[gjm11]

Suppose I claim that some programming language achieves performance "as fast as C". Then, unless I very clearly and explicitly say otherwise, it will be assumed that if I write kinda-C-like code in that language I will get performance similar to comparable code actually written in C.

But that doesn't at all follow from compiling to C. I gave one example above; here's another. Perhaps my programming language has arbitrary-length integers and the C code it produces for a simple loop looks something like this:

    bignum_t i = bignum_from_int(0);
    bignum_t x = bignum_from_int(0);
    while (i < bignum_from_int(1000000)) {  
      bignum_add_in_place(x, bignum_bitand(i, bignum_from_int(1)));  
    }

Corresponding code in "normal" C would use machine integers and the compiler might well be able to understand the loop well enough to eliminate it altogether. Code like the above would run much, much slower, despite being technically in C.