[LLVMdev] [RFC][PATCH] Adding absd/hadd/sad intrinsics

[Renato Golin]

On 6 May 2015 at 06:43, Shahid, Asghar-ahmad
<Asghar-ahmad.Shahid at amd.com> wrote:
> That’s right. I agree with your *pattern vs complexity* thinking.
>
> So I would drop llvm.sad() and go ahead with the remaining two.
>
> Does it make sense in general?

We strive to keep things simple.

Creating too many intrinsics makes the IR brittle and hard to
optimise. Creating pattern matching rules can be complex, but they're
generally preferred to adding intrinsics. But creating a new pattern
matching engine just for the sake of one case is too much, so we end
up using heuristics instead.

However, heuristics are like connective tissue. They look functional,
but they add up to a big and ineffective blob, which ends up being
less effective every new data you add. So, in the end, having a plan
to generate a more efficient pattern matching structure could very
well reduce the amount of code, if (and only if), the plan accounts
for most current cases as well as the new ones.

For the time being, if you can get away with heuristics, and that
fills your allocated time for this task, that it's the best way
forward for now.

cheers,
--renato