[Issue 15873] In order to implement std.simd, compile time info about CPU specifics is needed

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=15873

Marco Leise <Marco.Leise at gmx.de> changed:

           What    |Removed                     |Added
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                 CC|                            |Marco.Leise at gmx.de

--- Comment #7 from Marco Leise <Marco.Leise at gmx.de> ---
My concern is with "fast.json" where the call site reads

  auto json = parseJSON(...);

and I feel that

  import core.cpuid;
  if (sse42)
    handleJson!true();
  else
    handleJson!false();

  void handleJson(bool sse42)()
  {
    auto json = parseJSON!sse42(...);
  }

is just not palatable. ('handleJson' being needed, since the return value would
be a RAII struct with compile-time specialization.) Importing core.cpuid,
figuring out which flag to use and set as a template argument and writing a
switch-case or if-else is not economically reasonable, so to speak when you
could enable SSE4 globally and often implicitly (-march=native). Also in my
case DMD wont profit, because it's inline assembly doesn't inline (making it
too slow) and GDC wont profit because it is not supported by core.cpuid,
leaving only LDC - but that's another story.

My argument here is that the one writing SIMD code is not necessarily the one
calling it. Compile-time information about the (implied) target enables us to
reduce the cognitive load for library users, and still make use of the latest
CPU features. This is working to great benefit with intrinsics in other
compilers (for popcnt, memcpy, etc.), but we can't imitate that. So we ended up
with runtime checks against a global variable in popcnt for what should be a
single instruction on recent CPUs and an additional "SSE4 only" _popcnt in
http://dlang.org/phobos/core_bitop.html#.popcnt

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