[LLVMdev] llc is taking a long time at -O1 to convert *.bc to *.s

[Jakob Stoklund Olesen]

On Feb 2, 2012, at 11:03 PM, Kodakara, Sreekumar V wrote:

> Hi All,
>  
> I am using llvm-3.0 release version for compiling some files. I have a cpp file on which I did a few custom passes. After that I used llc to convert the *.bc file to *.s file at –O1 optimization level. llc took ~2630 seconds to perform this conversion. I timed llc and these are the top three passes that is taking most of the time.
>  
>  ---User Time---   --System Time--   --User+System--   ---Wall Time---  --- Name ---
>   1816.1735 ( 77.8%)  23.3215 ( 69.7%)  1839.4950 ( 77.6%)  1839.4132 ( 77.6%)  Simple Register Coalescing
>   493.5948 ( 21.1%)   9.1846 ( 27.4%)  502.7794 ( 21.2%)  502.7681 ( 21.2%)  X86 DAG->DAG Instruction Selection
>    5.7244 (  0.2%)   0.0000 (  0.0%)   5.7244 (  0.2%)   5.7204 (  0.2%)  Machine Instruction LICM
>  
> I am running this in a fairly new Xeon server. Furthermore, at –O0, llc takes less than 20 seconds to complete. Is this behavior expected and is there a way to disable Simple Register Coalescing pass without switching to –O0 optimization level to get around this problem?

Without coalescing you will get really bad code, but the llc option to turn it off is -join-intervals=0.

There is a good chance the performance problem has already been fixed on trunk.

/jakob

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