As I understand it, Intel will also sell you a compiler that can optimize better than GCC because they know internal microarchitecture details. To me, that seems like a more clear-cut by-design conflict of interest between chip designer and user than these undocumented instructions.
Intel's compiler is basically free for plebs but not for datacentres.
LLVM and GCC are good enough (brilliant, even) for general purpose compilation, but for a (say) supercomputer which is guarded by guns having Intel on the phone can be worth it.
Clang and GCC regularly go toe-to-toe with or exceed ICC and have plenty of architectural cost models for modern systems, in part thanks to the extensive documentation and free tools from Intel themselves. ICC does have some gimmick features like GPU offload, and (last I saw) much better auto-vectorization. But as far as run of the mill code goes, you aren't going to get some magical double-digit performance uplift on average scalar code that can only be attained by ICC and/or Intel doing subterfuge.
Beyond that, making a claim of a conflict of interest requires more than just "It seems like these words fit and sound right, to me." Because "Microprocessor company writes well optimized code for their microprocessors and sells it" hardly seems like a conflict of interest at all in this case, it literally seems like a company selling a product that goes with their existing product. Absolutely everybody does this, including AMD, IBM, Nvidia, Apple, etc. It is not a "conflict of interest" for Intel to both design a microprocessor and also sell code that works well on it. What might be a conflict of interest is if they said "We have an obligation to release these specifications, but also we can make lots of money off them if if we keep them private despite that existing obligation, so we'll just keep them private." Which is a much stronger claim. (In fact even the infamous "cripple AMD" legal SNAFU over ICC wasn't a conflict of interest, it was brought as an antitrust suit, AFAIK.) It's not like Twilio has a "conflict of interest" when they implement new features in their product portfolio, build them on Twilio's servers, and integrate them with other Twilio products.
In general, Intel is ahead of almost every other vendor when it comes to publicly-available information regarding performance optimization (first and third party), by a landslide.
GCC and Clang both generally match ICC’s performance, and in my experience beat it more often than they lose to it. ICC’s only real advantage is MKL and other bundled libraries, which are generally better than what free software has to offer.
