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

[Akira Hatanaka]

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.


This is the list of ideas I have considered or implemented that can
possibly solve my problem:


1. In SROA::getAnalysisUsage, always require DominatorTreeWrapperPass. This
will enable SROA::promoteAllocas to use mem2reg, which is fast because it
caches the per basic-block ordering of the relevant loads and stores. If
it's important to avoid always computing the dominator tree, computing it
conditionally based on whether there is a huge basic block in the function
is another idea, but I am not sure if that is possible (I don't think this
is currently supported).


This brings down the compilation time (using clang -emit-llvm) from 350s to
30s (it still takes about 23s to do GVN). It also might fix PR17855 (the
program that used to take 65s to compile now takes just 11s):


http://llvm.org/bugs/show_bug.cgi?id=17855


2. Cache the ordering of loads and stores in LoadAndStorePromoter::run. I
don't know how hard it would be to implement this, but I think it would be
as fast as 1 (using mem2reg).


3. Insert a pass that splits huge basic blocks into smaller blocks prior to
SROA and recombine them after the pass is run.


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*/;

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.
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