[LLVMdev] Slow jitter.

[Dan Gohman]

On Aug 26, 2009, at 8:32 AM, Chris Lattner <clattner at apple.com> wrote:

> On Aug 26, 2009, at 7:47 AM, Óscar Fuentes wrote:
>>> Also if you use -time-passes with llc it should show which pass in
>>> llc
>>> takes so much time.
>>
>> These are the three main culprits for llc -O0
>>
>>  ---User Time---   --System Time--   --User+System--   ---Wall
>> Time---  --- Name ---
>> 10.9531 ( 30.0%)   0.4687 ( 58.8%)  11.4218 ( 30.6%)  11.5468
>> ( 30.6%)  X86 DAG->DAG Instruction Selection
>> 10.2500 ( 28.0%)   0.0156 (  1.9%)  10.2656 ( 27.5%)  10.2500
>> ( 27.2%)  Live Variable Analysis
>>  4.8593 ( 13.3%)   0.0000 (  0.0%)   4.8593 ( 13.0%)   4.8593
>> ( 12.9%)  Linear Scan Register Allocator
>>
>> And there for -pre-RA-sched=fast -regalloc=simple -O0 code.bc
>>
>> 10.7187 ( 45.4%)   0.4375 ( 60.8%)  11.1562 ( 45.8%)  11.1718
>> ( 45.4%)  X86 DAG->DAG Instruction Selection
>>  7.4687 ( 31.6%)   0.0156 (  2.1%)   7.4843 ( 30.7%)   7.5312
>> ( 30.6%)  Simple Register Allocator
>>  1.9531 (  8.2%)   0.1406 ( 19.5%)   2.0937 (  8.6%)   2.1093
>> (  8.5%)  X86 Intel-Style Assembly Printer
>>
>> I suppose we can't get rid of instruction selection :-)
>
> Pass -fast-isel to speed up instruction selection.
>
> Dan, I think that this should be made "non hidden" and updated (from
> llc --help):
>
>   -fast-isel                              - Enable the experimental
> "fast" instruction selector

It's turned on by -O0.  And I guess it's not so "experimental" at this  
point :).  It hasn't been tuned for a wide variety of applications yet  
though.

An interesting option to add is -fast-isel-verbose, which prints out  
LLVM instructions that aren't going down the fast path.  If there's  
something that shows up a lot, it may be worthwhile looking into why  
the front-end is using it, or looking into adding support for that  
instruction to the fast path.

LLVM has made progress in this area, but there's more to be done.

Dan