On December 1, 2003 05:53 pm, Lev Walkin wrote: > Geoff Thorpe wrote: > > As part of the general hackathon/audit we're doing in crypto/bn/ I > > once again came across the curious zeroing code in bn_expand2, only > > this time I figured it was high time for me to actually ask you about > > it. :-) > > > > I understand the desire to cater for CPU pipelining with the 8-wise > > loop unrolling, but is this a better solution than just using > > memset() and letting the compiler take care of the same sort of > > thing? > > It's #ifdef'ed already, so you may've as well tried it out and check > performance with profiler.
Um, not in bn_expand2() it's not - though there are "#if 1" casings around similar code for BN_copy() et al. I'm looking at CVS by the way, so if you have stable releases or older snapshots, you may be seeing different code than I am. > Even if profiling would show you that memset() is faster, it may be by > order of magnitude slower on the machine very next to you (which has > different version of C library or different compiler settings). Well this of course is the kind of question that packagers must grapple with all the time. Do I build with "-march=<something-decent>" or do I maintain support for toasters? This isn't really what interests me in this case, though it may of course have interested whoever coded this. > Bottom line: try it out. Well I'm more interested in code maintainability than clock-tick comparisons of memset-vs-direct-assignments. The thing is, if there's a CPU out there that has (or will have) instruction-level support to memset a region of memory, then our direct assignments will buy us nothing. Conversely, if someone's building without optimisation for lcd-cpus, then calling memset() may suck in the most attrocious ways. That's an SEP. From a code maintenance point of view (something we are actively trying to improve inside crypto/bn/ right now), we already use memset()s, memcpy()s, and memmove()s in many places. So basically I'm curious to know if there is some real-world issue that justifies these bn-specific loops, or whether "it seemed like a good idea at the time". Cheers, Geoff -- Geoff Thorpe [EMAIL PROTECTED] http://www.geoffthorpe.net/ ______________________________________________________________________ OpenSSL Project http://www.openssl.org Development Mailing List [EMAIL PROTECTED] Automated List Manager [EMAIL PROTECTED]
