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Date: 9 Feb 2015 11:45:42 -0500
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From: "George Spelvin" <linux@horizon.com>
To: linux@horizon.com, linux@rasmusvillemoes.dk
Subject: Re: [PATCH v3 1/3] lib: find_*_bit reimplementation
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Sorry, I screwed up the bit-twiddling while messing with various options.
I was trying to get size == 32 to work; that should have been:

> 	tmp &= (2UL << ((size-1) % BITS_PER_LONG)) - 1;	/* Mask last word */

And you're right that LAST_WORD_MASK is a good wrapper.

Vasrious working solutions include:
#define LAST_WORD_MASK(bits) 	((2UL << (bits-1) % BITS_PER_LONG) - 1)
#define LAST_WORD_MASK(bits) 	~(~0UL << bits % BITS_PER_LONG)
#define LAST_WORD_MASK(bits) 	(~0UL >> -bits % BITS_PER_LONG)

I'm not sure which generates the nicest code.  It's 4 instructions
each way, with the last being 1 byte smaller:

0000000000000000 <lwm1>:
   0:   8d 4f ff                lea    -0x1(%rdi),%ecx
   3:   b8 02 00 00 00          mov    $0x2,%eax
   8:   48 d3 e0                shl    %cl,%rax
   b:   48 83 e8 01             sub    $0x1,%rax
   f:   c3                      retq   

0000000000000010 <lwm2>:
  10:   48 c7 c0 ff ff ff ff    mov    $0xffffffffffffffff,%rax
  17:   89 f9                   mov    %edi,%ecx
  19:   48 d3 e0                shl    %cl,%rax
  1c:   48 f7 d0                not    %rax
  1f:   c3                      retq   

0000000000000020 <lwm3>:
  20:   89 f9                   mov    %edi,%ecx
  22:   f7 d9                   neg    %ecx
  24:   48 c7 c0 ff ff ff ff    mov    $0xffffffffffffffff,%rax
  2b:   48 d3 e8                shr    %cl,%rax
  2e:   c3                      retq   


> Also, I think it is best to handle size==0 appropriately, meaning that
> one cannot dereference addr in any way (and certainly not addr[-1]).

Ah, okay; l I figured that was a safe case to omit.  But your solution is nicer
than mine overall.

It may be that omitting the mask *is* safe, but it's a lot of wading through
callers to prove it.
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