[LLVMdev] SROA is slow when compiling a large basic block

[Evan Cheng]

On May 21, 2014, at 3:05 PM, Philip Reames <listmail at philipreames.com> wrote:

> 
> On 05/15/2014 11:12 AM, Akira Hatanaka wrote:
>> On Thu, May 15, 2014 at 9:31 AM, Philip Reames <listmail at philipreames.com> wrote:
>> On 05/14/2014 06:02 PM, Akira Hatanaka wrote:
>>> I would like to get feedback from the community on how I can speed up the compilation of a function that has one huge basic block consisting of over 150K instructions. It takes about 10 minutes for clang to produce the object file and the function that is taking up the majority of the time is LoadAndStorePromoter::run, which is called when SROA is run:
>>> 
>>>     // Otherwise, we have mixed loads and stores (or just a bunch of stores).
>>>     // Since SSAUpdater is purely for cross-block values, we need to determine
>>>     // the order of these instructions in the block.  If the first use in the
>>>     // block is a load, then it uses the live in value.  The last store defines
>>>     // the live out value.  We handle this by doing a linear scan of the block.
>>>     Value *StoredValue = nullptr;
>>>     for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ++II) {
>>> 
>>> If I understand this code correctly. LoadAndStorePromoter::run is called once per every promotable alloca and iterates over the whole list to determine the order of loads and stores in the basic block that access the alloca.
>> I'm a bit confused about the cost here.  Based on the code, I see us walking each basic block at most once per alloca.  Even at 150K instructions, I wouldn't expect the loop to take *that* long.  How many allocas are you dealing with?
>> 
>> I do see an outer loop in SROA::performPromotion.  Are you possibly triggering a large number of iterations there?  That would make the inner loop appear much more expensive.  
>> 
>> 
>> The unoptimized bitcode (compiled with clang -O0) has a large number of allocas (over 7000), which I think is also contributing to the long compilation time.
> I suspect that many things in the optimizer aren't going to react well to that many allocas.  :)  Where are they all coming from?  And why hasn't mem2reg removed most of them?  I suspect you'd be better off investing effort to address this issue then optimizing SROA.  
>> 
>> But making LoadAndStorePromoter::run faster is sufficient to reduce the compilation time (at least in my case).
> I'm not objecting to this.  I'd be happy to see something along these lines land, I'm just not sure it solves the root cause of your issues.  
>>  ...
>> 
>>> 4. Change the definition of class Instruction in IR/Instruction.h so that determining whether one instruction precedes another becomes more efficient. My current implementation brings down the compilation time to just 6s.
>>> 
>>> I won't go into much detail about the change I made, but it basically replaces Instruction's pointer to its parent BasicBlock with another data structure. I haven't done much investigation into how this will affect the common case.
>>> 
>>> This will also help speed up the execution of code in other places. For example, these two functions are called when GVN::processLoad is called and iterate over a basic block's instruction list to determine the order of two (load and stores) instructions.:
>>> 
>>> llvm::isPotentiallyReachable
>>> 
>>>     // Linear scan, start at 'A', see whether we hit 'B' or the end first.
>>>     for (BasicBlock::const_iterator I = A, E = BB->end(); I != E; ++I) {
>>>       if (&*I == B)
>>>         return true;
>>>     }
>>> 
>>> DominatorTree::dominates
>>> 
>>>   // Loop through the basic block until we find Def or User.
>>>   BasicBlock::const_iterator I = DefBB->begin();
>>>   for (; &*I != Def && &*I != User; ++I)
>>>     /*empty*/;
>> I'd be curious to see a proposal here.  I've been bitten by the DT::dominates issue myself.  
>>> 
>> 
>> I was hesitant to make changes in classes such as BasicBlock and Instruction just to make it faster to compile a basic block with 150K+ instructions (which I think is a pathological case). If there are other use cases that need discovering orders of instructions in a basic block, it might be worth considering fixing those classes.
>>  
>> The attached patch is the current implementation I have. Basically I am trying to make function Instruction::precede fast and use it to speed up functions that are currently iterating over the whole instruction list to determine the order of instructions in a basic block. 
> I get where you're going with this, but I don't think this is the right approach.  In particular, you're introducing extra indirections along some fairly common paths and the caching scheme around sublist is non-obvious.  

Akira, I think this is worth solving, but probably not for this specific issue. I don't think it's uncommon for optimizations to want to understand the relatively ordering of two instructions.

I don't claim to know the right solution. Philip, is your concern of Akira's approach about performance in the common case? Or is is about the complexity of the scheme? Akira, if would be good to have some analysis of the efficiency of various operations related to instructions. It's good to make the 'proceeds' query fast, but not at the expensive of insertion, deletion, etc.

Evan


> 
> I do like the extraction of the 'preceeds' interface.  It gives us a single place to optimize.  
> 
> Looking at the problem, I don't see an easy answer here.  If we weren't using linked lists, this would be easy, but with that constraint, everything I come up with seems nasty.  
> 
> One somewhat less nasty (but still bad) scheme I came up with would be to use the traits of the InstList in basic block to create a listener which ignores all updates except when order is interesting (i.e. inside the SROA code).  This seems really bug prone though.  I am not in favour of actually implementing such a thing.  
> 
> Another scheme might be to have an Analysis which caches the answer "does X proceed Y".  By default, nothing would preserve this, and we could teach certain passes to be aware of it.  Again, this feels dirty.  
> 
> I don't really have a good solution here.  
> 
>> 
>>> 5. Simply give up doing SROA (and GVN::processLoad) if there is a huge basic block that can potentially take a long time to compile.
>> Until we have something better, this might be a necessary hack.  Whether it's actually worth introducing depends on how long Chandler thinks the mem2reg version will take to land.  
>> 
>> 
>> I tried this idea today, and unfortunately it didn't reduce the compilation time. Skipping SROA leaves a lot of loads/stores in the code that could have been promoted, and that places the burden on other passes that are run later, such as GVN and DSE.
> Makes some sense.  See my point above about lots of allocas stressing the optimizer.  :)
> 
> p.s. Any chance of bugs being filed with test cases for each of those?  :)
> 
> Philip
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