, Jul 09, 2006 at 10:40:06PM +0200, Rutger Nijlunsing wrote: > > So if a userspace progam ever uses volatile, it's almost certainly a > > bug, one way or another. > > Without 'volatile' and disabling optimizations altogether, how do we > prevent gcc from optimizing away pointers? As can be seen on > http://wiki.rubygarden.org/Ruby/page/show/GCAndExtensions (at > 'Compiler over-optimisations and "volatile"'), volatile is used to > prevent a specific type of optimization. This is because of the > garbage collector, which scans the stack and registers to find > referenced objects. So you don't want local variables containing > references to objects optimized away.
Well, if you look at the Wiki, it admits that this is a bug:
(Warning: This section is not strictly correct. volatile instructs the C compiler that it should not do certain optimisations to code that accesses the variable - the value cannot be stored in a register and must be read from memory each time it is accessed. It is still perfectly legal for the compiler to overwrite the VALUE's stack location with other data, if the compiler decides there are no further uses of the VALUE. Fortunately, a side effect of volatile in common C compilers like GCC and Visual Studio is to prevent the dangerous optimisation described above. The Ruby source itself uses volatile for this purpose, so it is an "accepted hack" for Ruby C extensions.)
"Accepted hack" is basically another way of saying bug. Some day GCC will be made smart enough to optimize the use of str on the stack, and then the Ruby will be screwed. (Mental note to self: don't use Ruby in any future project.)
This is really an architectural bug. RSTRING() should have explicitly bumped a use pointer, which the C code should have then explicitly decremented, to protect the underlying pointer from getting GC'ed unexpectedly. It would have made RSTRING() more difficult to use, but that's the price you pay when you try to graft a garbage-collected paradigm into C code, since the C language really was never designed for it.
So this would tend to confirm the rule of thumb: use of "volatile" in a userspace progam tends to indicate a bug.
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