[PATCH] D11605: [LoopUnswitch] Add block frequency analysis to recognize hot/cold regions

[David Li via llvm-commits]

davidxl added inline comments.

================
Comment at: lib/Transforms/Scalar/LoopUnswitch.cpp:88
@@ +87,3 @@
+static cl::opt<unsigned>
+ColdFrequency("loop-unswitch-cold-block-frequency", cl::init(20), cl::Hidden,
+    cl::desc("Block frequency to be considered as cold. Non-trivial "
----------------
reames wrote:
> I think the default here should be much lower, but we can tune this in separate changes.  My guess here would be something like 1/100 rather than 20/100.  I also think we should have an absolute cutoff, but again, we can tune separately.    
This is not frequency. Maybe call it 'ColdnessThreshold' or something.

================
Comment at: lib/Transforms/Scalar/LoopUnswitch.cpp:89
@@ +88,3 @@
+ColdFrequency("loop-unswitch-cold-block-frequency", cl::init(20), cl::Hidden,
+    cl::desc("Block frequency to be considered as cold. Non-trivial "
+             "unswitches are not applied to cold blocks."));
----------------
It is the 'Coldness threshold in percent. The loop header frequency (relative to the entry frequency) is compared with the threshold to determine if non-trivial unswitching should be enabled.'

================
Comment at: lib/Transforms/Scalar/LoopUnswitch.cpp:441
@@ +440,3 @@
+
+  if (LoopUnswitchWithBlockFrequency) {
+    BranchProbabilityInfo BPI(*F, *LI);
----------------
Note that this also triggers the check when PGO is not enabled (e.g, with static prediction).

To check if PGO is on, do:

  HasProfileData = F.getEntryCount().hasValue();
  if (HasProfileData) {

..


http://reviews.llvm.org/D11605