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[LowerExpectIntrinsic] make default likely/unlikely ratio bigger

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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

llvm-svn: 267615
This commit is contained in:
Sanjay Patel 2016-04-26 22:23:38 +00:00
parent 8b0e33adba
commit 65b8c3537e
2 changed files with 22 additions and 10 deletions
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24
lib/Transforms/Scalar/LowerExpectIntrinsic.cpp
View File

@ -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());

8
test/Transforms/LowerExpectIntrinsic/basic.ll
View File

@ -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}