Deoptimizing a C++ program

Re:Deoptimizing a C++ program by nkurz on Sunday May 22, 2016 @05:06PM (#11750294)

I'm finding this more difficult than I expected, at least if I interpret the problem as finding "deoptimizations" that are specific to modern Intel processors. Finding ways to slow things down through changing the algorithm, increasing precision, or adding unnecessary work are easy, but I haven't yet found anything that would be a plausible optimization made by a beginning programmer that hurts performance more on modern Intel than would be expected.

In the answers, I think Peter covers most of the likely attacks. One that isn't mentioned is "choosing the wrong compiler". Not just compiler options, but compiler. While there isn't a "best" compiler for everything, for a given program often one will perform better than the others. Especially if one is targetting recent Intel CPUs, Intel's compiler can occasionally beat GCC and Clang by surprisingly large amounts. This appears to be one of those times.

Here are the times I see for for this example on a 3.4 GHz Skylake running 64-bit Linux when compiled with different modern compilers:

  icpc 16.03:  0.87 s
  clang++ 3.8: 1.76 s
  g++ 5.3.0:   1.73 s

All were compiled with "-march=native -g -Ofast", which I think causes all of the compilers to use the equivalent of "-ffast-math". At least, I'm pretty sure icpc uses it by default, and I didn't see any difference in performance when I added "-ffast-math" or "-funsafe-math-optimizations" to clang++ and g++. So in this case, I (provocatively) would suggest that using clang++ or g++ would be a subtle 2x deoptimization. There are definitely reasons to prefer them, but for this example that choice looks to come at a large cost in performance.

If you want to do your own comparison, Version 17 of the Intel compiler is currently available as a free beta for Windows, Mac, and Linux: https://software.intel.com/en-us/articles/intel-parallel-stu.... I don't know how they are defining the license duration, but when I downloaded yesterday it gave me a license valid until October 2016. Separately, I'd be interested to see what MSVC does with this on a comparable machine.

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Re:Deoptimizing a C++ program by dsharlet on Sunday May 22, 2016 @05:39PM (#11750397)

Without digging into it too much, I would bet that most of the improvement from ICC comes from their awesome implementations of transcendental functions used in this program (exp, sqrt, etc.).

Still, the general point is good. last time I tried ICC on a heavily vectorized (with intrinsics) program, ICC was a 30% boost over clang or gcc.

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Re:Deoptimizing a C++ program by nkurz on Sunday May 22, 2016 @07:19PM (#11750719)

This was my first guess, but as I dig a bit deeper I'm less sure. It looks like the difference is the implementation of log and exp, but both of them are linked to the same libm from glibc. The difference is that Intel is inlining an AVX2 optimized version that uses FMA, but Clang and GCC are using the compiled function from libm.

The Ubuntu packages I'm using are compiled appear to have been compiled to support AVX, but not AVX2, and hence no FMA. So while GCC and Clang are much slower, this is really an issue of having suboptimal system libraries. The speedup is real, but it's not clear that either GCC or Clang are actually to blame.

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Re:Deoptimizing a C++ program by slavik81 on Sunday May 22, 2016 @08:30PM (#11750992)

Not an expert on this, but I think the compiler has to inline the function for that to work well. No matter how good the system library implementation is, the function signature is defined one scalar float, which limits vectorization.

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Re:Deoptimizing a C++ program by pcordes on Sunday May 22, 2016 @11:02PM (#11751560)

That's correct; I'm sure icc has vectorized versions of those functions to inline. Each monte-carlo iteration is independent, so the code vectorizes very easily if you have a vectorized RNG and vectorized exp() and log(). (vector sqrt() is available in hardware, so even gcc and clang will vectorize that, but not exp / log even with -ffast-math)

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Re:Deoptimizing a C++ program by nkurz on Monday May 23, 2016 @12:55AM (#11751964)

OK, I was able to test this more rigorously. Yes, the vast majority of the difference between Intel and GCC is just the better implementations of exp() and log(). These are not vector implementations that calculate multiple results simulataneously, just scalar implementations that make better use of the available instruction set.

When I can get g++ to use Intel's libimf instead of glibc's libm, it's only ~5% slower than icpc. I'd guess much of the remaining difference is function call overhead vs inlining. Clang still lags by another 10%, but I think that's probably because I haven't figured out quite the right incantation to get it to use libimf without also disabling some other useful optimization.

Here are the commands that I ended up with:

clang++ -fno-finite-math-only -march=native -Wall -Wextra -g -O3 option.cc -o option -Wl,-rpath=/opt/intel/compilers_and_libraries/linux/lib/intel64 -L/opt/intel/compilers_and_libraries/linux/lib/intel64 -limf -lintlc

g++ -fno-finite-math-only -march=native -Wall -Wextra -g -Ofast option.cc -o option -Wl,-rpath=/opt/intel/compilers_and_libraries/linux/lib/intel64 -L/opt/intel/compilers_and_libraries/linux/lib/intel64 -limf -lintlc

icpc -march=native -Wall -Wextra -g -Ofast option.cc -o option

The "-fno-finite-math-only" disables an otherwise good clang++ and g++ optimization that requires a __finite_exp() function that libimf does not have. clang++ seems to also need to switch from -Ofast to -O3 to make this stick.

What I don't know yet is whether recompiling glibc (or upgrading to the most recent glibc) will produce better performance out-of-the-box on recent Intel. My searches aren't turning up much information --- anyone know?

gcc and clang will vectorize that, but not exp / log

I don't know how well it's currently working, but it looks like this might have changed recently: https://sourceware.org/glibc/wiki/libmvec

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Re:Deoptimizing a C++ program by danieltillett on Sunday May 22, 2016 @05:16PM (#11750312)

I use icc on my main application which is cpu limited and it halves the run time compared to either clang or gcc. Intel has some very clever complier engineers.

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Re:Deoptimizing a C++ program by pcordes on Thursday May 26, 2016 @03:56PM (#11780968)

I added a link to this post in an update to my answer (where I added a section about causing AVX<->SSE transition stalls, which is actually highly plausible).

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