Messages in this thread | | | From | "Alexander van Heukelum" <> | Subject | Re: [PATCH] asm-generic/bitops/fls64.h | Date | Tue, 13 May 2008 16:24:04 +0200 |
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On Tue, 13 May 2008 14:58:39 +0100, "Russell King" <rmk+lkml@arm.linux.org.uk> said: > On Tue, May 13, 2008 at 03:24:13PM +0200, Alexander van Heukelum wrote: > > On Tue, 13 May 2008 16:29:04 +0400, "Nickolay Vinogradov" > > <nickolay@protei.ru> said: > > > Alexander van Heukelum пишет: > > > > > > > Hi Nickolay, > > > > > > > > The change is ok, I guess, but the cast should be a no-op (fls > > > > takes an int, which is always 32 bit in linux). What is the problem > > > > you are seeing? Does fls64() return a wrong value in some cases? If > > > > so, what cpu? Which values? > > > > > > > > Why would this be a bug on big endian systems only? There is no > > > > pointer magic involved, so the compiler should take care of the > > > > casts in a correct way. > > > > > > > > Maybe you see a compiler warning? Which compiler version? > > > > > > > > (also note that current (development) kernels now have separate > > > > versions for 32-bit and 64-bit environments.) > > > > > > Because fls() is a macro for asm-arm: > > > > > > #define fls(x) \ > > > ( __builtin_constant_p(x) ? constant_fls(x) : \ > > > ({ int __r; asm("clz\t%0, %1" : "=r"(__r) : "r"(x) : "cc"); > > > 32-__r; }) ) > > > > > > We can fix it right here: > > No. "fls" is for finding the last set bit in an _int_. It is not > supposed to have random crap passed to it, such as types longer than > sizeof(int). > > If you're going to pass long long (64-bit) arguments to fls, and then > cast them to a u32, you're truncating the value, and you'll get the > wrong answer if bit 33 or greater is set. If you don't actually care > about the upper bits, don't pass a 64-bit quantity to fls(). > > If you want to use fls with a long long, use fls64 instead. Or for top > marks, use a u64 and fls64.
But that was the problem we began with: the generic fls64 passes an u64 to fls. Nickolay's original patch solves that by putting a cast to u32 in fls64. I did not, however, understand why the cast was needed. It should not be needed for correctness, imho, because fls is expected to behave as if it was a function "int fls(int)", like ffs. Values passed to fls should thus be truncated if necessary.
Making the truncation explicit in fls64 is still a good idea, though.
Greetings, Alexander
> -- > Russell King > Linux kernel 2.6 ARM Linux - http://www.arm.linux.org.uk/ > maintainer of: -- Alexander van Heukelum heukelum@fastmail.fm
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