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[SimplifyCFG] When only one case value is missing, replace default with that case
Eliminating the default branch will provide more opportunities for optimization, such as lookup tables.
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3 files changed

+86
-26
lines changed

3 files changed

+86
-26
lines changed

‎llvm/lib/Transforms/Utils/SimplifyCFG.cpp

+28-7
Original file line numberDiff line numberDiff line change
@@ -5414,11 +5414,13 @@ static bool CasesAreContiguous(SmallVectorImpl<ConstantInt *> &Cases) {
54145414
}
54155415

54165416
static void createUnreachableSwitchDefault(SwitchInst *Switch,
5417-
DomTreeUpdater *DTU) {
5417+
DomTreeUpdater *DTU,
5418+
bool RemoveOrigDefaultBlock = true) {
54185419
LLVM_DEBUG(dbgs() << "SimplifyCFG: switch default is dead.\n");
54195420
auto *BB = Switch->getParent();
54205421
auto *OrigDefaultBlock = Switch->getDefaultDest();
5421-
OrigDefaultBlock->removePredecessor(BB);
5422+
if (RemoveOrigDefaultBlock)
5423+
OrigDefaultBlock->removePredecessor(BB);
54225424
BasicBlock *NewDefaultBlock = BasicBlock::Create(
54235425
BB->getContext(), BB->getName() + ".unreachabledefault", BB->getParent(),
54245426
OrigDefaultBlock);
@@ -5427,7 +5429,8 @@ static void createUnreachableSwitchDefault(SwitchInst *Switch,
54275429
if (DTU) {
54285430
SmallVector<DominatorTree::UpdateType, 2> Updates;
54295431
Updates.push_back({DominatorTree::Insert, BB, &*NewDefaultBlock});
5430-
if (!is_contained(successors(BB), OrigDefaultBlock))
5432+
if (RemoveOrigDefaultBlock &&
5433+
!is_contained(successors(BB), OrigDefaultBlock))
54315434
Updates.push_back({DominatorTree::Delete, BB, &*OrigDefaultBlock});
54325435
DTU->applyUpdates(Updates);
54335436
}
@@ -5609,10 +5612,28 @@ static bool eliminateDeadSwitchCases(SwitchInst *SI, DomTreeUpdater *DTU,
56095612
Known.getBitWidth() - (Known.Zero | Known.One).popcount();
56105613
assert(NumUnknownBits <= Known.getBitWidth());
56115614
if (HasDefault && DeadCases.empty() &&
5612-
NumUnknownBits < 64 /* avoid overflow */ &&
5613-
SI->getNumCases() == (1ULL << NumUnknownBits)) {
5614-
createUnreachableSwitchDefault(SI, DTU);
5615-
return true;
5615+
NumUnknownBits < 64 /* avoid overflow */) {
5616+
uint64_t AllNumCases = 1ULL << NumUnknownBits;
5617+
if (SI->getNumCases() == AllNumCases) {
5618+
createUnreachableSwitchDefault(SI, DTU);
5619+
return true;
5620+
}
5621+
// When only one case value is missing, replace default with that case.
5622+
// Eliminating the default branch will provide more opportunities for
5623+
// optimization, such as lookup tables.
5624+
if (SI->getNumCases() == AllNumCases - 1) {
5625+
assert(NumUnknownBits > 1 && "Should be canonicalized to a branch");
5626+
uint64_t MissingCaseVal = 0;
5627+
for (const auto &Case : SI->cases())
5628+
MissingCaseVal ^= Case.getCaseValue()->getValue().getLimitedValue();
5629+
auto *MissingCase =
5630+
cast<ConstantInt>(ConstantInt::get(Cond->getType(), MissingCaseVal));
5631+
SwitchInstProfUpdateWrapper SIW(*SI);
5632+
SIW.addCase(MissingCase, SI->getDefaultDest(), SIW.getSuccessorWeight(0));
5633+
createUnreachableSwitchDefault(SI, DTU, /*RemoveOrigDefaultBlock*/ false);
5634+
SIW.setSuccessorWeight(0, 0);
5635+
return true;
5636+
}
56165637
}
56175638

56185639
if (DeadCases.empty())

‎llvm/test/Transforms/SimplifyCFG/X86/switch-dead-default-lookup-table.ll renamed to ‎llvm/test/Transforms/SimplifyCFG/switch-dead-default-lookup-table.ll

+5-16
Original file line numberDiff line numberDiff line change
@@ -2,22 +2,15 @@
22
; RUN: opt %s -S -passes='simplifycfg<switch-to-lookup>' -simplifycfg-require-and-preserve-domtree=1 -switch-range-to-icmp | FileCheck %s
33

44
target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
5-
target triple = "x86_64-unknown-linux-gnu"
65

76
define i64 @test_1(i64 %0) {
87
; CHECK-LABEL: define i64 @test_1(
98
; CHECK-SAME: i64 [[TMP0:%.*]]) {
10-
; CHECK-NEXT: [[TMP2:%.*]] = urem i64 [[TMP0]], 4
11-
; CHECK-NEXT: [[SWITCH_TABLEIDX:%.*]] = sub i64 [[TMP2]], 1
12-
; CHECK-NEXT: [[TMP3:%.*]] = icmp ult i64 [[SWITCH_TABLEIDX]], 3
13-
; CHECK-NEXT: br i1 [[TMP3]], label [[SWITCH_LOOKUP:%.*]], label [[TMP4:%.*]]
14-
; CHECK: switch.lookup:
15-
; CHECK-NEXT: [[SWITCH_GEP:%.*]] = getelementptr inbounds [3 x i64], ptr @switch.table.test_1, i32 0, i64 [[SWITCH_TABLEIDX]]
9+
; CHECK-NEXT: switch.lookup:
10+
; CHECK-NEXT: [[TMP1:%.*]] = urem i64 [[TMP0]], 4
11+
; CHECK-NEXT: [[SWITCH_GEP:%.*]] = getelementptr inbounds [4 x i64], ptr @switch.table.test_1, i32 0, i64 [[TMP1]]
1612
; CHECK-NEXT: [[SWITCH_LOAD:%.*]] = load i64, ptr [[SWITCH_GEP]], align 8
17-
; CHECK-NEXT: br label [[TMP4]]
18-
; CHECK: 4:
19-
; CHECK-NEXT: [[DOT0:%.*]] = phi i64 [ 0, [[TMP1:%.*]] ], [ [[SWITCH_LOAD]], [[SWITCH_LOOKUP]] ]
20-
; CHECK-NEXT: ret i64 [[DOT0]]
13+
; CHECK-NEXT: ret i64 [[SWITCH_LOAD]]
2114
;
2215
%2 = urem i64 %0, 4
2316
switch i64 %2, label %5 [
@@ -43,11 +36,7 @@ define i64 @test_2(i64 %0) {
4336
; CHECK-SAME: i64 [[TMP0:%.*]]) {
4437
; CHECK-NEXT: switch.lookup:
4538
; CHECK-NEXT: [[TMP1:%.*]] = urem i64 [[TMP0]], 4
46-
; CHECK-NEXT: [[SWITCH_TABLEIDX:%.*]] = sub i64 [[TMP1]], 1
47-
; CHECK-NEXT: [[TMP2:%.*]] = icmp ult i64 [[SWITCH_TABLEIDX]], 3
48-
; CHECK-NEXT: [[SWITCH_OFFSET:%.*]] = add nsw i64 [[SWITCH_TABLEIDX]], 1
49-
; CHECK-NEXT: [[SPEC_SELECT:%.*]] = select i1 [[TMP2]], i64 [[SWITCH_OFFSET]], i64 0
50-
; CHECK-NEXT: ret i64 [[SPEC_SELECT]]
39+
; CHECK-NEXT: ret i64 [[TMP1]]
5140
;
5241
%2 = urem i64 %0, 4
5342
switch i64 %2, label %6 [

‎llvm/test/Transforms/SimplifyCFG/switch-dead-default.ll

+53-3
Original file line numberDiff line numberDiff line change
@@ -79,15 +79,15 @@ default:
7979
ret void
8080
}
8181

82-
; This one is a negative test - we know the value of the default,
83-
; but that's about it
82+
; We can replace the default branch with case 3 since it is the only case that is missing.
8483
define void @test3(i2 %a) {
8584
; CHECK-LABEL: define void @test3(
8685
; CHECK-SAME: i2 [[A:%.*]]) {
87-
; CHECK-NEXT: switch i2 [[A]], label [[DEFAULT:%.*]] [
86+
; CHECK-NEXT: switch i2 [[A]], label [[DOTUNREACHABLEDEFAULT:%.*]] [
8887
; CHECK-NEXT: i2 0, label [[CASE0:%.*]]
8988
; CHECK-NEXT: i2 1, label [[CASE1:%.*]]
9089
; CHECK-NEXT: i2 -2, label [[CASE2:%.*]]
90+
; CHECK-NEXT: i2 -1, label [[DEFAULT:%.*]]
9191
; CHECK-NEXT: ]
9292
; CHECK: common.ret:
9393
; CHECK-NEXT: ret void
@@ -100,6 +100,8 @@ define void @test3(i2 %a) {
100100
; CHECK: case2:
101101
; CHECK-NEXT: call void @foo(i32 2)
102102
; CHECK-NEXT: br label [[COMMON_RET]]
103+
; CHECK: .unreachabledefault:
104+
; CHECK-NEXT: unreachable
103105
; CHECK: default:
104106
; CHECK-NEXT: call void @foo(i32 3)
105107
; CHECK-NEXT: br label [[COMMON_RET]]
@@ -122,6 +124,50 @@ default:
122124
ret void
123125
}
124126

127+
define void @test3_prof(i2 %a) {
128+
; CHECK-LABEL: define void @test3_prof(
129+
; CHECK-SAME: i2 [[A:%.*]]) {
130+
; CHECK-NEXT: switch i2 [[A]], label [[DOTUNREACHABLEDEFAULT:%.*]] [
131+
; CHECK-NEXT: i2 0, label [[CASE0:%.*]]
132+
; CHECK-NEXT: i2 1, label [[CASE1:%.*]]
133+
; CHECK-NEXT: i2 -2, label [[CASE2:%.*]]
134+
; CHECK-NEXT: i2 -1, label [[DEFAULT:%.*]]
135+
; CHECK-NEXT: ], !prof [[PROF0:![0-9]+]]
136+
; CHECK: common.ret:
137+
; CHECK-NEXT: ret void
138+
; CHECK: case0:
139+
; CHECK-NEXT: call void @foo(i32 0)
140+
; CHECK-NEXT: br label [[COMMON_RET:%.*]]
141+
; CHECK: case1:
142+
; CHECK-NEXT: call void @foo(i32 1)
143+
; CHECK-NEXT: br label [[COMMON_RET]]
144+
; CHECK: case2:
145+
; CHECK-NEXT: call void @foo(i32 2)
146+
; CHECK-NEXT: br label [[COMMON_RET]]
147+
; CHECK: .unreachabledefault:
148+
; CHECK-NEXT: unreachable
149+
; CHECK: default:
150+
; CHECK-NEXT: call void @foo(i32 3)
151+
; CHECK-NEXT: br label [[COMMON_RET]]
152+
;
153+
switch i2 %a, label %default [i2 0, label %case0
154+
i2 1, label %case1
155+
i2 2, label %case2], !prof !0
156+
157+
case0:
158+
call void @foo(i32 0)
159+
ret void
160+
case1:
161+
call void @foo(i32 1)
162+
ret void
163+
case2:
164+
call void @foo(i32 2)
165+
ret void
166+
default:
167+
call void @foo(i32 3)
168+
ret void
169+
}
170+
125171
; Negative test - check for possible overflow when computing
126172
; number of possible cases.
127173
define void @test4(i128 %a) {
@@ -267,3 +313,7 @@ default:
267313

268314
declare void @llvm.assume(i1)
269315

316+
!0 = !{!"branch_weights", i32 8, i32 4, i32 2, i32 1}
317+
;.
318+
; CHECK: [[PROF0]] = !{!"branch_weights", i32 0, i32 4, i32 2, i32 1, i32 8}
319+
;.

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