Re: [PATCH 2/8] lib/lzo: clean-up by introducing COPY16

[Markus F.X.J. Oberhumer]

*NOT* acked by Markus Oberhumer, original author of the LZO data compression
library and author of the current Linux kernel LZO implementation.

Please see my email reply to the patch series header for more info.


On 2018-11-30 15:26, Dave Rodgman wrote:
> From: Matt Sealey <matt.sealey@xxxxxxx>
> 
> Most compilers should be able to merge adjacent loads/stores of sizes
> which are less than but effect a multiple of a machine word size (in
> effect a memcpy() of a constant amount). However the semantics of the
> macro are that it just does the copy, the pointer increment is in the
> code, hence we see
> 
>     *a = *b
>     a += 8
>     b += 8
>     *a = *b
>     a += 8
>     b += 8
> 
> This introduces a dependency between the two groups of statements which
> seems to defeat said compiler optimizers and generate some very strange
> sequences of addition and subtraction of address offsets (i.e. it is
> overcomplicated).
> 
> Since COPY8 is only ever used to copy amounts of 16 bytes (in pairs),
> just define COPY16 as COPY8,COPY8. We leave the definition to preserve
> the need to do unaligned accesses to machine-sized words per the
> original code intent, we just don't use it in the code proper.
> 
> COPY16 then gives us code like:
> 
>     *a = *b
>     *(a+8) = *(b+8)
>     a += 16
>     b += 16
> 
> This seems to allow compilers to generate much better code by using
> base register writeback or simply positively incrementing offsets which
> seems to positively affect performance. It is, at least, fewer
> instructions to do the same job.
> 
> Link: http://lkml.kernel.org/r/20181127161913.23863-3-dave.rodgman@xxxxxxx
> Signed-off-by: Matt Sealey <matt.sealey@xxxxxxx>
> Signed-off-by: Dave Rodgman <dave.rodgman@xxxxxxx>
> Cc: David S. Miller <davem@xxxxxxxxxxxxx>
> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> Cc: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
> Cc: Markus F.X.J. Oberhumer <markus@xxxxxxxxxxxxx>
> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> Cc: Nitin Gupta <nitingupta910@xxxxxxxxx>
> Cc: Richard Purdie <rpurdie@xxxxxxxxxxxxxx>
> Cc: Sergey Senozhatsky <sergey.senozhatsky.work@xxxxxxxxx>
> Cc: Sonny Rao <sonnyrao@xxxxxxxxxx>
> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Stephen Rothwell <sfr@xxxxxxxxxxxxxxxx>
> ---
>  lib/lzo/lzo1x_compress.c        |  9 +++------
>  lib/lzo/lzo1x_decompress_safe.c | 18 ++++++------------
>  lib/lzo/lzodefs.h               |  3 +++
>  3 files changed, 12 insertions(+), 18 deletions(-)
> 
> diff --git a/lib/lzo/lzo1x_compress.c b/lib/lzo/lzo1x_compress.c
> index 236eb21167b5..82fb5571ce5e 100644
> --- a/lib/lzo/lzo1x_compress.c
> +++ b/lib/lzo/lzo1x_compress.c
> @@ -60,8 +60,7 @@ lzo1x_1_do_compress(const unsigned char *in, size_t in_len,
>  				op += t;
>  			} else if (t <= 16) {
>  				*op++ = (t - 3);
> -				COPY8(op, ii);
> -				COPY8(op + 8, ii + 8);
> +				COPY16(op, ii);
>  				op += t;
>  			} else {
>  				if (t <= 18) {
> @@ -76,8 +75,7 @@ lzo1x_1_do_compress(const unsigned char *in, size_t in_len,
>  					*op++ = tt;
>  				}
>  				do {
> -					COPY8(op, ii);
> -					COPY8(op + 8, ii + 8);
> +					COPY16(op, ii);
>  					op += 16;
>  					ii += 16;
>  					t -= 16;
> @@ -255,8 +253,7 @@ int lzo1x_1_compress(const unsigned char *in, size_t in_len,
>  			*op++ = tt;
>  		}
>  		if (t >= 16) do {
> -			COPY8(op, ii);
> -			COPY8(op + 8, ii + 8);
> +			COPY16(op, ii);
>  			op += 16;
>  			ii += 16;
>  			t -= 16;
> diff --git a/lib/lzo/lzo1x_decompress_safe.c b/lib/lzo/lzo1x_decompress_safe.c
> index a1c387f6afba..aa95d3066b7d 100644
> --- a/lib/lzo/lzo1x_decompress_safe.c
> +++ b/lib/lzo/lzo1x_decompress_safe.c
> @@ -86,12 +86,9 @@ int lzo1x_decompress_safe(const unsigned char *in, size_t in_len,
>  					const unsigned char *ie = ip + t;
>  					unsigned char *oe = op + t;
>  					do {
> -						COPY8(op, ip);
> -						op += 8;
> -						ip += 8;
> -						COPY8(op, ip);
> -						op += 8;
> -						ip += 8;
> +						COPY16(op, ip);
> +						op += 16;
> +						ip += 16;
>  					} while (ip < ie);
>  					ip = ie;
>  					op = oe;
> @@ -187,12 +184,9 @@ int lzo1x_decompress_safe(const unsigned char *in, size_t in_len,
>  			unsigned char *oe = op + t;
>  			if (likely(HAVE_OP(t + 15))) {
>  				do {
> -					COPY8(op, m_pos);
> -					op += 8;
> -					m_pos += 8;
> -					COPY8(op, m_pos);
> -					op += 8;
> -					m_pos += 8;
> +					COPY16(op, m_pos);
> +					op += 16;
> +					m_pos += 16;
>  				} while (op < oe);
>  				op = oe;
>  				if (HAVE_IP(6)) {
> diff --git a/lib/lzo/lzodefs.h b/lib/lzo/lzodefs.h
> index 497f9c9f03a8..e1b3cf6459a9 100644
> --- a/lib/lzo/lzodefs.h
> +++ b/lib/lzo/lzodefs.h
> @@ -23,6 +23,9 @@
>  		COPY4(dst, src); COPY4((dst) + 4, (src) + 4)
>  #endif
>  
> +#define COPY16(dst, src) \
> +	do { COPY8(dst, src); COPY8((dst) + 8, (src) + 8); } while (0)
> +
>  #if defined(__BIG_ENDIAN) && defined(__LITTLE_ENDIAN)
>  #error "conflicting endian definitions"
>  #elif defined(CONFIG_X86_64)
> 


-- 
Markus Oberhumer, <markus@xxxxxxxxxxxxx>, http://www.oberhumer.com/