phc-discussions - Re: [PHC] wider integer multiply on 32-bit x86

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Date: Tue, 4 Mar 2014 18:41:44 -0500
From: Bill Cox <waywardgeek@...il.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] wider integer multiply on 32-bit x86

On Mon, Mar 3, 2014 at 9:13 PM, Solar Designer <solar@...nwall.com> wrote:
> Hi,
>
> Normally, on 32-bit x86 without SSE2 (thus, on Pentium 3 and older, or
> when code is compiled such that SSE2 is not enabled) the widest integer
> multiply available is 32x32->64, via the [I]MUL instruction.  There are
> two problems with this: the instruction uses the specific EDX:EAX
> registers, so we can't have more than one such multiply in progress
> until we've read/replaced at least the EAX contents(*), and 32x32->64 is
> not very wide.

You've talked me into 32x32->64 rather than 32x32->32.  It's slightly
slower, but not enough to justify sticking with 32x32->32.  The
slowdown is because in 64-bit mode, I have to right-shift the high
32-bits down to the low 32-bits to add it into a 32-bit register.
That's pretty much the only difference.  It runs fast in 64-bit and
32-bit compiled versions.

Thanks for the tip!

Bill