Re: _BitInt(N)

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From	Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups	comp.lang.c
Subject	Re: _BitInt(N)
Date	Mon, 24 Nov 2025 05:06:33 -0800
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David Brown <david.brown@hesbynett.no> writes:
[...]
> Yes, exactly.  At the call site, the size of the _BitInt type is
> always a known compile-time constant, so it can easily be passed on.
> Thus :
>
> 	_BitInt(N) x;
> 	_BitInt(M) y;
> 	_BitInt(NM) z = x * y;
>
> can be implemented as something like :
>
> 	__bit_int_signed_mult(NM, (unsigned char *) &z,
> 			N, (const unsigned char *) &x,
> 			M, (const unsigned char *) &y);

That looks like it's supposed to avoid overflow (I'm assuming NM is N + M), but
it wouldn't work.  The type of a C expression is almost always determined
by the expression itself, regardless of the context in which it appears.
The type of x * y is _BitInt(max(N, M)), not _BitInt(N+M), so it can
overflow even if the full result would fit into z.

You can do this instead (not tested):

    _BitInt(N) x;
    _BitInt(M) y;
    _Bit_Int(N+M) z = (_BitInt(N+M))x * y;

(I'm assuming N+M is sufficient, but I might have missed an off-by-one
error somewhere.)

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
void Void(void) { Void(); } /* The recursive call of the void */

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