> You shouldn't need to declare __builtin_* functions anyway. And if a > function can be represented directly with GNU C vector extensions, it's > preferred to implement it that way inline in the header rather than having > built-in functions duplicating existing GNU C functionality. (Look at > what AArch64 arm_neon.h does where possible, and what ARM arm_neon.h has > been moved towards lately. I don't now what the msa.h functions do, so I > don't know if this actually applies to any of them - but it's something to > consider, so that built-in functions are only defined where actually > needed.)
In the aarch64 arm_neon.h header there are a decent number of inline asm implementations too instead of builtins. It is not immediately obvious to me as to what the deciding factor is between adding a builtin and using inline asm when vector extensions do not support the operation. Do you happen to know why inline asm is used in places? This looks like a reasonable idea to use GNU extensions where available. The down-side to this approach is that it may be necessary to write quite dis-similar headers for LLVM vs GCC which I think is part of the reason why the header is written as it is. I don't know if that is a good reason to require builtins or not though. Regards, Matthew
