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Switch lowering: Take branch weight into account when ordering for fall-through

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Previously, the code would try to put a fall-through case last,
even if that meant moving a case with much higher branch weight
further down the chain.

Ordering by branch weight is most important, putting a fall-through
block last is secondary.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@235942 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Hans Wennborg 2015-04-27 23:35:22 +00:00
parent ae51ba7ea1
commit b176a4f2e4
5 changed files with 40 additions and 8 deletions
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7
lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
View File

@ -7678,11 +7678,12 @@ void SelectionDAGBuilder::lowerWorkItem(SwitchWorkListItem W, Value *Cond,
return a.Weight > b.Weight;
});
// Rearrange the case blocks so that the last one falls through if possible.
// Start at the bottom as that's the case with the lowest weight.
// FIXME: Take branch probability into account.
// Rearrange the case blocks so that the last one falls through if possible
// without without changing the order of weights.
for (CaseClusterIt I = W.LastCluster; I > W.FirstCluster; ) {
--I;
if (I->Weight > W.LastCluster->Weight)
break;
if (I->Kind == CC_Range && I->MBB == NextMBB) {
std::swap(*I, *W.LastCluster);
break;

6
test/CodeGen/ARM/arm-and-tst-peephole.ll
View File

@ -49,14 +49,14 @@ tailrecurse.switch: ; preds = %tailrecurse
; V8-NEXT: beq
; V8-NEXT: %tailrecurse.switch
; V8: cmp
; V8-NEXT: beq
; V8-NEXT: bne
; V8-NEXT: b
; The trailing space in the last line checks that the branch is unconditional
switch i32 %and, label %sw.epilog [
i32 1, label %sw.bb
i32 3, label %sw.bb6
i32 2, label %sw.bb8
]
], !prof !1
sw.bb: ; preds = %tailrecurse.switch, %tailrecurse
%shl = shl i32 %acc.tr, 1
@ -134,3 +134,5 @@ bb4: ; preds = %bb2
return: ; preds = %bb2, %bb, %entry
ret i8 1
}
!1 = !{!"branch_weights", i32 1, i32 1, i32 3, i32 2 }

4
test/CodeGen/Generic/MachineBranchProb.ll
View File

@ -17,9 +17,9 @@ entry:
; CHECK: BB#0: derived from LLVM BB %entry
; CHECK: Successors according to CFG: BB#2(64) BB#4(14)
; CHECK: BB#4: derived from LLVM BB %entry
; CHECK: Successors according to CFG: BB#1(4) BB#5(10)
; CHECK: Successors according to CFG: BB#1(10) BB#5(4)
; CHECK: BB#5: derived from LLVM BB %entry
; CHECK: Successors according to CFG: BB#1(10) BB#3(7)
; CHECK: Successors according to CFG: BB#1(4) BB#3(7)
sw.bb:
br label %return

2
test/CodeGen/X86/ragreedy-hoist-spill.ll
View File

@ -30,7 +30,7 @@ if.then:
if.end:
switch i64 undef, label %if.end25 [
i64 0, label %if.then4
i64 1, label %land.lhs.true14
i64 1, label %if.end25
]
if.then4:

29
test/CodeGen/X86/switch.ll
View File

@ -413,3 +413,32 @@ return: ret void
i32 4294967295, i32 2, i32 4294967295,
; Cases 2,5,8,9:
i32 3, i32 3, i32 3, i32 3}
define void @order_by_weight_and_fallthrough(i32 %x) {
entry:
switch i32 %x, label %return [
i32 100, label %bb1
i32 200, label %bb0
i32 300, label %bb0
], !prof !2
bb0: tail call void @g(i32 0) br label %return
bb1: tail call void @g(i32 1) br label %return
return: ret void
; Case 200 has the highest weight and should come first. 100 and 300 have the
; same weight, but 300 goes to the 'next' block, so should be last.
; CHECK-LABEL: order_by_weight_and_fallthrough
; CHECK: cmpl $200
; CHECK: cmpl $100
; CHECK: cmpl $300
}
!2 = !{!"branch_weights",
; Default:
i32 1,
; Case 100:
i32 10,
; Case 200:
i32 1000,
; Case 300:
i32 10}