Just a personal anecdote, but it regularly makes a substantial (like 2x+) difference for me in tight linear algebra loops. It seems to be required if you want to coax clang to emit vfmad instructions for common C linear algebra idioms. Narrow use case, I know, but it can definitely make a big difference in some domains.
I'll take your word for it. I guess I live in a world where the code I see is rarely tight algebra loops, but I can see that in certain domains you would definitely care.
The developer must also be smart enough not to do the mistake to lie to the compiler, by actually doing aliasing with restricted variables as it is UB and nasal dragons beware.
C compilers don't validate correct use of restrict.
