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 <[email protected]> wrote: > > On Apr 3, 2015, at 11:41 AM, Tony Arcieri <[email protected]> wrote: > > On Fri, Apr 3, 2015 at 11:35 AM, Irene Knapp <[email protected]> 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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