Date: Fri, 10 Aug 2012 14:06:13 -0400 From: Rich Felker <dalias@...ifal.cx> To: musl@...ts.openwall.com Subject: Re: crypt* files in crypt directory On Fri, Aug 10, 2012 at 09:04:35PM +0400, Solar Designer wrote: > > > why increase ptr at the begining? > > > it seems the idiomatic way would be > > > > > > *ptr++ = L; > > > *ptr++ = R; > > > > For me, making this change makes it 5% faster. I suspect the > > difference comes from the fact that gcc is not smart enough to move > > the ptr+=2; across the rest of the loop body, and the fact that it > > gets spilled to the stack and reloaded for *both* points of usage > > rather than just one. The original version may perform better on > > machines with A LOT more registers, but I'm doubtful... > > The spilling theory makes sense to me, but it does not fully explain the > 5% difference - I think it could explain a 1% difference or so. More > likely there's some change in register allocation overall, not only for > ptr - or something like it. Indeed, that's possible too. I haven't read the asm diff. > Anyhow, this does not match my test results so far, for different > revisions of this code. What compiler, options, architecture, CPU? gcc 4.6.3, -O3, generic/i486 code generation, no tuning for my cpu, which is Atom. Rich
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