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[QUOTE="Kevin Bracey, post: 2407392"]
In message <xxpis425bxi.fsf@usrts005.corpusers.net>

I think you may have a valid point there. Given that it was signed
arithmetic, then technically speaking you invoke undefined behaviour by
overflow. So the compiler is free to do what it wants.

However, for unsigned arithmetic, the standard does require results to be
reduced modulo (TYPE_MAX+1).

Also, nearly all implementations document signed arithmetic overflow as being
handled in a modulo fashion, effectively "defining" the undefined behaviour.
So your compiler may still be violating its documentation.

For what it's worth, assuming you do want a 16x16->32 multiply, the way I'd
write it in C would be:

acc += (INT32_T) x[n] * y[n];

That promotes x[n] to INT32_T, and hence y[n] as well (because the operands
must match), so you invoke a 32x32->32 multiply. C does not have the concept
of an operation that produces a result wider than its operands.

However, hopefully a compiler will spot when a semantic 32x32->32 multiply is
actually of the form:

(16-bit signed quantity widened to 32 bits)
    x (16-bit signed quantity widened to 32 bits) -> 32 bits

and reduce that into a signed 16x16->32 multiply operation, if that's
sensible for the target architecture.
[/QUOTE]

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