I performed a micro-benchmark on my dual epyc (zen 2) server and version 1 wins for small values of n.
20 runs: n version min median mean max stddev noise% ----------------------------------------------------------------------- n=1 version1 2.440 2.440 2.450 2.550 0.024 4.5% n=1 version2 4.260 4.280 4.277 4.290 0.007 0.7% n=2 version1 2.740 2.750 2.757 2.880 0.029 5.1% n=2 version2 3.970 3.980 3.980 4.020 0.010 1.3% n=4 version1 4.580 4.595 4.649 4.910 0.094 7.2% n=4 version2 5.780 5.815 5.809 5.820 0.013 0.7% But, micro-benchmarks always make me nervous, so I looked at the actual instruction cost for my platform given the version 1 and version 2 code. If we count cpu cycles using the AMD Zen 2 instruction latency/throughput tables: version 1 (loop body) has a critical path of ~5-6 cycles per iteration. version 2 (loop body) has ~3-4 cycles per iteration. The problem for version 2 is that the call to memcpy is ~24-30 cycles due to the stub + function call + return and branch predictor pressure on first call. This probably results in ~2.5 ns per iteration cost for version 2. So, no I wouldn't call it an optimization. But, it will be interesting to hear other opinions on this. --bg On Mon, Mar 9, 2026 at 10:25 AM Ranier Vilela <[email protected]> wrote: > > > Em seg., 9 de mar. de 2026 às 11:47, Bryan Green <[email protected]> > escreveu: > >> I created an example that is a little bit closer to the actual code and >> changed the compiler from C++ to C. >> >> It is interesting the optimization that the compiler has chosen for >> version 1 versus version 2. One calls >> memcpy and one doesn't. There is a good chance the inlining of memcpy as >> SSE+scalar per iteration >> will be faster for syscache scans-- which I believe are usually small >> (1-4 keys?). >> > I doubt the inline version is better. > Clang is supported too and the code generated is much better with memcpy > one call outside of the loop. > > >> >> Probably the only reason to do this patch would be if N is normally large >> or if this is considered an >> improvement in code clarity without a detrimental impact on small N >> syscache scans. >> I realize you only said "possible small optimization". It might be >> worthwhile to benchmark the code for >> different values of n to determine if there is a tipping point either way? >> > In your opinion, shouldn't this be considered an optimization, even a > small one? > > best regards, > Ranier Vilela >
