[llvm] r357067 - Re-commit r355490 "[CodeGen] Omit range checks from jump tables when lowering switches with unreachable default"
Hans Wennborg via llvm-commits
llvm-commits at lists.llvm.org
Wed Mar 27 07:10:11 PDT 2019
Author: hans
Date: Wed Mar 27 07:10:11 2019
New Revision: 357067
URL: http://llvm.org/viewvc/llvm-project?rev=357067&view=rev
Log:
Re-commit r355490 "[CodeGen] Omit range checks from jump tables when lowering switches with unreachable default"
Original commit by Ayonam Ray.
This commit adds a regression test for the issue discovered in the
previous commit: that the range check for the jump table can only be
omitted if the fall-through destination of the jump table is
unreachable, which isn't necessarily true just because the default of
the switch is unreachable.
This addresses the missing optimization in PR41242.
> 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.
>
> Differential Revision: https://reviews.llvm.org/D52002
> Reviewers: Hans Wennborg, Eli Freidman, Roman Lebedev
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
llvm/trunk/test/CodeGen/X86/pr38743.ll
llvm/trunk/test/CodeGen/X86/switch-jump-table.ll
Modified: llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp?rev=357067&r1=357066&r2=357067&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp (original)
+++ llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp Wed Mar 27 07:10:11 2019
@@ -2366,9 +2366,7 @@ void SelectionDAGBuilder::visitJumpTable
MachineBasicBlock *SwitchBB) {
SDLoc dl = getCurSDLoc();
- // Subtract the lowest switch case value from the value being switched on and
- // conditional branch to default mbb if the result is greater than the
- // difference between smallest and largest cases.
+ // Subtract the lowest switch case value from the value being switched on.
SDValue SwitchOp = getValue(JTH.SValue);
EVT VT = SwitchOp.getValueType();
SDValue Sub = DAG.getNode(ISD::SUB, dl, VT, SwitchOp,
@@ -2388,24 +2386,33 @@ 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 {
+ // Avoid emitting unnecessary branches to the next block.
+ if (JT.MBB != NextBlock(SwitchBB))
+ DAG.setRoot(DAG.getNode(ISD::BR, dl, MVT::Other, CopyTo,
+ DAG.getBasicBlock(JT.MBB)));
+ else
+ DAG.setRoot(CopyTo);
+ }
}
/// Create a LOAD_STACK_GUARD node, and let it carry the target specific global
@@ -10335,7 +10342,15 @@ void SelectionDAGBuilder::lowerWorkItem(
}
}
- addSuccessorWithProb(CurMBB, Fallthrough, FallthroughProb);
+ if (Fallthrough == DefaultMBB &&
+ isa<UnreachableInst>(
+ DefaultMBB->getBasicBlock()->getFirstNonPHIOrDbg())) {
+ // Skip the range check if the fallthrough block is unreachable.
+ JTH->OmitRangeCheck = true;
+ }
+
+ if (!JTH->OmitRangeCheck)
+ addSuccessorWithProb(CurMBB, Fallthrough, FallthroughProb);
addSuccessorWithProb(CurMBB, JumpMBB, JumpProb);
CurMBB->normalizeSuccProbs();
@@ -10649,38 +10664,6 @@ 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=357067&r1=357066&r2=357067&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.h (original)
+++ llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.h Wed Mar 27 07:10:11 2019
@@ -278,11 +278,12 @@ private:
const Value *SValue;
MachineBasicBlock *HeaderBB;
bool Emitted;
+ bool OmitRangeCheck;
JumpTableHeader(APInt F, APInt L, const Value *SV, MachineBasicBlock *H,
bool E = false)
: First(std::move(F)), Last(std::move(L)), SValue(SV), HeaderBB(H),
- Emitted(E) {}
+ Emitted(E), OmitRangeCheck(false) {}
};
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=357067&view=auto
==============================================================================
--- llvm/trunk/test/CodeGen/AArch64/switch-unreachable-default.ll (added)
+++ llvm/trunk/test/CodeGen/AArch64/switch-unreachable-default.ll Wed Mar 27 07:10:11 2019
@@ -0,0 +1,99 @@
+; 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: ldrb {{w[0-9]+}}, [{{x[0-9]+}}, {{x[0-9]+}}]
+; CHECK: add {{x[0-9]+}}, {{x[0-9]+}}, {{x[0-9]+}}, lsl #2
+; 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)
+
+
+
+define i32 @reachable_fallthrough(i32 %x) {
+entry:
+ switch i32 %x, label %def [
+ i32 1, label %bb1
+ i32 8, label %bb2
+ i32 16, label %bb3
+ i32 32, label %bb4
+ i32 64, label %bb5
+ ]
+
+; The switch is lowered with a jump table for cases 1--32 and case 64 checked
+; separately. Even though the default of switch is unreachable, the fall-through
+; for the jump table *is* reachable so the range check must be emitted.
+;
+; CHECK-LABEL: reachable_fallthrough
+; CHECK: sub
+; CHECK: cmp
+; CHECK: b.hi
+;
+; TODO: Drop the cmp for the 64 case since its fallthrough is unreachable.
+
+
+def: unreachable
+bb1: br label %return
+bb2: br label %return
+bb3: br label %return
+bb4: br label %return
+bb5: br label %return
+
+return:
+ %p = phi i32 [ 3, %bb1 ], [ 2, %bb2 ], [ 1, %bb3 ], [ 0, %bb4 ], [ 0, %bb5 ]
+ ret i32 %p
+}
Modified: llvm/trunk/test/CodeGen/X86/pr38743.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/X86/pr38743.ll?rev=357067&r1=357066&r2=357067&view=diff
==============================================================================
--- llvm/trunk/test/CodeGen/X86/pr38743.ll (original)
+++ llvm/trunk/test/CodeGen/X86/pr38743.ll Wed Mar 27 07:10:11 2019
@@ -19,26 +19,24 @@ declare void @llvm.memcpy.p0i8.p0i8.i64(
define void @pr38743(i32 %a0) #1 align 2 {
; CHECK-LABEL: pr38743:
; CHECK: # %bb.0: # %bb
-; CHECK-NEXT: cmpl $3, %edi
-; CHECK-NEXT: je .LBB0_4
-; CHECK-NEXT: # %bb.1: # %bb
-; CHECK-NEXT: cmpl $1, %edi
-; CHECK-NEXT: je .LBB0_2
-; CHECK-NEXT: # %bb.3: # %bb5
+; CHECK-NEXT: # kill: def $edi killed $edi def $rdi
+; CHECK-NEXT: decl %edi
+; CHECK-NEXT: jmpq *.LJTI0_0(,%rdi,8)
+; CHECK-NEXT: .LBB0_2: # %bb5
; CHECK-NEXT: movzwl .str.17+{{.*}}(%rip), %eax
; CHECK-NEXT: movw %ax, -{{[0-9]+}}(%rsp)
; CHECK-NEXT: movq {{.*}}(%rip), %rax
-; CHECK-NEXT: jmp .LBB0_5
-; CHECK-NEXT: .LBB0_4: # %bb8
-; CHECK-NEXT: movq .str.18+{{.*}}(%rip), %rax
+; CHECK-NEXT: jmp .LBB0_4
+; CHECK-NEXT: .LBB0_1: # %bb2
+; CHECK-NEXT: movq .str.16+{{.*}}(%rip), %rax
; CHECK-NEXT: movq %rax, -{{[0-9]+}}(%rsp)
; CHECK-NEXT: movq {{.*}}(%rip), %rax
-; CHECK-NEXT: jmp .LBB0_5
-; CHECK-NEXT: .LBB0_2: # %bb2
-; CHECK-NEXT: movq .str.16+{{.*}}(%rip), %rax
+; CHECK-NEXT: jmp .LBB0_4
+; CHECK-NEXT: .LBB0_3: # %bb8
+; CHECK-NEXT: movq .str.18+{{.*}}(%rip), %rax
; CHECK-NEXT: movq %rax, -{{[0-9]+}}(%rsp)
; CHECK-NEXT: movq {{.*}}(%rip), %rax
-; CHECK-NEXT: .LBB0_5: # %bb12
+; CHECK-NEXT: .LBB0_4: # %bb12
; CHECK-NEXT: movq %rax, -{{[0-9]+}}(%rsp)
; CHECK-NEXT: movq -{{[0-9]+}}(%rsp), %rax
; CHECK-NEXT: movq %rax, (%rax)
Modified: llvm/trunk/test/CodeGen/X86/switch-jump-table.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/X86/switch-jump-table.ll?rev=357067&r1=357066&r2=357067&view=diff
==============================================================================
--- llvm/trunk/test/CodeGen/X86/switch-jump-table.ll (original)
+++ llvm/trunk/test/CodeGen/X86/switch-jump-table.ll Wed Mar 27 07:10:11 2019
@@ -2,14 +2,12 @@
; RUN: llc -mtriple=i686-pc-gnu-linux -print-machineinstrs=expand-isel-pseudos %s -o /dev/null 2>&1 | FileCheck %s -check-prefix=CHECK-JT-PROB
-; An unreachable default destination is replaced with the most popular case label.
+; An unreachable default destination is ignored and no compare and branch
+; is generated for the default values.
define void @foo(i32 %x, i32* %to) {
; CHECK-LABEL: foo:
; CHECK: movl 4(%esp), [[REG:%e[a-z]{2}]]
-; CHECK: cmpl $3, [[REG]]
-; CHECK: ja .LBB0_6
-; CHECK-NEXT: # %bb.1:
; CHECK-NEXT: jmpl *.LJTI0_0(,[[REG]],4)
; CHECK: movl $4
; CHECK: retl
@@ -45,10 +43,12 @@ default:
; The jump table has four entries.
; CHECK-LABEL: .LJTI0_0:
+; CHECK-NEXT: .long .LBB0_1
; CHECK-NEXT: .long .LBB0_2
; CHECK-NEXT: .long .LBB0_3
; CHECK-NEXT: .long .LBB0_4
; CHECK-NEXT: .long .LBB0_5
+; CHECK-NEXT: .long .LBB0_5
}
; Check if branch probabilities are correctly assigned to the jump table.
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