[llvm-commits] [llvm] r98023 - /llvm/trunk/lib/CodeGen/SimpleRegisterCoalescing.cpp

[Evan Cheng]

On Mar 16, 2010, at 6:41 PM, Chris Lattner wrote:

> 
> On Mar 16, 2010, at 6:37 PM, Jakob Stoklund Olesen wrote:
> 
>>>> 
>>>> The attached graph shows the coalescer runtime as a function of the array length in my tiny test function (inlining disabled). The patch kicks in at N=500 and reduces runtime to nothing.
>>> 
>>> Is it possible to fix or improve the N^2 algorithm?
>> 
>> Yes, I think so. From the end of SimpleJoin:
>> 
>> // Update the liveintervals of sub-registers.
>> if (TargetRegisterInfo::isPhysicalRegister(LHS.reg))
>>   for (const unsigned *AS = tri_->getSubRegisters(LHS.reg); *AS; ++AS)
>>     li_->getOrCreateInterval(*AS).MergeInClobberRanges(*li_, LHS,
>>                                                   li_->getVNInfoAllocator());
>> 
>> 
>> MergeInClobberRanges() is linear in the size of LHS, and SimpleJoin() is called once for each LiveRange in LHS -> quadratic runtime.
>> 
>> However, SimpleJoin() is called to merge a trivial RHS into a complex LHS. It may be as simple as calling MergeClobberRanges(RHS) instead. I would have to read the code more closely.
> 
> Nice, definitely sounds like a worthwhile experiment.
> 
>> Ideally, I want to avoid physreg joining altogether, and use better hinting instead. Half of the coalescer would go away, and all of the bugs ;-)

That's the proper way to fix this. But we are ready to just do this yet. At least not with the short comings of linear scan.

Evan

> 
> cool, do that too! ;-)
> 
> -Chris
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