[Rafael Avila de Espindola]

Cool. Note that I only hit the bug. I am not current on the selection dag code. Chandler or Jim are probably the best to review the patch.

Sent from my iPhone

> On Sep 5, 2014, at 6:08, Jiangning Liu <liujiangning1 at gmail.com> wrote:
> 
> Hi Rafael,
> 
> I think I've got a solution to fix this slowdown issue. I personally think the ISEL infrastructure needs to be improved, and CopyValueToVirtualRegisters in SelectionDAGBuilder is called too many times.
> 
> Now I change my algorithm by making an early decision before ISEL and store the info into FuncInfo. We can do this because deciding preferred sext/zext doesn't depend on SDNode but LLVM IR. This way, we will be able to calculate the info once and use it many times in real ISEL stage.
> 
> I will sent out a patch update later on, and my initial experiment shows that huge case you gave me can finish in 6 minutes now. It's really a good test case to measure compile-time. :-)
> 
> Thanks,
> -Jiangning
> 
> 
> 
> 2014-09-05 11:31 GMT+08:00 Jiangning Liu <liujiangning1 at gmail.com>:
>> Hi Rafael,
>> 
>> Attached is that test case, but I can't see slowdown with it.
>> 
>> Thanks,
>> -Jiangning
>> 
>> 
>> 
>> 2014-09-04 21:55 GMT+08:00 Rafael Espíndola <rafael.espindola at gmail.com>:
>>> Can you put that testcase somewhere?
>>> 
>>> On 4 September 2014 01:19, Jiangning Liu <liujiangning1 at gmail.com> wrote:
>>> > 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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