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[InstCombine] Fix infinite loop due to bitcast <-> phi transforms
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Fix for https://bugs.llvm.org/show_bug.cgi?id=44245.

The optimizeBitCastFromPhi() and FoldPHIArgOpIntoPHI() end up
fighting against each other, because optimizeBitCastFromPhi()
assumes that bitcasts of loads will get folded. This doesn't
happen here, because a dangling phi node prevents the one-use
fold in https://github.com/llvm/llvm-project/blob/master/llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp#L620-L628 from triggering.

This patch fixes the issue by explicitly performing the load
combine as part of the bitcast of phi transform. Other attempts
to force the load to be combined first were ultimately too
unreliable.

Differential Revision: https://reviews.llvm.org/D71164
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@nikic @tmandry
nikic authored and tmandry committed Jan 16, 2020
1 parent 24100f0 commit 8813358
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11 changes: 8 additions & 3 deletions llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -2234,9 +2234,14 @@ Instruction *InstCombiner::optimizeBitCastFromPhi(CastInst &CI, PHINode *PN) {
if (auto *C = dyn_cast<Constant>(V)) {
NewV = ConstantExpr::getBitCast(C, DestTy);
} else if (auto *LI = dyn_cast<LoadInst>(V)) {
Builder.SetInsertPoint(LI->getNextNode());
NewV = Builder.CreateBitCast(LI, DestTy);
Worklist.Add(LI);
// Explicitly perform load combine to make sure no opposing transform
// can remove the bitcast in the meantime and trigger an infinite loop.
Builder.SetInsertPoint(LI);
NewV = combineLoadToNewType(*LI, DestTy);
// Remove the old load and its use in the old phi, which itself becomes
// dead once the whole transform finishes.
replaceInstUsesWith(*LI, UndefValue::get(LI->getType()));
eraseInstFromFunction(*LI);
} else if (auto *BCI = dyn_cast<BitCastInst>(V)) {
NewV = BCI->getOperand(0);
} else if (auto *PrevPN = dyn_cast<PHINode>(V)) {
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192 changes: 192 additions & 0 deletions llvm/test/Transforms/InstCombine/pr44245.ll
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Original file line number Diff line number Diff line change
@@ -0,0 +1,192 @@
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
; RUN: opt -S -instcombine -instcombine-infinite-loop-threshold=2 < %s | FileCheck %s

; This used to cause on infinite instcombine loop.

define void @test(i1 %c) {
; CHECK-LABEL: @test(
; CHECK-NEXT: bb16:
; CHECK-NEXT: br i1 [[C:%.*]], label [[BB17:%.*]], label [[BB24:%.*]]
; CHECK: bb17:
; CHECK-NEXT: [[TMP0:%.*]] = phi i8* [ [[TMP1:%.*]], [[BB47:%.*]] ], [ undef, [[BB16:%.*]] ]
; CHECK-NEXT: store i8* [[TMP0]], i8** undef, align 8
; CHECK-NEXT: ret void
; CHECK: bb24:
; CHECK-NEXT: br i1 [[C]], label [[BB44:%.*]], label [[BB49:%.*]]
; CHECK: bb44:
; CHECK-NEXT: [[TMP467:%.*]] = load i8*, i8** inttoptr (i64 16 to i8**), align 16
; CHECK-NEXT: br label [[BB47]]
; CHECK: bb47:
; CHECK-NEXT: [[TMP1]] = phi i8* [ [[TMP2:%.*]], [[BB150:%.*]] ], [ [[TMP1221:%.*]], [[BB119:%.*]] ], [ [[TMP1032:%.*]], [[BB101:%.*]] ], [ [[TMP933:%.*]], [[BB91:%.*]] ], [ [[TMP834:%.*]], [[BB81:%.*]] ], [ [[TMP705:%.*]], [[BB67:%.*]] ], [ [[TMP586:%.*]], [[BB56:%.*]] ], [ [[TMP467]], [[BB44]] ]
; CHECK-NEXT: br label [[BB17]]
; CHECK: bb49:
; CHECK-NEXT: br i1 [[C]], label [[BB56]], label [[BB59:%.*]]
; CHECK: bb56:
; CHECK-NEXT: [[TMP586]] = load i8*, i8** inttoptr (i64 16 to i8**), align 16
; CHECK-NEXT: br label [[BB47]]
; CHECK: bb59:
; CHECK-NEXT: br i1 [[C]], label [[BB67]], label [[BB71:%.*]]
; CHECK: bb67:
; CHECK-NEXT: [[TMP705]] = load i8*, i8** inttoptr (i64 16 to i8**), align 16
; CHECK-NEXT: br label [[BB47]]
; CHECK: bb71:
; CHECK-NEXT: br i1 [[C]], label [[BB81]], label [[BB84:%.*]]
; CHECK: bb81:
; CHECK-NEXT: [[TMP834]] = load i8*, i8** inttoptr (i64 16 to i8**), align 16
; CHECK-NEXT: br label [[BB47]]
; CHECK: bb84:
; CHECK-NEXT: br i1 [[C]], label [[BB91]], label [[BB94:%.*]]
; CHECK: bb91:
; CHECK-NEXT: [[TMP933]] = load i8*, i8** inttoptr (i64 16 to i8**), align 16
; CHECK-NEXT: br label [[BB47]]
; CHECK: bb94:
; CHECK-NEXT: br i1 [[C]], label [[BB101]], label [[BB104:%.*]]
; CHECK: bb101:
; CHECK-NEXT: [[TMP1032]] = load i8*, i8** inttoptr (i64 16 to i8**), align 16
; CHECK-NEXT: br label [[BB47]]
; CHECK: bb104:
; CHECK-NEXT: br i1 [[C]], label [[BB119]], label [[BB123:%.*]]
; CHECK: bb119:
; CHECK-NEXT: [[TMP1221]] = load i8*, i8** inttoptr (i64 16 to i8**), align 16
; CHECK-NEXT: br label [[BB47]]
; CHECK: bb123:
; CHECK-NEXT: br i1 [[C]], label [[BB147:%.*]], label [[BB152:%.*]]
; CHECK: bb147:
; CHECK-NEXT: [[TMP1499:%.*]] = load i8*, i8** inttoptr (i64 16 to i8**), align 16
; CHECK-NEXT: br label [[BB150]]
; CHECK: bb150:
; CHECK-NEXT: [[TMP2]] = phi i8* [ [[TMP1848:%.*]], [[BB152]] ], [ [[TMP1499]], [[BB147]] ]
; CHECK-NEXT: br label [[BB47]]
; CHECK: bb152:
; CHECK-NEXT: [[TMP1848]] = load i8*, i8** inttoptr (i64 16 to i8**), align 16
; CHECK-NEXT: call void undef()
; CHECK-NEXT: br label [[BB150]]
;
bb16: ; preds = %bb
br i1 %c, label %bb17, label %bb24

bb17: ; preds = %bb47, %bb17
%0 = phi i8* [ %1, %bb47 ], [ undef, %bb16 ]
store i8* %0, i8** undef, align 8
ret void

bb24: ; preds = %bb24
br i1 %c, label %bb44, label %bb49

bb44: ; preds = %bb43
%tmp46 = load i64*, i64** inttoptr (i64 16 to i64**), align 16
br label %bb47

bb47: ; preds = %bb150, %bb119, %bb101, %bb91, %bb81, %bb67, %bb56, %bb44
%.in1 = phi i64* [ %.in, %bb150 ], [ %tmp122, %bb119 ], [ %tmp103, %bb101 ], [ %tmp93, %bb91 ], [ %tmp83, %bb81 ], [ %tmp70, %bb67 ], [ %tmp58, %bb56 ], [ %tmp46, %bb44 ]
%1 = bitcast i64* %.in1 to i8*
br label %bb17

bb49: ; preds = %bb49
br i1 %c, label %bb56, label %bb59

bb56: ; preds = %bb55
%tmp58 = load i64*, i64** inttoptr (i64 16 to i64**), align 16
br label %bb47

bb59: ; preds = %bb59
br i1 %c, label %bb67, label %bb71

bb67: ; preds = %bb66
%tmp70 = load i64*, i64** inttoptr (i64 16 to i64**), align 16
br label %bb47

bb71: ; preds = %bb71
br i1 %c, label %bb81, label %bb84

bb81: ; preds = %bb80
%tmp83 = load i64*, i64** inttoptr (i64 16 to i64**), align 16
br label %bb47

bb84: ; preds = %bb84
br i1 %c, label %bb91, label %bb94

bb91: ; preds = %bb90
%tmp93 = load i64*, i64** inttoptr (i64 16 to i64**), align 16
br label %bb47

bb94: ; preds = %bb94
br i1 %c, label %bb101, label %bb104

bb101: ; preds = %bb100
%tmp103 = load i64*, i64** inttoptr (i64 16 to i64**), align 16
br label %bb47

bb104: ; preds = %bb104
br i1 %c, label %bb119, label %bb123

bb119: ; preds = %bb118
%tmp122 = load i64*, i64** inttoptr (i64 16 to i64**), align 16
br label %bb47

bb123: ; preds = %bb123
br i1 %c, label %bb147, label %bb152

bb147: ; preds = %bb146
%tmp149 = load i64*, i64** inttoptr (i64 16 to i64**), align 16
br label %bb150

bb150: ; preds = %bb152, %bb147
%.in = phi i64* [ %tmp184, %bb152 ], [ %tmp149, %bb147 ]
br label %bb47

bb152: ; preds = %bb146
%tmp184 = load i64*, i64** inttoptr (i64 16 to i64**), align 16
call void undef()
br label %bb150
}

; This used to cause an instcombine loop when the problem above was
; addressed in a non-robust fashion.

%type_1 = type {}
%type_2 = type {}
%type_3 = type {}

define void @test_2(i1 %c) local_unnamed_addr {
; CHECK-LABEL: @test_2(
; CHECK-NEXT: entry:
; CHECK-NEXT: br label [[WHILE_COND:%.*]]
; CHECK: while.cond:
; CHECK-NEXT: br label [[FOR_COND:%.*]]
; CHECK: for.cond:
; CHECK-NEXT: br i1 [[C:%.*]], label [[COND_TRUE133:%.*]], label [[COND_FALSE138:%.*]]
; CHECK: cond.true133:
; CHECK-NEXT: store %type_3* undef, %type_3** null, align 536870912
; CHECK-NEXT: br label [[COND_END144:%.*]]
; CHECK: cond.false138:
; CHECK-NEXT: store %type_3* undef, %type_3** null, align 536870912
; CHECK-NEXT: br label [[COND_END144]]
; CHECK: cond.end144:
; CHECK-NEXT: br label [[WHILE_COND]]
;
entry:
br label %while.cond

while.cond: ; preds = %cond.end144, %entry
%link.0 = phi %type_2* [ undef, %entry ], [ %cond145, %cond.end144 ]
%os115 = bitcast %type_2* %link.0 to %type_3*
%ou116 = getelementptr inbounds %type_3, %type_3* %os115, i32 0
%os1117 = bitcast %type_3* %ou116 to %type_1*
br label %for.cond

for.cond: ; preds = %while.cond
br i1 %c, label %cond.true133, label %cond.false138

cond.true133: ; preds = %sw.epilog
%0 = load %type_2*, %type_2** undef, align 8
br label %cond.end144

cond.false138: ; preds = %sw.epilog
%1 = load %type_2*, %type_2** undef, align 8
br label %cond.end144

cond.end144: ; preds = %cond.false138, %cond.true133
%cond145 = phi %type_2* [ %0, %cond.true133 ], [ %1, %cond.false138 ]
br label %while.cond
}

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