[LLVMdev] some superoptimizer results

[Sean Silva]

Are you taking into account critical path length? Because e.g. for:

%0:i64 = var
%1:i1 = slt 18446744073709551615:i64, %0
%2:i64 = subnsw 0:i64, %0
%3:i64 = select %1, %0, %2
infer %3
%4:i64 = ashr %0, 63:i64
%5:i64 = add %0, %4
%6:i64 = xor %5, %4
result %6

In the former case, the cmp and sub are independent, so can be executed in
parallel, while in the latter case all 3 instructions are dependent. So the
former case can execute in 2 cycles while the latter takes 3. Modern OoO
chips do in fact exploit this kind of thing.

-- Sean Silva


On Wed, Jul 22, 2015 at 10:15 AM, John Regehr <regehr at cs.utah.edu> wrote:

> We (the folks working on Souper) would appreciate any feedback on these
> IR-level superoptimizer results:
>
>   http://blog.regehr.org/extra_files/souper-jul-15.html
>
> My impression is that while there's clearly plenty of material in here
> that doesn't want to get implemented in an opt pass, there are a number of
> gems hiding in there that are worth implementing.
>
> Blog post containing additional explanation and caveats is here:
>
>   http://blog.regehr.org/archives/1252
>
> Thanks!
>
> John
>
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