[llvm] r216066 - Optimize ZERO_EXTEND and SIGN_EXTEND in both SelectionDAG Builder and type

[Rafael Espíndola]

On 29 August 2014 05:16, Jiangning Liu <liujiangning1 at gmail.com> wrote:
> Hi Rafael and Bob,
>
> The case you gave is really huge! :-)

Yes, sorry, it is the LTO of clang :-)

> I tried and it turned out it is not a infinite loop, and it can finish in
> ~70 minutes.
>
> I tried llc command line option -time-passes, and it shows
>
> ==-------------------------------------------------------------------------===
>                       ... Pass execution timing report ...
> ===-------------------------------------------------------------------------===
>   Total Execution Time: 4125.4617 seconds (4124.7082 wall clock)
>
>    ---User Time---   --System Time--   --User+System--   ---Wall Time---
> --- Name ---
>   3911.0328 ( 95.1%)   8.5007 ( 65.8%)  3919.5335 ( 95.0%)  3920.7144 (
> 95.1%)  X86 DAG->DAG Instruction Selection
>   47.5946 (  1.2%)   0.6397 (  5.0%)  48.2343 (  1.2%)  48.1823 (  1.2%)
> Greedy Register Allocator
>   16.7073 (  0.4%)   0.0244 (  0.2%)  16.7317 (  0.4%)  16.7890 (  0.4%)
> Simple Register Coalescing
>   11.6154 (  0.3%)   0.0164 (  0.1%)  11.6318 (  0.3%)  11.7178 (  0.3%)
> Machine Instruction Scheduler
>   10.8118 (  0.3%)   0.0677 (  0.5%)  10.8794 (  0.3%)  10.3740 (  0.3%)
> Loop Strength Reduction
>
> So the problem is around "X86 DAG->DAG Instruction Selection".
>
> I tried to capture "hot" sports using debugger, but I failed, and it seems
> the time is accumulated somewhere.
>
> Do you have any suggestions?

You can try running llvm-extract with every function and then running
llc on the result (which will have only one function). Hopefully you
will find a much smaller testcase that way.

> And I'm wondering if this is a x86 specific issue or the slowdown can also
> exposed for other targets like aarch64?

Hard to tell without a smaller testcase.

Cheers,
Rafael