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Re: [PATCH] Simplify strncat.
- From: OndÅej BÃlka <neleai at seznam dot cz>
- To: Adhemerval Zanella <azanella at linux dot vnet dot ibm dot com>
- Cc: libc-alpha at sourceware dot org
- Date: Thu, 18 Dec 2014 20:59:41 +0100
- Subject: Re: [PATCH] Simplify strncat.
- Authentication-results: sourceware.org; auth=none
- References: <20141216202438 dot GA5612 at domone> <20141216203638 dot D237C2C2448 at topped-with-meat dot com> <20141217163206 dot GA26604 at domone> <5491CBFD dot 60503 at linux dot vnet dot ibm dot com>
On Wed, Dec 17, 2014 at 04:31:25PM -0200, Adhemerval Zanella wrote:
> On 17-12-2014 14:32, OndÅej BÃlka wrote:
> > On Tue, Dec 16, 2014 at 12:36:38PM -0800, Roland McGrath wrote:
> >> Use '\0', not '\000'.
> >> IIRC we have a general policy about having a benchtests case and citing
> >> numbers on that.
> > That is not policy as benchtest results on string function are still
> > meaningless. Here I could simply improve benchtest score by inlining
> > strnlen and memcpy implementations, for example with
> > #define memcpy memcpy2
> > #include <string/memcpy.c>
> > As it avoids call overhead. However it would be big mistake to do that
> > "optimization" as strncat is cold function while memcpy is likely in
> > memory it degrade performance due to additonal icache misses.
> This is a fair criticize, but benchtests from string functions are far for
> 'meanigless'. On powerpc side, for instance, I noted unaligned cases
> that current code for strcpy was outperforming. It leads to check more
> result using different workloads and profiler and code a better strategy.
> Which resulted in a better implementation in the end (patch just posted).
Which is quite dangerous, if benchmark is unreliable you cannot be sure
if that improvement is real or will turn into regression. That could now
easily be case as function in benchtest predicts all branches perfectly
which makes path fast in benchtest does not happen in reality. I will post
several changes and we will see truth (I did not read patch so I cannot say
what will happen.)