[PATCH] D158777: [ConstraintElim] Support decrementing inductions with step -1.

[Florian Hahn via Phabricator via llvm-commits]

fhahn created this revision.
fhahn added reviewers: zjaffal, nikic, rapidsna, antoniofrighetto.
Herald added subscribers: StephenFan, hiraditya.
Herald added a project: All.
fhahn requested review of this revision.
Herald added a project: LLVM.

Extend the logic in addInfoForInductions to support decrementing
inductions with a step of -1.

NOTE: I still need to add small tests for the patch.

Depends on D152730 <https://reviews.llvm.org/D152730>.

Fixes #64881.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D158777

Files:
  llvm/lib/Transforms/Scalar/ConstraintElimination.cpp
  llvm/test/Transforms/PhaseOrdering/loop-access-checks.ll


Index: llvm/test/Transforms/PhaseOrdering/loop-access-checks.ll
===================================================================
--- llvm/test/Transforms/PhaseOrdering/loop-access-checks.ll
+++ llvm/test/Transforms/PhaseOrdering/loop-access-checks.ll
@@ -270,7 +270,7 @@
 
 define void @monkey(ptr noundef %arr, i32 noundef %len) {
 ; CHECK-LABEL: define void @monkey
-; CHECK-SAME: (ptr nocapture noundef [[ARR:%.*]], i32 noundef [[LEN:%.*]]) local_unnamed_addr {
+; CHECK-SAME: (ptr nocapture noundef [[ARR:%.*]], i32 noundef [[LEN:%.*]]) local_unnamed_addr #[[ATTR1:[0-9]+]] {
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[CMP8:%.*]] = icmp ugt i32 [[LEN]], 1
 ; CHECK-NEXT:    br i1 [[CMP8]], label [[FOR_BODY4_PREHEADER:%.*]], label [[FOR_COND_CLEANUP:%.*]]
@@ -284,13 +284,7 @@
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp ult i32 [[INC]], [[LEN]]
 ; CHECK-NEXT:    br i1 [[CMP]], label [[FOR_BODY4_PREHEADER]], label [[FOR_COND_CLEANUP]]
 ; CHECK:       for.body4:
-; CHECK-NEXT:    [[K_07:%.*]] = phi i32 [ [[DEC:%.*]], [[AT_EXIT:%.*]] ], [ [[I_09]], [[FOR_BODY4_PREHEADER]] ]
-; CHECK-NEXT:    [[CMP_NOT_I:%.*]] = icmp ult i32 [[K_07]], [[LEN]]
-; CHECK-NEXT:    br i1 [[CMP_NOT_I]], label [[AT_EXIT]], label [[IF_THEN_I:%.*]]
-; CHECK:       if.then.i:
-; CHECK-NEXT:    tail call void @abort()
-; CHECK-NEXT:    unreachable
-; CHECK:       at.exit:
+; CHECK-NEXT:    [[K_07:%.*]] = phi i32 [ [[DEC:%.*]], [[FOR_BODY4]] ], [ [[I_09]], [[FOR_BODY4_PREHEADER]] ]
 ; CHECK-NEXT:    [[IDX_EXT_I:%.*]] = zext i32 [[K_07]] to i64
 ; CHECK-NEXT:    [[ADD_PTR_I:%.*]] = getelementptr inbounds i32, ptr [[ARR]], i64 [[IDX_EXT_I]]
 ; CHECK-NEXT:    [[TMP0:%.*]] = load i32, ptr [[ADD_PTR_I]], align 4
Index: llvm/lib/Transforms/Scalar/ConstraintElimination.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/ConstraintElimination.cpp
+++ llvm/lib/Transforms/Scalar/ConstraintElimination.cpp
@@ -830,7 +830,8 @@
     return;
 
   auto *AR = dyn_cast_or_null<SCEVAddRecExpr>(SE.getSCEV(PN));
-  if (!AR)
+  BasicBlock *LoopPred = L->getLoopPredecessor();
+  if (!AR || !LoopPred)
     return;
 
   const SCEV *StartSCEV = AR->getStart();
@@ -839,9 +840,11 @@
     StartValue = C->getValue();
   else if (auto *U = dyn_cast<SCEVUnknown>(StartSCEV))
     StartValue = U->getValue();
-
-  if (!StartValue)
-    return;
+  else {
+    StartValue = PN->getIncomingValueForBlock(LoopPred);
+    if (SE.getSCEV(StartValue) != AR->getStart())
+      return;
+  }
 
   Type *StepTy = AR->getType();
   const DataLayout &DL = BB.getModule()->getDataLayout();
@@ -853,6 +856,25 @@
   else
     return;
 
+  // Handle negative steps.
+  if (StepOffset.isNegative()) {
+    // TODO: Extend to allow steps > -1.
+    if (!(-StepOffset).isOne())
+      return;
+
+    // AR may wrap. Add StartValue >= PN conditional on B <= StartValue which
+    // guarantees that the loop exits before wrapping with a step of -1.
+    auto *DTN = DT.getNode(Succ);
+    WorkList.push_back(FactOrCheck::getFact(DTN, CmpInst::ICMP_UGE, StartValue,
+                                            PN, CmpInst::ICMP_ULE, B,
+                                            StartValue));
+    // AR may wrap. Add PN > B conditional on B <= StartValue which guarantees
+    // that the loop exits when reaching B with a step of -1.
+    WorkList.push_back(FactOrCheck::getFact(DTN, CmpInst::ICMP_UGT, PN, B,
+                                            CmpInst::ICMP_ULE, B, StartValue));
+    return;
+  }
+
   // Make sure AR either steps by 1 or that the value we compare against is a
   // GEP based on the same start value and all offsets are a multiple of the
   // step size, to guarantee that the induction will reach the value.


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