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Switch lowering: use profile info to build weight-balanced binary search trees

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This will cause hot nodes to appear closer to the root.

The literature says building the tree like this makes it a near-optimal (in
terms of search time given key frequencies) binary search tree. In LLVM's case,
we can do up to 3 comparisons in each leaf node, so it might be better to opt
for lower tree height in some cases; that's something to look into in the
future.

Differential Revision: http://reviews.llvm.org/D9318

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@236192 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Hans Wennborg 2015-04-30 00:57:37 +00:00
parent a607be94ca
commit 49baa9f896
3 changed files with 157 additions and 8 deletions
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42
lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
View File

@ -7986,14 +7986,41 @@ void SelectionDAGBuilder::splitWorkItem(SwitchWorkList &WorkList,
unsigned NumClusters = W.LastCluster - W.FirstCluster + 1;
assert(NumClusters >= 2 && "Too small to split!");
// FIXME: When we have profile info, we might want to balance the tree based
// on weights instead of node count.
// Balance the tree based on branch weights to create a near-optimal (in terms
// of search time given key frequency) binary search tree. See e.g. Kurt
// Mehlhorn "Nearly Optimal Binary Search Trees" (1975).
CaseClusterIt LastLeft = W.FirstCluster;
CaseClusterIt FirstRight = W.LastCluster;
uint32_t LeftWeight = LastLeft->Weight;
uint32_t RightWeight = FirstRight->Weight;
// Move LastLeft and FirstRight towards each other from opposite directions to
// find a partitioning of the clusters which balances the weight on both
// sides.
while (LastLeft + 1 < FirstRight) {
// Zero-weight nodes would cause skewed trees since they don't affect
// LeftWeight or RightWeight.
assert(LastLeft->Weight != 0);
assert(FirstRight->Weight != 0);
if (LeftWeight < RightWeight)
LeftWeight += (++LastLeft)->Weight;
else
RightWeight += (--FirstRight)->Weight;
}
assert(LastLeft + 1 == FirstRight);
assert(LastLeft >= W.FirstCluster);
assert(FirstRight <= W.LastCluster);
// Use the first element on the right as pivot since we will make less-than
// comparisons against it.
CaseClusterIt PivotCluster = FirstRight;
assert(PivotCluster > W.FirstCluster);
assert(PivotCluster <= W.LastCluster);
CaseClusterIt PivotCluster = W.FirstCluster + NumClusters / 2;
CaseClusterIt FirstLeft = W.FirstCluster;
CaseClusterIt LastLeft = PivotCluster - 1;
CaseClusterIt FirstRight = PivotCluster;
CaseClusterIt LastRight = W.LastCluster;
const ConstantInt *Pivot = PivotCluster->Low;
// New blocks will be inserted immediately after the current one.
@ -8032,7 +8059,8 @@ void SelectionDAGBuilder::splitWorkItem(SwitchWorkList &WorkList,
}
// Create the CaseBlock record that will be used to lower the branch.
CaseBlock CB(ISD::SETLT, Cond, Pivot, nullptr, LeftMBB, RightMBB, W.MBB);
CaseBlock CB(ISD::SETLT, Cond, Pivot, nullptr, LeftMBB, RightMBB, W.MBB,
LeftWeight, RightWeight);
if (W.MBB == SwitchMBB)
visitSwitchCase(CB, SwitchMBB);
@ -8048,7 +8076,7 @@ void SelectionDAGBuilder::visitSwitch(const SwitchInst &SI) {
for (auto I : SI.cases()) {
MachineBasicBlock *Succ = FuncInfo.MBBMap[I.getCaseSuccessor()];
const ConstantInt *CaseVal = I.getCaseValue();
uint32_t Weight = 0; // FIXME: Use 1 instead?
uint32_t Weight = 1;
if (BPI) {
Weight = BPI->getEdgeWeight(SI.getParent(), I.getSuccessorIndex());
assert(Weight <= UINT32_MAX / SI.getNumSuccessors());

40
test/CodeGen/Generic/MachineBranchProb.ll
View File

@ -5,7 +5,7 @@
; Make sure we have the correct weight attached to each successor.
define i32 @test2(i32 %x) nounwind uwtable readnone ssp {
; CHECK: Machine code for function test2:
; CHECK-LABEL: Machine code for function test2:
entry:
%conv = sext i32 %x to i64
switch i64 %conv, label %return [
@ -33,3 +33,41 @@ return:
}
!0 = !{!"branch_weights", i32 7, i32 6, i32 4, i32 4, i32 64}
declare void @g(i32)
define void @left_leaning_weight_balanced_tree(i32 %x) {
entry:
switch i32 %x, label %return [
i32 0, label %bb0
i32 10, label %bb1
i32 20, label %bb2
i32 30, label %bb3
i32 40, label %bb4
i32 50, label %bb5
], !prof !1
bb0: tail call void @g(i32 0) br label %return
bb1: tail call void @g(i32 1) br label %return
bb2: tail call void @g(i32 2) br label %return
bb3: tail call void @g(i32 3) br label %return
bb4: tail call void @g(i32 4) br label %return
bb5: tail call void @g(i32 5) br label %return
return: ret void
; Check that we set branch weights on the pivot cmp instruction correctly.
; Cases {0,10,20,30} go on the left with weight 13; cases {40,50} go on the
; right with weight 20.
;
; CHECK-LABEL: Machine code for function left_leaning_weight_balanced_tree:
; CHECK: BB#0: derived from LLVM BB %entry
; CHECK-NOT: Successors
; CHECK: Successors according to CFG: BB#8(13) BB#9(20)
}
!1 = !{!"branch_weights",
; Default:
i32 1,
; Case 0, 10, 20:
i32 10, i32 1, i32 1,
; Case 30, 40, 50:
i32 1, i32 10, i32 10}

83
test/CodeGen/X86/switch.ll
View File

@ -442,3 +442,86 @@ return: ret void
i32 1000,
; Case 300:
i32 10}
define void @zero_weight_tree(i32 %x) {
entry:
switch i32 %x, label %return [
i32 0, label %bb0
i32 10, label %bb1
i32 20, label %bb2
i32 30, label %bb3
i32 40, label %bb4
i32 50, label %bb5
], !prof !3
bb0: tail call void @g(i32 0) br label %return
bb1: tail call void @g(i32 1) br label %return
bb2: tail call void @g(i32 2) br label %return
bb3: tail call void @g(i32 3) br label %return
bb4: tail call void @g(i32 4) br label %return
bb5: tail call void @g(i32 5) br label %return
return: ret void
; Make sure to pick a pivot in the middle also with zero-weight cases.
; CHECK-LABEL: zero_weight_tree
; CHECK-NOT: cmpl
; CHECK: cmpl $29
}
!3 = !{!"branch_weights", i32 1, i32 10, i32 0, i32 0, i32 0, i32 0, i32 10}
define void @left_leaning_weight_balanced_tree(i32 %x) {
entry:
switch i32 %x, label %return [
i32 0, label %bb0
i32 10, label %bb1
i32 20, label %bb2
i32 30, label %bb3
i32 40, label %bb4
i32 50, label %bb5
], !prof !4
bb0: tail call void @g(i32 0) br label %return
bb1: tail call void @g(i32 1) br label %return
bb2: tail call void @g(i32 2) br label %return
bb3: tail call void @g(i32 3) br label %return
bb4: tail call void @g(i32 4) br label %return
bb5: tail call void @g(i32 5) br label %return
return: ret void
; To balance the tree by weight, the pivot is shifted to the right, moving hot
; cases closer to the root.
; CHECK-LABEL: left_leaning_weight_balanced_tree
; CHECK-NOT: cmpl
; CHECK: cmpl $39
}
!4 = !{!"branch_weights", i32 1, i32 10, i32 1, i32 1, i32 1, i32 10, i32 10}
define void @jump_table_affects_balance(i32 %x) {
entry:
switch i32 %x, label %return [
; Jump table:
i32 0, label %bb0
i32 1, label %bb1
i32 2, label %bb2
i32 3, label %bb3
i32 100, label %bb0
i32 200, label %bb1
i32 300, label %bb2
]
bb0: tail call void @g(i32 0) br label %return
bb1: tail call void @g(i32 1) br label %return
bb2: tail call void @g(i32 2) br label %return
bb3: tail call void @g(i32 3) br label %return
return: ret void
; CHECK-LABEL: jump_table_affects_balance
; If the tree were balanced based on number of clusters, {0-3,100} would go on
; the left and {200,300} on the right. However, the jump table weights as much
; as its components, so 100 is selected as the pivot.
; CHECK-NOT: cmpl
; CHECK: cmpl $99
}