Messages in this thread | | | Date | Tue, 28 Feb 2006 01:04:01 -0500 | From | Chuck Ebbert <> | Subject | Re: [patch] i386: make bitops safe |
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In-Reply-To: <20060228005436.GA24895@redhat.com>
On Mon, 27 Feb 2006 at 16:54:36 -0800, Richard Henderson wrote:
> On Tue, Feb 28, 2006 at 12:47:22AM +0100, Andi Kleen wrote: > > I remember asking rth about this at some point and IIRC > > he expressed doubts if it would actually do what expected. Richard? > > It's a bit dicey to be sure. GCC may or may not be able to look > through the size of the array and not kill things beyond it. If > one could be *sure* of some actual maximum index, this would be > fine, but I don't think you can. >
In theory the bit offset could be from -2**31 to 2**31 - 1
> One could reasonably argue that if you used a structure with a > flexible array member, that GCC could not look through that. But > again I'm not 100% positive this is handled properly.
This seems to work but causes more problems than it solves:
#define vaddr ((volatile long *) addr) static inline void set_bit(int nr, volatile unsigned long * addr) { __asm__ __volatile__( "lock ; " "btsl %2,%1" :"+m" (*(vaddr + (nr>>5))) :"m" (*vaddr),"Ir" (nr) ); }
First, it generates the byte offset nr>>5 and puts it in a register even though it will never be used in the asm. I can't find a constraint that says "I'll be accessing this address but I don't need you to generate it for me." Second, the compiler thinks *vaddr will be read when it really won't (unless nr>>5 == 0 in which case constraint 0 takes care of it.)
Generated code when nr is a variable:
movl nr,%edx movl %edx,%eax sarl $5,%eax sall $2,%eax lock ; btsl %edx,addr
This causes a register reload afterward (assuming all regs are busy) and can cause a function to use more stack space. That plus the three extra instructions made me go with the full memory clobber instead.
-- Chuck "Equations are the Devil's sentences." --Stephen Colbert
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