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

[Jiangning Liu]

Hi Rafael,


2014-08-29 19:10 GMT+08:00 Rafael Espíndola <rafael.espindola at gmail.com>:

> 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.
>

Thanks for your suggestion. I tried this method, and successfully
extracted 27041 functions from that huge file. However, I failed to
reproduce a small case containing a single function which can reproduce the
slowdown. The slowest function I find
is _ZN5clang15StmtVisitorBaseINS_8make_ptrENS_13ASTStmtWriterEvE5VisitEPNS_4StmtE.bc,
but it can finish in 16 seconds on my x86 box.

So it seems there are some module passes triggering the slowdown issue...

Thanks,
-Jiangning


>
> > 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
>
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