I've seen people imply that latest generation c compilers (gcc,clang etc) generate quite a lot better code than these previous generation compilers. Partly I guess turbo Pascal didn't to much auto vectorization for example. Unsure about loop unrolling.

I'm sure it helped that Pascal had a IMNHO saner calling convention (more like the current amd64 convention) - so would utilise register for passing arguments rather than always using the stack.

I doubt vectorization (automatic or not) was relevant during Turbo Pascal's era.

True, but it's relevant today (since around 1996 with the mmx instruction set). And if some people are arguing today that you can have single pass, fast and simple, and get ~max performance on modern architecture.

I'm not saying you can't, I'm just saying that "as fast machine code as early 90s era c compilers" might not translate to as fast as today's c++ compilers machine code.