Re: [Emacs-diffs] trunk r115095: Simplify, port and tune bool vector implementation.

[Daniel Colascione]

>  (bits_word_to_host_endian): Prefer if to #if.  Don't assume
>  chars are narrower than ints.

We will never run on machines where sizeof(char) == sizeof(int).

-      changed |= adata[i] ^ mword;
+      if (! changed)
+       changed = adata[i] != mword;

That's not an optimization. Branches are expensive on misprediction, and 
this branch will frequently be mispredicted. ALU operations are cheap, 
especially when they're done on registers and don't introduce new data 
hazards. It's not trivial for an optimizer to notice that using 
bitwise-OR here is safe. Please change the code back to the way it was 
before.

+       * data.c (bool_vector_binop_driver): Don't assume bit vectors always
+       contain at least one word.

Zero-length bool vectors are _extremely_ uncommon. Please don't reduce 
efficiency in the general case.

Also, from a previous patch:

>  (Fbool_vector_count_matches_at): Don't assume CHAR_BIT == 8.

We write "CHAR_BIT" instead of "8" for clarity, not because the number 
of bits per byte might change. In general-purpose machines, CHAR_BIT 
will be 8 and might as well be a physical constant.

       mword >>= offset;
+      mword |= (bits_word) 1 << (BITS_PER_BITS_WORD - offset);
       count = count_trailing_zero_bits (mword);
-      count = min (count, BITS_PER_BITS_WORD - offset);
       pos++;

That's a good change: cutting off the bit count is better than clamping 
the result of the count operatin. Thanks. Can you comment the line you 
added, though? Its purpose isn't immediately obvious.