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From: mitchalsup@aol.com (MitchAlsup1)
Newsgroups: comp.arch
Subject: Re: Cray style vectors
Date: Tue, 12 Mar 2024 17:13:38 +0000
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Terje Mathisen wrote:

> Tim Rentsch wrote:
>> Terje Mathisen <terje.mathisen@tmsw.no> writes:
>> 
>>> If I really had to write a 64x64->128 MUL, with no widening MUL or
>>> MULH which returns the high half, then I would punt and do it using
>>> 32-bit parts (all variables are u64):  [...]
>> 
>> I wrote some code along the same lines.  A difference is you
>> are considering unsigned multiplication, and I am considering
>> signed multiplication.
>> 
> Signed mul is just a special case of unsigned mul, right?

The low order N bits are all the same while the higher order N bits
are different; where N is operand size.

> I.e. in case of a signed widening mul, you'd first extract the signs, 
> convert the inputs to unsigned, then do the unsigned widening mul, 
> before finally resotirng the sign as the XOR of the input signs?

SW may have to do that, HW does not.

> There is a small gotcha if either of the inputs are of the 0x80000000 
> form, i.e. MININT, but the naive iabs() conversion will do the right 
> thing by leaving the input unchanged.

> At the other end there cannot be any issues since restoring a negative 
> output sign cannot overflow/fail.

> Terje