Revert D71539 "Recommit "[SCEV] Look through single value PHIs.""

This reverts commit 11dccf8.

A bootstrapped clang crashes (due to ArrayRef::front called on an empty
ArrayRef) when compiling some files.  Very strangely, this only reproduces with
modules.

```
13 0x0000564d3349e968 llvm::ArrayRef<llvm::BasicBlock*>::front() const /proc/self/cwd/llvm/include/llvm/ADT/ArrayRef.h:160:7
14 0x0000564d3349e896 llvm::LoopBase<llvm::BasicBlock, llvm::Loop>::getHeader() const /proc/self/cwd/llvm/include/llvm/Analysis/LoopInfo.h:104:50
15 0x0000564d3349fd9d llvm::LoopBase<llvm::BasicBlock, llvm::Loop>::getLoopLatch() const /proc/self/cwd/llvm/include/llvm/Analysis/LoopInfoImpl.h:210:11
16 0x0000564d33593c8a llvm::ScalarEvolution::computeBackedgeTakenCount(llvm::Loop const*, bool) /proc/self/cwd/llvm/lib/Analysis/ScalarEvolution.cpp:6933:15
17 0x0000564d33592ebc llvm::ScalarEvolution::getBackedgeTakenInfo(llvm::Loop const*) /proc/self/cwd/llvm/lib/Analysis/ScalarEvolution.cpp:0:30
18 0x0000564d33593a54 llvm::ScalarEvolution::getBackedgeTakenCount(llvm::Loop const*, llvm::ScalarEvolution::ExitCountKind) /proc/self/cwd/llvm/lib/Analysis/ScalarEvolution.cpp:6487:36
19 0x0000564d32be2402 llvm::ScalarEvolution::getConstantMaxBackedgeTakenCount(llvm::Loop const*) /proc/self/cwd/llvm/include/llvm/Analysis/ScalarEvolution.h:768:5
20 0x0000564d33590807 llvm::ScalarEvolution::getRangeRef(llvm::SCEV const*, llvm::ScalarEvolution::RangeSignHint) /proc/self/cwd/llvm/lib/Analysis/ScalarEvolution.cpp:5495:19
21 0x0000564d320abab7 llvm::ScalarEvolution::getSignedRange(llvm::SCEV const*) /proc/self/cwd/llvm/include/llvm/Analysis/ScalarEvolution.h:840:12
22 0x0000564d335a03aa llvm::ScalarEvolution::isKnownPredicateViaConstantRanges(llvm::CmpInst::Predicate, llvm::SCEV const*, llvm::SCEV const*) /proc/self/cwd/llvm/lib/Analysis/ScalarEvolution.cpp:9239:60
23 0x0000564d33586a80 llvm::ScalarEvolution::isKnownViaNonRecursiveReasoning(llvm::CmpInst::Predicate, llvm::SCEV const*, llvm::SCEV const*) /proc/self/cwd/llvm/lib/Analysis/ScalarEvolution.cpp:10284:60
```

Author: MaskRay

llvm/lib/Analysis/ScalarEvolution.cpp

@@ -5114,8 +5114,13 @@ const SCEV *ScalarEvolution::createNodeForPHI(PHINode *PN) {
   if (const SCEV *S = createNodeFromSelectLikePHI(PN))
     return S;
 
+  // If the PHI has a single incoming value, follow that value, unless the
+  // PHI's incoming blocks are in a different loop, in which case doing so
+  // risks breaking LCSSA form. Instcombine would normally zap these, but
+  // it doesn't have DominatorTree information, so it may miss cases.
   if (Value *V = SimplifyInstruction(PN, {getDataLayout(), &TLI, &DT, &AC}))
-    return getSCEV(V);
+    if (LI.replacementPreservesLCSSAForm(PN, V))
+      return getSCEV(V);
 
   // If it's not a loop phi, we can't handle it yet.
   return getUnknown(PN);

llvm/test/Analysis/ScalarEvolution/solve-quadratic-i1.ll

@@ -58,9 +58,9 @@ b2:                                               ; preds = %b1
 ; CHECK-NEXT:   %v6 = add nuw nsw i32 %v1, 1
 ; CHECK-NEXT:   -->  {4,+,1}<nuw><nsw><%b1> U: [4,7) S: [4,7)         Exits: 6                LoopDispositions: { %b1: Computable }
 ; CHECK-NEXT:   %v7 = phi i32 [ %v1, %b1 ]
-; CHECK-NEXT:   -->  {3,+,1}<nuw><nsw><%b1> U: [3,6) S: [3,6)  -->  5 U: [5,6) S: [5,6)
+; CHECK-NEXT:   -->  %v7 U: [3,6) S: [3,6)
 ; CHECK-NEXT:   %v8 = phi i16 [ %v3, %b1 ]
-; CHECK-NEXT:   -->  {3,+,4,+,1}<%b1> U: full-set S: full-set  -->  12 U: [12,13) S: [12,13)
+; CHECK-NEXT:   -->  %v8 U: full-set S: full-set
 ; CHECK-NEXT: Determining loop execution counts for: @f1
 ; CHECK-NEXT: Loop %b3: <multiple exits> Unpredictable backedge-taken count.
 ; CHECK-NEXT: Loop %b3: Unpredictable max backedge-taken count.

llvm/test/Analysis/ScalarEvolution/solve-quadratic-overflow.ll

@@ -13,11 +13,11 @@
 ; CHECK-NEXT:   %v3 = mul i16 %v2, %v2
 ; CHECK-NEXT:   -->  {1,+,3,+,2}<%b1> U: full-set S: full-set         Exits: 0               LoopDispositions: { %b1: Computable }
 ; CHECK-NEXT:   %v5 = phi i16 [ %v2, %b1 ]
-; CHECK-NEXT:   -->  {-1,+,-1}<%b1> U: [-256,0) S: [-256,0)  -->  -256 U: [-256,-255) S: [-256,-255)
+; CHECK-NEXT:   -->  %v5 U: [-256,0) S: [-256,0)
 ; CHECK-NEXT:   %v6 = phi i16 [ %v3, %b1 ]
-; CHECK-NEXT:   -->  {1,+,3,+,2}<%b1> U: full-set S: full-set  -->  0 U: [0,1) S: [0,1)
+; CHECK-NEXT:   -->  %v6 U: full-set S: full-set
 ; CHECK-NEXT:   %v7 = sext i16 %v5 to i32
-; CHECK-NEXT:   -->  {-1,+,-1}<nsw><%b1> U: [-256,0) S: [-256,0)  -->  -256 U: [-256,-255) S: [-256,-255)
+; CHECK-NEXT:   -->  (sext i16 %v5 to i32) U: [-256,0) S: [-256,0)
 ; CHECK-NEXT: Determining loop execution counts for: @f0
 ; CHECK-NEXT: Loop %b1: backedge-taken count is 255
 ; CHECK-NEXT: Loop %b1: max backedge-taken count is 255

llvm/test/Transforms/LoopStrengthReduce/funclet.ll

@@ -15,21 +15,25 @@ define void @f() personality i32 (...)* @_except_handler3 {
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    br label [[THROW:%.*]]
 ; CHECK:       throw:
+; CHECK-NEXT:    [[TMP96:%.*]] = getelementptr inbounds i8, i8* undef, i32 1
 ; CHECK-NEXT:    invoke void @reserve()
 ; CHECK-NEXT:    to label [[THROW]] unwind label [[PAD:%.*]]
 ; CHECK:       pad:
+; CHECK-NEXT:    [[PHI2:%.*]] = phi i8* [ [[TMP96]], [[THROW]] ]
 ; CHECK-NEXT:    [[CS:%.*]] = catchswitch within none [label %unreachable] unwind label [[BLAH2:%.*]]
 ; CHECK:       unreachable:
 ; CHECK-NEXT:    [[TMP0:%.*]] = catchpad within [[CS]] []
 ; CHECK-NEXT:    unreachable
 ; CHECK:       blah2:
 ; CHECK-NEXT:    [[CLEANUPPADI4_I_I_I:%.*]] = cleanuppad within none []
+; CHECK-NEXT:    [[PHI21:%.*]] = ptrtoint i8* [[PHI2]] to i32
+; CHECK-NEXT:    [[TMP1:%.*]] = sub i32 1, [[PHI21]]
+; CHECK-NEXT:    [[SCEVGEP:%.*]] = getelementptr i8, i8* undef, i32 [[TMP1]]
 ; CHECK-NEXT:    br label [[LOOP_BODY:%.*]]
 ; CHECK:       loop_body:
-; CHECK-NEXT:    [[LSR_IV:%.*]] = phi i32 [ [[LSR_IV_NEXT:%.*]], [[ITER:%.*]] ], [ 0, [[BLAH2]] ]
-; CHECK-NEXT:    [[LSR_IV_NEXT]] = add nuw nsw i32 [[LSR_IV]], -1
-; CHECK-NEXT:    [[LSR_IV_NEXT1:%.*]] = inttoptr i32 [[LSR_IV_NEXT]] to i8*
-; CHECK-NEXT:    [[TMP100:%.*]] = icmp eq i8* [[LSR_IV_NEXT1]], null
+; CHECK-NEXT:    [[LSR_IV:%.*]] = phi i8* [ [[SCEVGEP2:%.*]], [[ITER:%.*]] ], [ [[SCEVGEP]], [[BLAH2]] ]
+; CHECK-NEXT:    [[SCEVGEP2]] = getelementptr i8, i8* [[LSR_IV]], i32 -1
+; CHECK-NEXT:    [[TMP100:%.*]] = icmp eq i8* [[SCEVGEP2]], null
 ; CHECK-NEXT:    br i1 [[TMP100]], label [[UNWIND_OUT:%.*]], label [[ITER]]
 ; CHECK:       iter:
 ; CHECK-NEXT:    br i1 true, label [[UNWIND_OUT]], label [[LOOP_BODY]]
@@ -74,25 +78,29 @@ define void @g() personality i32 (...)* @_except_handler3 {
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    br label [[THROW:%.*]]
 ; CHECK:       throw:
+; CHECK-NEXT:    [[TMP96:%.*]] = getelementptr inbounds i8, i8* undef, i32 1
 ; CHECK-NEXT:    invoke void @reserve()
 ; CHECK-NEXT:    to label [[THROW]] unwind label [[PAD:%.*]]
 ; CHECK:       pad:
+; CHECK-NEXT:    [[PHI2:%.*]] = phi i8* [ [[TMP96]], [[THROW]] ]
 ; CHECK-NEXT:    [[CS:%.*]] = catchswitch within none [label [[UNREACHABLE:%.*]], label %blah] unwind to caller
 ; CHECK:       unreachable:
 ; CHECK-NEXT:    [[TMP0:%.*]] = catchpad within [[CS]] []
 ; CHECK-NEXT:    unreachable
 ; CHECK:       blah:
 ; CHECK-NEXT:    [[CATCHPAD:%.*]] = catchpad within [[CS]] []
+; CHECK-NEXT:    [[PHI21:%.*]] = ptrtoint i8* [[PHI2]] to i32
+; CHECK-NEXT:    [[TMP1:%.*]] = sub i32 1, [[PHI21]]
+; CHECK-NEXT:    [[SCEVGEP:%.*]] = getelementptr i8, i8* undef, i32 [[TMP1]]
 ; CHECK-NEXT:    br label [[LOOP_BODY:%.*]]
 ; CHECK:       unwind_out:
 ; CHECK-NEXT:    catchret from [[CATCHPAD]] to label [[LEAVE:%.*]]
 ; CHECK:       leave:
 ; CHECK-NEXT:    ret void
 ; CHECK:       loop_body:
-; CHECK-NEXT:    [[LSR_IV:%.*]] = phi i32 [ [[LSR_IV_NEXT:%.*]], [[ITER:%.*]] ], [ 0, [[BLAH:%.*]] ]
-; CHECK-NEXT:    [[LSR_IV_NEXT]] = add nuw nsw i32 [[LSR_IV]], -1
-; CHECK-NEXT:    [[LSR_IV_NEXT1:%.*]] = inttoptr i32 [[LSR_IV_NEXT]] to i8*
-; CHECK-NEXT:    [[TMP100:%.*]] = icmp eq i8* [[LSR_IV_NEXT1]], null
+; CHECK-NEXT:    [[LSR_IV:%.*]] = phi i8* [ [[SCEVGEP2:%.*]], [[ITER:%.*]] ], [ [[SCEVGEP]], [[BLAH:%.*]] ]
+; CHECK-NEXT:    [[SCEVGEP2]] = getelementptr i8, i8* [[LSR_IV]], i32 -1
+; CHECK-NEXT:    [[TMP100:%.*]] = icmp eq i8* [[SCEVGEP2]], null
 ; CHECK-NEXT:    br i1 [[TMP100]], label [[UNWIND_OUT:%.*]], label [[ITER]]
 ; CHECK:       iter:
 ; CHECK-NEXT:    br i1 true, label [[UNWIND_OUT]], label [[LOOP_BODY]]
@@ -138,6 +146,7 @@ define void @h() personality i32 (...)* @_except_handler3 {
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    br label [[THROW:%.*]]
 ; CHECK:       throw:
+; CHECK-NEXT:    [[TMP96:%.*]] = getelementptr inbounds i8, i8* undef, i32 1
 ; CHECK-NEXT:    invoke void @reserve()
 ; CHECK-NEXT:    to label [[THROW]] unwind label [[PAD:%.*]]
 ; CHECK:       pad:
@@ -146,17 +155,20 @@ define void @h() personality i32 (...)* @_except_handler3 {
 ; CHECK-NEXT:    [[TMP0:%.*]] = catchpad within [[CS]] []
 ; CHECK-NEXT:    unreachable
 ; CHECK:       blug:
+; CHECK-NEXT:    [[PHI2:%.*]] = phi i8* [ [[TMP96]], [[PAD]] ]
 ; CHECK-NEXT:    [[CATCHPAD:%.*]] = catchpad within [[CS]] []
+; CHECK-NEXT:    [[PHI21:%.*]] = ptrtoint i8* [[PHI2]] to i32
+; CHECK-NEXT:    [[TMP1:%.*]] = sub i32 1, [[PHI21]]
+; CHECK-NEXT:    [[SCEVGEP:%.*]] = getelementptr i8, i8* undef, i32 [[TMP1]]
 ; CHECK-NEXT:    br label [[LOOP_BODY:%.*]]
 ; CHECK:       unwind_out:
 ; CHECK-NEXT:    catchret from [[CATCHPAD]] to label [[LEAVE:%.*]]
 ; CHECK:       leave:
 ; CHECK-NEXT:    ret void
 ; CHECK:       loop_body:
-; CHECK-NEXT:    [[LSR_IV:%.*]] = phi i32 [ [[LSR_IV_NEXT:%.*]], [[ITER:%.*]] ], [ 0, [[BLUG:%.*]] ]
-; CHECK-NEXT:    [[LSR_IV_NEXT]] = add nuw nsw i32 [[LSR_IV]], -1
-; CHECK-NEXT:    [[LSR_IV_NEXT1:%.*]] = inttoptr i32 [[LSR_IV_NEXT]] to i8*
-; CHECK-NEXT:    [[TMP100:%.*]] = icmp eq i8* [[LSR_IV_NEXT1]], null
+; CHECK-NEXT:    [[LSR_IV:%.*]] = phi i8* [ [[SCEVGEP2:%.*]], [[ITER:%.*]] ], [ [[SCEVGEP]], [[BLUG:%.*]] ]
+; CHECK-NEXT:    [[SCEVGEP2]] = getelementptr i8, i8* [[LSR_IV]], i32 -1
+; CHECK-NEXT:    [[TMP100:%.*]] = icmp eq i8* [[SCEVGEP2]], null
 ; CHECK-NEXT:    br i1 [[TMP100]], label [[UNWIND_OUT:%.*]], label [[ITER]]
 ; CHECK:       iter:
 ; CHECK-NEXT:    br i1 true, label [[UNWIND_OUT]], label [[LOOP_BODY]]
@@ -202,9 +214,11 @@ define void @i() personality i32 (...)* @_except_handler3 {
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    br label [[THROW:%.*]]
 ; CHECK:       throw:
+; CHECK-NEXT:    [[TMP96:%.*]] = getelementptr inbounds i8, i8* undef, i32 1
 ; CHECK-NEXT:    invoke void @reserve()
 ; CHECK-NEXT:    to label [[THROW]] unwind label [[CATCHPAD:%.*]]
 ; CHECK:       catchpad:
+; CHECK-NEXT:    [[PHI2:%.*]] = phi i8* [ [[TMP96]], [[THROW]] ]
 ; CHECK-NEXT:    [[CS:%.*]] = catchswitch within none [label %cp_body] unwind label [[CLEANUPPAD:%.*]]
 ; CHECK:       cp_body:
 ; CHECK-NEXT:    [[TMP0:%.*]] = catchpad within [[CS]] []
@@ -213,12 +227,14 @@ define void @i() personality i32 (...)* @_except_handler3 {
 ; CHECK-NEXT:    [[TMP1:%.*]] = cleanuppad within none []
 ; CHECK-NEXT:    br label [[LOOP_HEAD]]
 ; CHECK:       loop_head:
+; CHECK-NEXT:    [[PHI21:%.*]] = ptrtoint i8* [[PHI2]] to i32
+; CHECK-NEXT:    [[TMP2:%.*]] = sub i32 1, [[PHI21]]
+; CHECK-NEXT:    [[SCEVGEP:%.*]] = getelementptr i8, i8* undef, i32 [[TMP2]]
 ; CHECK-NEXT:    br label [[LOOP_BODY:%.*]]
 ; CHECK:       loop_body:
-; CHECK-NEXT:    [[LSR_IV:%.*]] = phi i32 [ [[LSR_IV_NEXT:%.*]], [[ITER:%.*]] ], [ 0, [[LOOP_HEAD]] ]
-; CHECK-NEXT:    [[LSR_IV_NEXT]] = add nuw nsw i32 [[LSR_IV]], -1
-; CHECK-NEXT:    [[LSR_IV_NEXT1:%.*]] = inttoptr i32 [[LSR_IV_NEXT]] to i8*
-; CHECK-NEXT:    [[TMP100:%.*]] = icmp eq i8* [[LSR_IV_NEXT1]], null
+; CHECK-NEXT:    [[LSR_IV:%.*]] = phi i8* [ [[SCEVGEP2:%.*]], [[ITER:%.*]] ], [ [[SCEVGEP]], [[LOOP_HEAD]] ]
+; CHECK-NEXT:    [[SCEVGEP2]] = getelementptr i8, i8* [[LSR_IV]], i32 -1
+; CHECK-NEXT:    [[TMP100:%.*]] = icmp eq i8* [[SCEVGEP2]], null
 ; CHECK-NEXT:    br i1 [[TMP100]], label [[UNWIND_OUT:%.*]], label [[ITER]]
 ; CHECK:       iter:
 ; CHECK-NEXT:    br i1 true, label [[UNWIND_OUT]], label [[LOOP_BODY]]