On Wed, 20 Feb 2019, Warren D Smith wrote:
> but if I try to replace that with the nicer (since more portable)
>    c = __builtin_shuffle(a, b);
> then
> error: use of unknown builtin '__builtin_shuffle'
> [-Wimplicit-function-declaration]

Most likely you're on OS X and the 'gcc' command actually invokes Clang/LLVM.
Clang does not implement this builtin (there's __builtin_shufflevector with
a different interface — see Clang documentation for details).

Alexander