[MBP] add comments and bug fix

Document the new parameter and threshod computation
model.  Also fix a bug when the threshold parameter
is set to be different from the default.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@272749 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/CodeGen/MachineBlockPlacement.cpp b/lib/CodeGen/MachineBlockPlacement.cpp
index 7379a3099b1b6de545273f35222107c8f5629d97..332755d0ff4b47cb074eafd8f1ca502193db105c 100644 (file)
--- a/lib/CodeGen/MachineBlockPlacement.cpp
+++ b/lib/CodeGen/MachineBlockPlacement.cpp
@@ -530,9 +530,19 @@ static BranchProbability getLayoutSuccessorProbThreshold(
   if (BB->succ_size() == 2) {
     const MachineBasicBlock *Succ1 = *BB->succ_begin();
     const MachineBasicBlock *Succ2 = *(BB->succ_begin() + 1);
-    if (Succ1->isSuccessor(Succ2) || Succ2->isSuccessor(Succ1))
-      return BranchProbability(
-          200 - 2 * ProfileLikelyProb, 200 - ProfileLikelyProb);
+    if (Succ1->isSuccessor(Succ2) || Succ2->isSuccessor(Succ1)) {
+      /* See case 1 below for the cost analysis. For BB->Succ to
+       * be taken with smaller cost, the following needs to hold:
+       *   Prob(BB->Succ) > 2* Prob(BB->Pred)
+       *   So the threshold T
+       *   T = 2 * (1-Prob(BB->Pred). Since T + Prob(BB->Pred) == 1,
+       * We have  T + T/2 = 1, i.e. T = 2/3. Also adding user specified
+       * branch bias, we have
+       *   T = (2/3)*(ProfileLikelyProb/50)
+       *     = (2*ProfileLikelyProb)/150)
+       */
+      return BranchProbability(2 * ProfileLikelyProb, 150);
+    }
   }
   return BranchProbability(ProfileLikelyProb, 100);
 }

diff --git a/lib/CodeGen/MachineBranchProbabilityInfo.cpp b/lib/CodeGen/MachineBranchProbabilityInfo.cpp
index 9e14cb621dd09b6c008104f76bbc2826df5e4942..fe73406183743e6fd0c19ed57b258bd0e017575d 100644 (file)
--- a/lib/CodeGen/MachineBranchProbabilityInfo.cpp
+++ b/lib/CodeGen/MachineBranchProbabilityInfo.cpp
@@ -24,15 +24,16 @@ INITIALIZE_PASS_BEGIN(MachineBranchProbabilityInfo, "machine-branch-prob",
 INITIALIZE_PASS_END(MachineBranchProbabilityInfo, "machine-branch-prob",
                     "Machine Branch Probability Analysis", false, true)
 
-cl::opt<unsigned> StaticLikelyProb(
-    "static-likely-prob",
-    cl::desc("branch probability threshold to be considered very likely"),
-    cl::init(80), cl::Hidden);
+cl::opt<unsigned>
+    StaticLikelyProb("static-likely-prob",
+                     cl::desc("branch probability threshold in percentage"
+                              "to be considered very likely"),
+                     cl::init(80), cl::Hidden);
 
 cl::opt<unsigned> ProfileLikelyProb(
     "profile-likely-prob",
-    cl::desc("branch probability threshold to be considered very likely "
-             "when profile is available"),
+    cl::desc("branch probability threshold in percentage to be considered"
+             " very likely when profile is available"),
     cl::init(51), cl::Hidden);
 
 char MachineBranchProbabilityInfo::ID = 0;