[InstCombine] Fix infinite loop due to bitcast <-> phi transforms

nikic · tmandry · commit 8813358d42d1 · 2020-01-16T15:49:30.000-08:00
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
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -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)) {
diff --git a/llvm/test/Transforms/InstCombine/pr44245.ll b/llvm/test/Transforms/InstCombine/pr44245.ll
@@ -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
+}
