[llvm-dev] _ExtInt, LLVM integers and constant time

Adrien Guinet via llvm-dev llvm-dev at lists.llvm.org
Wed Apr 22 09:35:22 PDT 2020

On 4/22/20 6:13 PM, Chris Lattner wrote:>> On Apr 22, 2020, at 12:24 AM, Roman Lebedev via
llvm-dev <llvm-dev at lists.llvm.org>
>> wrote:
>> On Wed, Apr 22, 2020 at 9:35 AM Adrien Guinet via llvm-dev <llvm-dev at lists.llvm.org
>> <mailto:llvm-dev at lists.llvm.org>> wrote:
>>> Hello everyone,
>>> After reading the nice blog post about _ExtInt, I was wondering whether operations
>>> on i128/i256 and more generally on integer types in LLVM are guaranteed to be
>>> constant time or not.
>> I don't believe there's any such guarantee even for normal 8/16/32/64 -bit integers.
> Right. I would expect this to be implementation / target dependent.  The maximum bit
> width of an integer may also be target specific.  For example, some targets may not
> provide a 1024 bit integer divide lib call, and may not want to open code it.

Okay that makes sense!

If we would like, at some point, to introduce such guarantees, that would imply adding a
"constant time" flag to the arithmetic operations at the LLVM IR level, and have the
backends honor it (which already seems the case), or fail if not possible ? .

The only use case I have in mind that would benefit from this is cryptographic code, but
there might be others.

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