[PATCH] D20991: Set machine block placement hot prob threshold for both static and runtime profile.
David Li via llvm-commits
llvm-commits at lists.llvm.org
Tue Jun 14 13:10:13 PDT 2016
davidxl added inline comments.
Comment at: lib/CodeGen/MachineBlockPlacement.cpp:596
@@ -595,2 +595,3 @@
- BranchProbability HotProb = getLayoutSuccessorProbThreshold();
+ BranchProbability HotProb = getLayoutSuccessorProbThreshold(
There is no need to pass this parameter -- MachineFunction is now a member field in the class.
Comment at: lib/CodeGen/MachineBlockPlacement.cpp:621
@@ +620,3 @@
+ // be added to PredEdgeFrequency to compare with CandidateEdgeFreq.
+ if (BB->isSuccessor(Pred))
+ PredEdgeFreq +=
It is probably not a good idea to bump the frequency of the alternate edge. Instead we should adjust the HotProb threshold computed by:
HotProb = getLayoutSuccessorProbThreshold(BB, Succ);
When Edge BB->Succ is examined, with PGO, the thresold should be adjusted to ~66% (compared to 50% default). In order words, in this shape, in order for BB->Succ to be considered, it needs at least 66% likely.
We should not change the non-PGO default (80%) now (can be done later -- but since it requires 'guessing' probbaility, a lot more perf testing is needed in order to change that)
More information about the llvm-commits