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Re: integer overflow during divl instruction

From: Jose E. Marchesi
Subject: Re: integer overflow during divl instruction
Date: Mon, 22 Feb 2021 17:19:07 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/28.0.50 (gnu/linux)

> Jose E. Marchesi wrote:
>> In Poke we have integer values of any number of bits from 1 to 63.  If
>> we were to define what happens when signed overflow occurs (like raising
>> an exception E_overflow) that would need to happen also when a, say,
>> 4-bit signed integer is overflown by an operation.
>> So we need something more complex than that.
> Addition and subtraction for a signed integer type with N bits (1 ≤ N ≤ 64)
> works like this:
>   add (x, y) := add_int64_t (x << (64-N), y << (64-N)) >> (64-N).
>   sub (x, y) := sub_int64_t (x << (64-N), y << (64-N)) >> (64-N).
>   mul (x, y) := mul_int64_t (x << (64-N), y) >> (64-N).
> where add_int64_t, sub_int64_t, mul_int64_t check for overflow (intprops.h
> and >> is a signed right-shift.

I just thought that we should also make casts to signed integral types

  8 as uint<3> -> This evals to (uint <3>) 0, and is ok.
  8 as int<3>  -> This should error on overflow.

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