1 files changed, 18 insertions, 6 deletions
diff --git a/llvm/lib/Transforms/Scalar/LowerExpectIntrinsic.cpp b/llvm/lib/Transforms/Scalar/LowerExpectIntrinsic.cpp
index 2ace902a7a1..40ba7447359 100644
--- a/llvm/lib/Transforms/Scalar/LowerExpectIntrinsic.cpp
+++ b/llvm/lib/Transforms/Scalar/LowerExpectIntrinsic.cpp
@@ -34,12 +34,24 @@ using namespace llvm;
 STATISTIC(ExpectIntrinsicsHandled,
           "Number of 'expect' intrinsic instructions handled");
 
-static cl::opt<uint32_t>
-LikelyBranchWeight("likely-branch-weight", cl::Hidden, cl::init(64),
-                   cl::desc("Weight of the branch likely to be taken (default = 64)"));
-static cl::opt<uint32_t>
-UnlikelyBranchWeight("unlikely-branch-weight", cl::Hidden, cl::init(4),
-                   cl::desc("Weight of the branch unlikely to be taken (default = 4)"));
+// These default values are chosen to represent an extremely skewed outcome for
+// a condition, but they leave some room for interpretation by later passes.
+//
+// If the documentation for __builtin_expect() was made explicit that it should
+// only be used in extreme cases, we could make this ratio higher. As it stands,
+// programmers may be using __builtin_expect() / llvm.expect to annotate that a
+// branch is likely or unlikely to be taken.
+//
+// There is a known dependency on this ratio in CodeGenPrepare when transforming
+// 'select' instructions. It may be worthwhile to hoist these values to some
+// shared space, so they can be used directly by other passes.
+
+static cl::opt<uint32_t> LikelyBranchWeight(
+    "likely-branch-weight", cl::Hidden, cl::init(2000),
+    cl::desc("Weight of the branch likely to be taken (default = 2000)"));
+static cl::opt<uint32_t> UnlikelyBranchWeight(
+    "unlikely-branch-weight", cl::Hidden, cl::init(1),
+    cl::desc("Weight of the branch unlikely to be taken (default = 1)"));
 
 static bool handleSwitchExpect(SwitchInst &SI) {
   CallInst *CI = dyn_cast<CallInst>(SI.getCondition());