[curves] Distribution-ready optimized code

[Irene Knapp]

Well, my revised suggestion is actually to skip its codegen and go directly
to the class that knows how to output x86-64.  It's conceptually an
internal class, but it's still part of the documented API (to the extent
that any of LLVM can be said to be documented).  You'd be hooking in as if
your code were the codegen, and yes, it'd be specific to the architecture
but that's what you wanted.

I definitely don't want to push this too hard; I understand that I'm not
the one who would actually be implementing any solution. :)  Coming from
the compilers background, it's the appealing route to me.

Irene

On Fri, Apr 3, 2015 at 11:48 AM Michael Hamburg <mike at shiftleft.org> wrote:

>
> On Apr 3, 2015, at 11:41 AM, Tony Arcieri <bascule at gmail.com> wrote:
>
> On Fri, Apr 3, 2015 at 11:35 AM, Irene Knapp <ireneista at gmail.com> wrote:
>
>> Surely, what you are describing is a lightweight tool that either
>> generates LLVM bitcode, or hooks into the LLVM backends at a slightly lower
>> level than that to output particular instructions when that's what you
>> really, really want - but I suspect its hinting system already makes that
>> unnecessary for this use-case.  LLVM bitcode is precisely this "mostly
>> concrete assembly" concept that you're describing.
>>
>
> The problem with using LLVM in this context is robust cryptographic
> implementations need to follow a very specific set of rules to avoid cache
> timing attacks, and LLVM is not designed to follow these rules:
>
> https://cryptocoding.net/index.php/Coding_rules
>
> LLVM has not been designed to support the generation of constant time code
> and is instead rather eager to do things like insert branches in otherwise
> branch free code if it thinks the code can be better optimized.
>
> --
> Tony Arcieri
>
>
> It may be that if your tool chooses carefully the optimization passes — or
> even avoids most of them entirely — you could get constant-time operation.
> But I don’t know enough about LLVM’s codegen to be sure one way or the
> other.  At least until recently, though, it was absolutely terrible at
> things like add-with-carry intrinsics.  (Not necessarily making them
> variable time, but lowering add; addc to add; setc; zext; add; add.)
>
> — Mike
>
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