Messages in this thread | | | Date | Mon, 9 Feb 2009 10:49:51 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [ltt-dev] [RFC git tree] Userspace RCU (urcu) for Linux (repost) |
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On Mon, Feb 09, 2009 at 10:37:42AM -0800, Paul E. McKenney wrote: > On Mon, Feb 09, 2009 at 01:13:41PM -0500, Mathieu Desnoyers wrote: > > * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
[ . . . ]
> > You know what ? Changing RCU_GP_CTR_BIT to 16 uses a > > testw %ax, %ax instead of a testb %al, %al. The trick here is that > > RCU_GP_CTR_BIT must be a multiple of 8 so we can use a full 8-bits, > > 16-bits or 32-bits bitmask for the lower order bits. > > > > On 64-bits, using a RCU_GP_CTR_BIT of 32 is also ok. It uses a testl. > > > > To provide 32-bits compability and allow the deepest nesting possible, I > > think it makes sense to use > > > > /* Use the amount of bits equal to half of the architecture long size */ > > #define RCU_GP_CTR_BIT (sizeof(long) << 2) > > You lost me on this one: > > sizeof(long) << 2 = 0x10 > > I could believe the following (run on a 32-bit machine): > > 1 << (sizeof(long) * 8 - 1) = 0x80000000 > > Or, if you were wanting to use a bit halfway up the word, perhaps this: > > 1 << (sizeof(long) * 4 - 1) = 0x8000 > > Or am I confused?
Well, I am at least partly confused. You were wanting a low-order bit, so you want to lose the "- 1" above. Here are some of the possibilities:
sizeof(long) = 0x4 sizeof(long) << 2 = 0x10 1 << (sizeof(long) * 8 - 1) = 0x80000000 1 << (sizeof(long) * 4) = 0x10000 1 << (sizeof(long) * 4 - 1) = 0x8000 1 << (sizeof(long) * 2) = 0x100 1 << (sizeof(long) * 2 - 1) = 0x80
My guess is that 1 << (sizeof(long) * 4) and 1 << (sizeof(long) * 2) are of the most interest.
Thanx, Paul
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