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[Caml-list] Ints and overflow...
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Date: 2001-07-15 (11:17)
From: Xavier Leroy <Xavier.Leroy@i...>
Subject: Re: [Caml-list] Ints and overflow...
> However, this has led me directly into the zone of integer overflow.
> Basically, I don't understand what I can expect to be portable, and
> I really don't understand what OCaml does -- what is the tagging 
> scheme, and how does that affect the overflow results I can expect?

As long as you're programming in well-typed Caml, the exact details of
the tagging scheme do not matter.  You get regular 2-complement modulo
arithmetic, just like in Java and most if not all C compilers, with 31
bits of precision, or 63 bits if you're lucky enought to run on a
64-bit processor.

> For example: Is the integer tag 0 or 1?

It's a 1 in the least significant bit, but again you can't observe
this from well-typed Caml.

> Does it vary by platform?

No, but same comment as above.

> Can the results vary by how Caml chooses to optimize adds?

Normally, no.  If it varied, that would be due to a bug in the tagging
optimization scheme of the ocamlopt compiler.

> Does Caml promise anything at all about the results of an
> overflowing integer add?

Yes: it wraps around, i.e. you get the result modulo 2^31 (or 2^63 on
a 64-bit platform).

Hope this answers your questions.

- Xavier Leroy
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