[llvm] r350186 - Omit range checks from jump tables when lowering switches with unreachable
Ayonam Ray via llvm-commits
llvm-commits at lists.llvm.org
Mon Dec 31 22:37:51 PST 2018
Author: ayonam
Date: Mon Dec 31 22:37:50 2018
New Revision: 350186
URL: http://llvm.org/viewvc/llvm-project?rev=350186&view=rev
Log:
Omit range checks from jump tables when lowering switches with unreachable
default
During the lowering of a switch that would result in the generation of a jump
table, a range check is performed before indexing into the jump table, for the
switch value being outside the jump table range and a conditional branch is
inserted to jump to the default block. In case the default block is
unreachable, this conditional jump can be omitted. This patch implements
omitting this conditional branch for unreachable defaults.
Review Reference: D52002
Added:
llvm/trunk/test/CodeGen/AArch64/switch-unreachable-default.ll
Modified:
llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.h
Modified: llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp?rev=350186&r1=350185&r2=350186&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp (original)
+++ llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp Mon Dec 31 22:37:50 2018
@@ -2189,24 +2189,30 @@ void SelectionDAGBuilder::visitJumpTable
JumpTableReg, SwitchOp);
JT.Reg = JumpTableReg;
- // Emit the range check for the jump table, and branch to the default block
- // for the switch statement if the value being switched on exceeds the largest
- // case in the switch.
- SDValue CMP = DAG.getSetCC(
- dl, TLI.getSetCCResultType(DAG.getDataLayout(), *DAG.getContext(),
- Sub.getValueType()),
- Sub, DAG.getConstant(JTH.Last - JTH.First, dl, VT), ISD::SETUGT);
+ if (!JTH.OmitRangeCheck) {
+ // Emit the range check for the jump table, and branch to the default block
+ // for the switch statement if the value being switched on exceeds the
+ // largest case in the switch.
+ SDValue CMP = DAG.getSetCC(
+ dl, TLI.getSetCCResultType(DAG.getDataLayout(), *DAG.getContext(),
+ Sub.getValueType()),
+ Sub, DAG.getConstant(JTH.Last - JTH.First, dl, VT), ISD::SETUGT);
+
+ SDValue BrCond = DAG.getNode(ISD::BRCOND, dl,
+ MVT::Other, CopyTo, CMP,
+ DAG.getBasicBlock(JT.Default));
+
+ // Avoid emitting unnecessary branches to the next block.
+ if (JT.MBB != NextBlock(SwitchBB))
+ BrCond = DAG.getNode(ISD::BR, dl, MVT::Other, BrCond,
+ DAG.getBasicBlock(JT.MBB));
- SDValue BrCond = DAG.getNode(ISD::BRCOND, dl,
- MVT::Other, CopyTo, CMP,
- DAG.getBasicBlock(JT.Default));
-
- // Avoid emitting unnecessary branches to the next block.
- if (JT.MBB != NextBlock(SwitchBB))
- BrCond = DAG.getNode(ISD::BR, dl, MVT::Other, BrCond,
- DAG.getBasicBlock(JT.MBB));
-
- DAG.setRoot(BrCond);
+ DAG.setRoot(BrCond);
+ } else {
+ SDValue BrCond = DAG.getNode(ISD::BR, dl, MVT::Other, CopyTo,
+ DAG.getBasicBlock(JT.MBB));
+ DAG.setRoot(BrCond);
+ }
}
/// Create a LOAD_STACK_GUARD node, and let it carry the target specific global
@@ -9558,10 +9564,13 @@ bool SelectionDAGBuilder::buildJumpTable
->createJumpTableIndex(Table);
// Set up the jump table info.
+ bool UnreachableDefault =
+ isa<UnreachableInst>(SI->getDefaultDest()->getFirstNonPHIOrDbg());
+ bool OmitRangeCheck = UnreachableDefault;
JumpTable JT(-1U, JTI, JumpTableMBB, nullptr);
JumpTableHeader JTH(Clusters[First].Low->getValue(),
Clusters[Last].High->getValue(), SI->getCondition(),
- nullptr, false);
+ nullptr, false, OmitRangeCheck);
JTCases.emplace_back(std::move(JTH), std::move(JT));
JTCluster = CaseCluster::jumpTable(Clusters[First].Low, Clusters[Last].High,
@@ -10298,6 +10307,7 @@ MachineBasicBlock *SelectionDAGBuilder::
const SwitchInst &SI, CaseClusterVector &Clusters,
BranchProbability &PeeledCaseProb) {
MachineBasicBlock *SwitchMBB = FuncInfo.MBB;
+
// Don't perform if there is only one cluster or optimizing for size.
if (SwitchPeelThreshold > 100 || !FuncInfo.BPI || Clusters.size() < 2 ||
TM.getOptLevel() == CodeGenOpt::None ||
@@ -10350,6 +10360,7 @@ void SelectionDAGBuilder::visitSwitch(co
// Extract cases from the switch.
BranchProbabilityInfo *BPI = FuncInfo.BPI;
CaseClusterVector Clusters;
+
Clusters.reserve(SI.getNumCases());
for (auto I : SI.cases()) {
MachineBasicBlock *Succ = FuncInfo.MBBMap[I.getCaseSuccessor()];
@@ -10367,38 +10378,6 @@ void SelectionDAGBuilder::visitSwitch(co
// if there are many clusters.
sortAndRangeify(Clusters);
- if (TM.getOptLevel() != CodeGenOpt::None) {
- // Replace an unreachable default with the most popular destination.
- // FIXME: Exploit unreachable default more aggressively.
- bool UnreachableDefault =
- isa<UnreachableInst>(SI.getDefaultDest()->getFirstNonPHIOrDbg());
- if (UnreachableDefault && !Clusters.empty()) {
- DenseMap<const BasicBlock *, unsigned> Popularity;
- unsigned MaxPop = 0;
- const BasicBlock *MaxBB = nullptr;
- for (auto I : SI.cases()) {
- const BasicBlock *BB = I.getCaseSuccessor();
- if (++Popularity[BB] > MaxPop) {
- MaxPop = Popularity[BB];
- MaxBB = BB;
- }
- }
- // Set new default.
- assert(MaxPop > 0 && MaxBB);
- DefaultMBB = FuncInfo.MBBMap[MaxBB];
-
- // Remove cases that were pointing to the destination that is now the
- // default.
- CaseClusterVector New;
- New.reserve(Clusters.size());
- for (CaseCluster &CC : Clusters) {
- if (CC.MBB != DefaultMBB)
- New.push_back(CC);
- }
- Clusters = std::move(New);
- }
- }
-
// The branch probablity of the peeled case.
BranchProbability PeeledCaseProb = BranchProbability::getZero();
MachineBasicBlock *PeeledSwitchMBB =
Modified: llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.h
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.h?rev=350186&r1=350185&r2=350186&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.h (original)
+++ llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.h Mon Dec 31 22:37:50 2018
@@ -279,11 +279,12 @@ private:
const Value *SValue;
MachineBasicBlock *HeaderBB;
bool Emitted;
+ bool OmitRangeCheck;
JumpTableHeader(APInt F, APInt L, const Value *SV, MachineBasicBlock *H,
- bool E = false)
+ bool E = false, bool ORC = false)
: First(std::move(F)), Last(std::move(L)), SValue(SV), HeaderBB(H),
- Emitted(E) {}
+ Emitted(E), OmitRangeCheck(ORC) {}
};
using JumpTableBlock = std::pair<JumpTableHeader, JumpTable>;
Added: llvm/trunk/test/CodeGen/AArch64/switch-unreachable-default.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/AArch64/switch-unreachable-default.ll?rev=350186&view=auto
==============================================================================
--- llvm/trunk/test/CodeGen/AArch64/switch-unreachable-default.ll (added)
+++ llvm/trunk/test/CodeGen/AArch64/switch-unreachable-default.ll Mon Dec 31 22:37:50 2018
@@ -0,0 +1,62 @@
+; RUN: llc -O3 -o - %s | FileCheck %s
+
+; Test that the output in the presence of an unreachable default does not have
+; a compare and branch at the top of the switch to handle the default case.
+
+target triple = "aarch64-unknown-linux-gnu"
+
+; Function Attrs: nounwind
+define void @fn(i4) {
+ switch i4 %0, label %default [
+ i4 0, label %case_0
+ i4 1, label %case_1
+ i4 2, label %case_2
+ i4 3, label %case_3
+ i4 4, label %case_4
+ i4 5, label %case_5
+ ]
+
+; CHECK-LABEL: fn:
+; CHECK-NOT: sub
+; CHECK-NOT: cmp
+; CHECK-NOT: b.hi
+; CHECK: ldr {{x[0-9]+}}, [{{x[0-9]+}}, {{x[0-9]+}}, lsl #3]
+; CHECK: br {{x[0-9]+}}
+
+default:
+ unreachable
+
+case_0:
+ tail call void @handle_case_00(i4 %0) #2
+ br label %return_label
+
+case_1:
+ tail call void @handle_case_01(i4 %0) #2
+ br label %return_label
+
+case_2:
+ tail call void @handle_case_02(i4 %0) #2
+ br label %return_label
+
+case_3:
+ tail call void @handle_case_03(i4 %0) #2
+ br label %return_label
+
+case_4:
+ tail call void @handle_case_04(i4 %0) #2
+ br label %return_label
+
+case_5:
+ tail call void @handle_case_05(i4 %0) #2
+ br label %return_label
+
+return_label:
+ ret void
+}
+
+declare void @handle_case_00(i4)
+declare void @handle_case_01(i4)
+declare void @handle_case_02(i4)
+declare void @handle_case_03(i4)
+declare void @handle_case_04(i4)
+declare void @handle_case_05(i4)
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