Re: [RFC PATCH 4/3] Fix previous patch and add header.

[Joseph Myers <joseph at codesourcery dot com>]

On Tue, 26 May 2015, Ondřej Bílka wrote:

> diff --git a/string/common.h b/string/common.h
> new file mode 100644
> index 0000000..84f9767
> --- /dev/null
> +++ b/string/common.h
> @@ -0,0 +1,35 @@
> +#include <stdint.h>

All files should start with a comment that first describes the file, then 
has the copyright and license notice.

> +/* Use vector arithmetic tricks. Idea is to take expression works on
> +   unsigned byte and evaluates 0 for nozero byte and nonzero on zero byte.
> +   Our expression is ((s + 127) ^ (~s)) & 128  
> +   Now we evaluate this expression on each byte in parallel and on first 
> +   nonzero byte our expression will have nonzero value. */

I think a more detailed, multi-paragraph comment carefully explaining the 
algorithm used (and why and how it works) would be better.  Like the one 
discussed in bug 5806, but of course without the mistake discussed there.

The principle of a common header with this logic is a good one - hopefully 
if it gets used everywhere this can resolve bug 5806 by eliminating all 
copies of the old comment.

(The patch submission should carefully discuss how the algorithm relates 
to the ones discussed in bug 5806 / 5807 or used in existing code.  But 
that's part of justifying the patches rather than for the comments in the 
new code.)

> +static __always_inline
> +unsigned long int 
> +contains_zero (unsigned long int s)
> +{
> +  return ((s + add) ^ ~s) & high_bits;

Instead of using unsigned long int, architectures should be able to 
configure the type used.  I expect that for ilp32 configurations on 64-bit 
architectures, where registers are 64-bit but unsigned long int is 32-bit, 
such as x32 and MIPS n32, it will be better to use unsigned long long int.

> +#define CROSS_PAGE(x, n) (((uintptr_t)x) % 4096 >= 4096 - n)

It might also make sense for the use of 4096 here to be configurable by 
architectures as well.

-- 
Joseph S. Myers
joseph@codesourcery.com