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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 13:53:28 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/28.0.50 (gnu/linux)

> Division by -1 is also known to cause signed integer overflow. In this
> case, on x86_64 CPUs, you don't even need a '-ftrapv' option. Test case:
> (poke) var x = -4611686018427387904;
> (poke) var y = 2*x;
> (poke) y / -1
> Gleitkomma-Ausnahme (Speicherabzug geschrieben)
> In the debugger:
> (poke) var x = -4611686018427387904;
> (poke) var y = 2*x;
> (poke) y / -1
> Thread 1 "poke" received signal SIGFPE, Arithmetic exception.
> 0x00007ffff7b8ea84 in pvm_execute_or_initialize (jitter_initialize=63,
> jitter_initial_program_point=0xac5f90,
>     jitter_original_state=0x632310) at ../../libpoke/pvm.jitter:2325
> 2325        PVM_CHECKED_BINOP (LONG, LONG, LONG, /);
> You should get away without a crash by using the INT_DIVIDE_OVERFLOW
> macro from Gnulib's intprops.h.

I don't think we can use these macros, which are tailored to C values
and their types.

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.

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