[LowerExpectIntrinsic] make default likely/unlikely ratio bigger

We need the default ratio to be sufficiently large that it triggers transforms
based on block frequency info (BFI) and plays well with the recently introduced
BranchProbability used by CGP.

Differential Revision: http://reviews.llvm.org/D19435

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@267615 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Transforms/Scalar/LowerExpectIntrinsic.cpp b/lib/Transforms/Scalar/LowerExpectIntrinsic.cpp
index 2ace902..40ba744 100644 (file)
--- a/lib/Transforms/Scalar/LowerExpectIntrinsic.cpp
+++ b/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());

diff --git a/test/Transforms/LowerExpectIntrinsic/basic.ll b/test/Transforms/LowerExpectIntrinsic/basic.ll
index 69e67cd..562db86 100644 (file)
--- a/test/Transforms/LowerExpectIntrinsic/basic.ll
+++ b/test/Transforms/LowerExpectIntrinsic/basic.ll
@@ -275,7 +275,7 @@ return:                                           ; preds = %if.end, %if.then
 
 declare i1 @llvm.expect.i1(i1, i1) nounwind readnone
 
-; CHECK: !0 = !{!"branch_weights", i32 64, i32 4}
-; CHECK: !1 = !{!"branch_weights", i32 4, i32 64}
-; CHECK: !2 = !{!"branch_weights", i32 4, i32 64, i32 4}
-; CHECK: !3 = !{!"branch_weights", i32 64, i32 4, i32 4}
+; CHECK: !0 = !{!"branch_weights", i32 2000, i32 1}
+; CHECK: !1 = !{!"branch_weights", i32 1, i32 2000}
+; CHECK: !2 = !{!"branch_weights", i32 1, i32 2000, i32 1}
+; CHECK: !3 = !{!"branch_weights", i32 2000, i32 1, i32 1}