Do you say that because you have coded those programs using SIMD and rustc nightly, and seen that using SIMD eliminates the performance difference ?

I have not personally done this, but the people who have identified that as the issue, and what we’re stabilizing is the exact same thing as what C and C++ compilers have, so there’s no reason to believe it would be different. Once it’s stable, we’ll all see!

Someone has repeatedly commented on /r/rust/ that differences are due not so much to SIMD but iirc to not triggering the same LLVM loop unrolling.

I think you're confusing loop unrolling and autovectorization. That said, it's kinda moot, once SIMD is stable, we'll find out :)

>> I think you're confusing loop unrolling and autovectorization. <<

Here's exactly what I was told -- "This was achieved by manually unrolling a 10-step loop, which compiler apparently could not optimize."