[llvm] r267615 - [LowerExpectIntrinsic] make default likely/unlikely ratio bigger

[Sanjay Patel via llvm-commits]

Author: spatel
Date: Tue Apr 26 17:23:38 2016
New Revision: 267615

URL: http://llvm.org/viewvc/llvm-project?rev=267615&view=rev
Log:
[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


Modified:
    llvm/trunk/lib/Transforms/Scalar/LowerExpectIntrinsic.cpp
    llvm/trunk/test/Transforms/LowerExpectIntrinsic/basic.ll

Modified: llvm/trunk/lib/Transforms/Scalar/LowerExpectIntrinsic.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/LowerExpectIntrinsic.cpp?rev=267615&r1=267614&r2=267615&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Scalar/LowerExpectIntrinsic.cpp (original)
+++ llvm/trunk/lib/Transforms/Scalar/LowerExpectIntrinsic.cpp Tue Apr 26 17:23:38 2016
@@ -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());

Modified: llvm/trunk/test/Transforms/LowerExpectIntrinsic/basic.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/LowerExpectIntrinsic/basic.ll?rev=267615&r1=267614&r2=267615&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/LowerExpectIntrinsic/basic.ll (original)
+++ llvm/trunk/test/Transforms/LowerExpectIntrinsic/basic.ll Tue Apr 26 17:23:38 2016
@@ -275,7 +275,7 @@ return:
 
 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}