If I recall correctly, C and C++ allow some types of pointer aliasing that fortran forbids. If you're reading from one buffer and writing to another, those buffers can overlap in C or C++, but can't in fortran, so a fortran compiler is allowed more leeway with the way instructions are ordered (and maybe elided? Not sure). In compute-intensive workloads, every little bit helps.
It's also worth noting that although modern C/C++ has the `restrict` keyword, compilers for those languages are generally worse at actually using that information. For example, there's been a long running series of LLVM bugs that has (several times) required Rust to stop emitting that metadata because it would actually miscompile fairly simple vector operations that used `restrict`[1]. I'm hopeful that Flang (the Fortran LLVM compiler) will shake most of those out, since there's a large body of Fortran code that relies on good aliasing optimizations.
Correct. Back in the days, it used to be that you could run Fortran 77 code through f2c and then compile with gcc using the assume-no-aliasing option, and you would get roughly the same performance as if you compiled with f77.
