[LLVMdev] Performance regression in the LiveIntevals phase

[Andrew Trick]

> On Oct 9, 2014, at 9:17 AM, Hal Finkel <hfinkel at anl.gov> wrote:
> 
> ----- Original Message -----
>> From: "Vaidas Gasiunas" <vaidas.gasiunas at sap.com>
>> To: "LLVM Dev (llvmdev at cs.uiuc.edu)" <llvmdev at cs.uiuc.edu>
>> Sent: Thursday, October 9, 2014 4:37:39 AM
>> Subject: [LLVMdev] Performance regression in the LiveIntevals phase
>> 
>> Some time ago we reported a compile-time performance regression in
>> the LiveIntervals analysis pass (see
>> http://llvm.org/bugs/show_bug.cgi?id=18580 ). We detected it at
>> first after migrating from LLVM 3.1 to 3.3, but the problem persists
>> also in 3.5. This regression is especially critical when compiling
>> long functions. In one of our benchmarks compile time goes from 200s
>> (in 3.1) up to 1500s (in 3.3).
>> 
>> 
>> 
>> We also managed to reproduce the regression with a C++ example using
>> clang. Here are the instructions:
>> 
>> 
>> 
>> Generate the example C++ file with the Perl script attached to the
>> Bug (5000 is a good size parameter to clearly see the regression):
>> 
>> perl create.pl example.cpp 5000
>> 
>> Generate LLVM IR code for the C++ file (the regression is in the
>> backend functionality, so we don't want to measure frontend
>> compilation)
>> 
>> llvm31/clang++ example.cpp -c -emit-llvm -o example.ir-31
>> 
>> llvm35/clang++ example.cpp -c -emit-llvm -o example.ir-35
>> 
>> 
>> 
>> Now you can run the actual performance measurements using llc:
>> 
>> llvm31/llc -cppgen=program -o=5000.asm example.ir-31
>> 
>> llvm35/llc -cppgen=program -o=5000.asm example.ir-35
>> 
>> or you can also compile example.ir-31 with llvm3.5:
>> 
>> llvm35/llc -cppgen=program -o=5000.asm example.ir-31
>> 
>> 
>> 
>> Our analysis had shown that the most of time is taken for generation
>> of live intervals for physical registers. In the biggest example,
>> the live interval of one of the physical registers consists of about
>> 500.000 live ranges. Insertions in the middle of the array of live
>> ranges cause quadratic algorithmic complexity, which is apparently
>> the main reason for the slow-down. I changed the implementation of
>> computeRegUnitInterval() so that it uses a set instead of the array,
>> and in this way managed to reduce the execution time of the
>> computeRegUnitInterval from 1200s down to about 1s. The fix is a bit
>> ugly, however, because I cannot completely switch to the set, since
>> further analyses are more efficient on the array. For that reason, I
>> flush the contents of the set into the array at the end of
>> computeRegUnitInterval. I also had to rewrite various operations on
>> the live interval so that they use the set structure if it is
>> available and the array otherwise.
>> 
>> If there is an interest, I can port the patch to the dev version of
>> llvm, but maybe someone has a better idea of how to deal with the
>> regression.
> 
> I think this would be interesting, please do. There are a few places in LLVM where we currently keep dual data structures, and needing another one is not necessarily bad (we have SetVector, for example).

Yes, it would be good to get a fix for this behavior in LLVM trunk even if it isn’t pretty. I’ve also seen extendToUses showing up on profiles. Your description of the fix sounds reasonable.

-Andy
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