[PATCH] D20991: Set machine block placement hot prob threshold for both static and runtime profile.

[David Li via llvm-commits]

davidxl added a comment.

The PGO related change is only half correct.  In fact the trace successor probability threshold should be computed based on existing parameters misfetch cost and jump instruction cost.

In  a diamond shaped CFG, the resulting succProb threshold is indeed 50% as the CFG is symmetric. For Triagular shaped CFG, the result will be very different because breaking topological order introduces more overhead compared with diamond shape case.

Given

     A
   /  |
  B  |
    \ |
     C

If Prob(A,B) is greater than Prob(A,C),  A B C should always be the right order. However, if if Prob(A,B) is lower than P(A,C), we need to  compare the cost of A B C vs A C B

For A B C, the cost is

Prob(A,C) * Misfetch_cost   due to the taken branch from A to C

For A C B, the cost is

Prob(A, B) *Misfetch_cost          // taken branch from A to B

  + Prob(A,B) * misfetch_cost      // taken branch (unconditional) from B back to C
  + Prob(A,B) * jump_inst_cost    //  one more unconditional jump instruction is introduced 

If jump instruction cost is assumed to be low (0), then  Prob(A,C) needs to be >66%  in order for A C B to be a better layout.   If jump instr cost is the same as misfetch, we get 75% threshold.

We need to introduce a helper function to compute the threshold on the fly based on the CFG and parameter.


================
Comment at: lib/CodeGen/MachineBlockPlacement.cpp:120
@@ +119,3 @@
+static cl::opt<unsigned> StaticHotProb(
+    "static-hot-prob",
+    cl::desc("Hot branch probability when only static profile is available."),
----------------
hot-->likely 

================
Comment at: lib/CodeGen/MachineBlockPlacement.cpp:125
@@ +124,3 @@
+static cl::opt<unsigned> ProfileHotProb(
+    "profile-hot-prob",
+    cl::desc("Hot branch probability when only runtime profile is available."),
----------------
hot --> likely


http://reviews.llvm.org/D20991