In this case, I’d expect constant folding to be the absolute minimum performed at all optimization levels. It is, in fact,—for integers. For (integer) vectors, it’s not, even though it’s much more important there. That’s why advising cryptographers who program using vector intrinsics (aka “assembly except you get a register allocator”) to compile with GCC at -O0 is such bad advice. (Just checked MSVC and it’s better there.)

There are, however, more unambiguous cases, where Intel documents an intrinsic to produce an instruction, but GCC does not in fact produce said instruction from said intrinsic unless optimization is enabled. (I just don’t remember them because constants in particular were so ridiculously bad in the specific case I hit.)