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In my understanding when you use intrinsics and build for a processor without support for the intrinsics then GCC for example will replace it with equivalent co

In my understanding when you use intrinsics and build for a processor without support for the intrinsics then GCC for example will replace it with equivalent code.

eesmith 8y ago
That is true. Here's a couple of negatives. First, you still need to build once for each architecture, either as different executables, or as different object files, and provide some dispatch mechanism to use the right one based on what hardware is available. Second, if the intrinsics aren't built-in then there may be faster alternatives than using the GCC emulated version.
- BeeOnRope 8y ago
  You must be thinking about GCC "builtins" because there is no emulation for x86 SIMD intrinsics (ie the things in <immintrin.h>).
  - eesmith 8y ago
    Oh, indeed I was. Thanks for pointing out my error. I was specifically thinking about POPCNT.
mcbain 8y ago
Unfortunately, no. That is the case with GCCs __builtin functions. With a few exceptions, intrinsics are basically macros for inline asm that the compiler can reason about. If on x86-64 you use a _mm256* intrinsic and compile without AVX support you just get a compile error, not a pair of equivalent SSE instructions.
- rurban 8y ago
  Even worse. You mostly get run-time errors when the built machine supported that feature, your machine doesn't, and the features aren't separated into multiversioning or loading different shared libs.