[Libre-soc-dev] SVP64 Vectorised add-carry => big int add

[Jacob Lifshay]

On Fri, Apr 15, 2022, 15:46 lkcl <luke.leighton at gmail.com> wrote:

> looking at the algorithm
> https://libre-soc.org/openpower/sv/bitmanip/appendix/
>
> i am not seeing the top 32 bit of the 64 bit mul actually being used
> except for overflow.
>

am i right in thinking that 64 bit mul could be used with 64 bit result not
> 128 bit?
>

no, you're not right...think of what happens when multiplying a big-int
with 32-bit words:
[0x12345678, 0x9abcdef0]
by a single word 0x1000 (which is half of what the inner loop does, q[i] is
the single word it multiplies by):
0x9abcdef0 * 0x1000 = 0x9abcdef00000
so it produces word 0xdef00000 with carry = 0x9abc (carry is the result of
mul-hi)
critically carry can't just be an overflow flag here
next 0x12345678 * 0x1000 + 0x9abc (carry) = 0x123456789abc
so it produces word 0x56789abc with carry = 0x1234
finally the product is:
[0x1234, 0x56789abc, 0xdef00000]

Jacob