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Re: [Emacs-diffs] trunk r115095: Simplify, port and tune bool vector imp
Re: [Emacs-diffs] trunk r115095: Simplify, port and tune bool vector implementation.
Sun, 17 Nov 2013 04:06:55 -0800
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> (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
+ * 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);
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.
- Re: [Emacs-diffs] trunk r115095: Simplify, port and tune bool vector implementation.,
Daniel Colascione <=