Having said that, compilers do pretty badly on C-to-simd optimisation. The best you get is loop vectorization if it's really simple logic. You can usually get some pretty good wins there. The fact you lay out your memory for simd usually is a win all of its own due to cache prefetching even if you don't actually use any simd instructions. Compilers need heuristics to manage cache when you know what you're trying to do, (eg when should it use non-temporal writes, for example?) Fast C code is written while having a really clear mental model of the underlying architecture and the assembly that the C will produce with -O3 (or whatever flag is relevant to your compiler) and then checked with -S or objdump -D, profiled with callgrind/cachegrind, perf, rdtsc etc...

The compiler really can't "Do it for you" You /can/ use a compiler as one of your tools when /you/ do it. As Randy Hyde points out you can always beat the compiler because you can use its generated assembly language in every case you can't beat, so the absolute worst you get is a tie.

So yeah, you can totally smoke clang, Intel, microsoft and gnu C compiler and get paid something for doing it in certain industries too. :-)

Mike Acton being aggressively opinionated on the subject, but the lecture is really good (despite/because of) the bits you'll disagree with and the manner he'll rub you the wrong way. https://www.youtube.com/watch?v=rX0ItVEVjHc

    [trent@ubuntu/ttypts/4(~s/wrk)%] ./wrk -c 1 -t 1 --latency -d 5 http://localhost:8080/Makefile 
    Running 5s test @ http://localhost:8080/Makefile
      1 threads and 1 connections
      Thread Stats   Avg      Stdev     Max   +/- Stdev
        Latency    35.00us    0.00us  35.00us  100.00%
        Req/Sec    10.00      0.00    10.00    100.00%
      Latency Distribution
         50%   35.00us
         75%   35.00us
         90%   35.00us
         99%   35.00us
      1 requests in 5.10s, 1.57KB read
    Requests/sec:      0.20
    Transfer/sec:     314.55B

    [trent@ubuntu/ttypts/4(~s/wrk)%] ./wrk -c 1000 -t 1 --latency -d 5 http://localhost:8080/Makefile
    Running 5s test @ http://localhost:8080/Makefile
      1 threads and 1000 connections
      Thread Stats   Avg      Stdev     Max   +/- Stdev
        Latency   307.93ms  552.88ms   1.63s    87.10%
        Req/Sec   414.29    439.07     1.34k    85.71%
      Latency Distribution
         50%    4.11ms
         75%  407.63ms
         90%    1.63s
             99%    1.63s
      1000 requests in 5.01s, 1.53MB read
      Socket errors: connect 0, read 41, write 0, timeout 0
    Requests/sec:    199.79
    Transfer/sec:    312.96KB