Comment by Const-me

8 years ago

> Using intrinsics certainly does not guarantee that exactly the corresponding instructions will be emitted in exactly the given order.

I hope they will fix the compilers to add such guarantees. Currently, every time compilers try to mess with manually-written intrinsics, they decrease performance.

See this bug about LLVM failing to emit the exact instructions, decreasing the performance: https://bugs.llvm.org/show_bug.cgi?id=26491

See this bug about VC++ failing to emit them in the given order, again decreasing performance: https://developercommunity.visualstudio.com/content/problem/...

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Const-me

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nikic  8 years ago

As usual, things aren't quite that simple. While it certainly can happen that the compiler can pessimize carefully crafted SIMD code by applying undesirable transformations, simply not optimizing intrinsics would lead to other problems.

If you consider SIMD code that spans more than just one small function, then it is quite useful if the compiler can SROA (e.g. you pass around an array of vectors instead of having 8 arguments to each function, but still want them to end up in registers eventually), LICM (you're calling a function that needs a constant vector in a loop) or otherwise optimize your code.

If you want the compiler to leave alone your intrinsics, link in an assembly file.

  • Const-me  8 years ago

    > it is quite useful if the compiler can SROA

    __attribute__((always_inline)) / __forceinline usually help.

    > LICM (you're calling a function that needs a constant vector in a loop)

    I can calculate that vector outside of the loop.

    > link in an assembly file.

    Way more complex, I need to write outer scalar code in assembly too, need to manually allocate registers, also C++ templates are sometimes very useful for that kind of code.

    In 99% of cases intrinsics are good enough for me, but I would love the compilers to leave alone my intrinsics.