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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
This commit is contained in:
Nikita Popov 2020-01-13 18:57:14 +01:00
parent 1b8770ebf1
commit c8831fb193
2 changed files with 200 additions and 3 deletions
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11
lib/Transforms/InstCombine/InstCombineCasts.cpp
View File

@ -2319,9 +2319,14 @@ 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)) {

192
test/Transforms/InstCombine/pr44245.ll Normal file
View File

@ -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
}