[PATCH] D10825: Improvement on computing edge probabilities when choosing the best successor in machine block placement.

David davidxl at google.com
Thu Jul 23 12:03:51 PDT 2015

davidxl added inline comments.

Comment at: lib/CodeGen/MachineBlockPlacement.cpp:367
@@ -365,3 +366,3 @@
   uint32_t SumWeight = MBPI->getSumForBlock(BB, WeightScale);
   DEBUG(dbgs() << "Attempting merge from: " << getBlockName(BB) << "\n");
   for (MachineBasicBlock *Succ : BB->successors()) {
congh wrote:
> davidxl wrote:
> > Do SumWeight adjustment here -- outside the main loop:
> > 
> > uint32_t SumWeight = ...
> > SumWeight = AdjustWeightSumExcludingInternalBackEdge(SumWeight, BB);
> The adjustment on SumWeight is done only when it has a successor outside of its loop that is also in BlockFilter (if the successor outside of the loop is not in BlockFilter, we are building chains for the loop). It makes more sense to calculate this adjusted weight inside of this loop. Note that when we build chains for the loop, we don't need this adjusted weight at all.
I think the adjustment can be done without even needing to have a top level for loop like

for (auto Succ: BB->successors()) {
   if (...) continue;
   for (auto SuccBB : BB->successors())
	​        if (MLI->getLoopFor(SuccBB) == L)
		​          SumWeight2 -= MBPI->getEdgeWeight(BB, SuccBB) / WeightScale;

but simply:

 for (auto SuccBB : BB->successors())
        // We find out that SuccBB is already in 'Chain', but it is not the end of 'Chain' -- it must be a block in the inner 
        // loop including BB that has already be laid out. Skip it 
        if (BlockToChain(SuccBB) == &Chain)   
		​   SumWeight -= MBPI->getEdgeWeight(BB, SuccBB) / WeightScale;

Comment at: lib/CodeGen/MachineBlockPlacement.cpp:386
@@ +385,3 @@
+    // when BB belongs to a loop but we are building chains for another loop
+    // (either outer or peer).  In this case, the edge from BB to the successor
+    // outside of the inner loop usually has a small probability, due to edges
congh wrote:
> davidxl wrote:
> > Do you have examples of the problem when chain formation is for a 'peer' loop?
> The following partial CFG shows such a case:
> ```
> A
>> B
>> C
>> D
> ```
> Here, B->C is an edge between two peer loops.
Ok.  A test case would be good.


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