Asking for clarification of the Long_val function that converts from OCaml integer to C integer

Learning
1

It is understood that machine representation of integers in OCaml is different from that in C; in particular, on 64-bit architecture, an OCaml integer occupies one word (i.e. 64 bits) with the least significant bit set to 1 and the top 63 bits encode an integer in two’s complement.

The Long_val function from caml/mlvalues.h implements a simple algorithm to convert an OCaml machine integer to an equivalent C machine integer: just a right shift of the word by one bit !

This is confusing.

Consider the OCaml runtime representation of the integer -1 (minus one): it is a 64-bit sequence of 1’s. I expect it also to be the machine representation of -1 in C, but by Long_val, we have to right shift this 64-bit sequence of 1’s by one bit to get a sequence 011…11 which consists of one 0 followed by sixty-three 1’s: this by no means is the 64-bit two’s complement encoding of -1. What is wrong here?

2

I think the right shifted bit sequence is actually 111…11, because value is a signed type, and right-shifting it will probably (apparently it depends on the compiler) perform a sign-extending right shift, i.e., arithmetic right shift.

3

Thanks ! Your reply reminded me that I shall just check GCC manual, and yes as you said, right shift does not necessarily pad 0’s. Below is the link, on Sec. 3.8 Bit Shifting

https://www.gnu.org/software/gnu-c-manual/gnu-c-manual.html#Bit-Shifting