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    • David Howells's avatar
      [PATCH] LOG2: Implement a general integer log2 facility in the kernel · f0d1b0b3
      David Howells authored
      This facility provides three entry points:
      
      	ilog2()		Log base 2 of unsigned long
      	ilog2_u32()	Log base 2 of u32
      	ilog2_u64()	Log base 2 of u64
      
      These facilities can either be used inside functions on dynamic data:
      
      	int do_something(long q)
      	{
      		...;
      		y = ilog2(x)
      		...;
      	}
      
      Or can be used to statically initialise global variables with constant values:
      
      	unsigned n = ilog2(27);
      
      When performing static initialisation, the compiler will report "error:
      initializer element is not constant" if asked to take a log of zero or of
      something not reducible to a constant.  They treat negative numbers as
      unsigned.
      
      When not dealing with a constant, they fall back to using fls() which permits
      them to use arch-specific log calculation instructions - such as BSR on
      x86/x86_64 or SCAN on FRV - if available.
      
      [akpm@osdl.org: MMC fix]
      Signed-off-by: default avatarDavid Howells <dhowells@redhat.com>
      Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
      Cc: Paul Mackerras <paulus@samba.org>
      Cc: Herbert Xu <herbert@gondor.apana.org.au>
      Cc: David Howells <dhowells@redhat.com>
      Cc: Wojtek Kaniewski <wojtekka@toxygen.net>
      Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
      Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
      f0d1b0b3
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