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Date: Tue, 11 Feb 2014 11:00:06 +0400
From: Solar Designer <>
Subject: Re: [PHC] multiply-hardening (Re: NoelKDF ready for submission)

On Mon, Feb 10, 2014 at 06:03:33PM -0500, Bill Cox wrote:
> On Sun, Feb 9, 2014 at 9:46 PM, Solar Designer <> wrote:
> > Do we have any 32x32->64 in Java(Script) at all, whether signed or
> > unsigned?  I'm not familiar with Java(Script).
> Java has 64-bit integer multiply, but it's signed.  There are no
> unsigned operations in Java, only signed.

If Java's multiplies are not widening, the sign bits on inputs don't
matter, and it's up to us how to treat the output.  So I see no problem
with this.  We can have 32x32->64 implemented using signed 64x64->64,
where the sign bits on 64-bit inputs would be zero anyway, or we could
have 32x32->32 or 64x64->64 treated by us as unsigned.  We'll also need
to be doing ADDs and bitwise ops on such "signed" values.  In C,
overflowing signed int is undefined behavior.  Is it defined in Java?
Apparently, it is OK in Java:

"When a mathematical operation cannot be represented using the supplied
integer types, Java's built-in integer operators silently wrap the
result without indicating overflow."

> JavaScript has only floating point, but the various JiT compilers all
> seem to have ways to infer integer operations.  I think it depends on
> the specific compiler, but my tests in Chrome's JavaScript without
> compiler-specific hints were very slow for integer operations (10X
> slower or more than C).  I have been told that with good hints, 2X is
> achievable, but I assume this is versus single-thread non-SIMD.

I think fast ints in JavaScript are accessed via the typed arrays
extension, and it appears to provide for SIMD support:

Unfortunately, it's up to 32-bit int only (both signed and unsigned),
and 32- and 64-bit float.

JavaScript's native numeric type is 64-bit float:

As a somewhat crazy option, we could consider 32x32->53, implementable
via any of: 32x32->64 int, 64x64->64 int, or 64-bit float, including in
SIMD form where available.  This could maximize the number of archs and
languages where this can be implemented efficiently, including with
SIMD, but it does add risk when implemented with floating-point.  Subtle
floating-point implementation bugs resulting in small precision errors
would be fatal.

I suppose an implementation in JavaScript without typed arrays could try
64-bit floating-point first, and upon failed login try recomputing the
client-side portion of hashing using emulated 32-bit ints and deriving
32x32->53 from those (less dependency on floating-point precision then).
It'd only retry login if the recomputed hash is different from what was
tried first.  Too crazy?  I probably wouldn't do something like that
myself, but providing the option (masking to 53 bits) for others to do
crazy things like that may be OK.

VMULPD working on 4 packed 64-bit floats is 5 cycles latency, 1 cycle or
better throughput (is listed at 0.63c throughput in
GenuineIntel00306C3_Haswell*.txt) on all CPUs supporting it (both Intel
and AMD), it seems.  Masking the right bits in inputs/outputs will add
some overhead, though.  There's no point in using it on 32-bit inputs,
since PMULUDQ is about as fast, is available starting with older CPUs,
and avoids the need to mask the inputs.

Just thinking aloud.

> I agree unsigned is prefered.  Java can add a few instructions to
> emulate unsigned if needed.

I think this is not needed - we can simply treat its signed ints as
unsigned, as long as we don't do right shift and division.

> The hybrid that uses 64-bit memory read/write, but only a 32x32->64
> multiplication had some extra overhead for converting from 64-bit to
> 32-bit.

Can you show the instructions that were generated for this?


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