[ConstraintElim] Support decrementing inductions with step -1. (#68644)

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

Fixes #64881.

Author: fhahn

llvm/lib/Transforms/Scalar/ConstraintElimination.cpp

@@ -889,6 +889,26 @@ void State::addInfoForInductions(BasicBlock &BB) {
   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.
+    WorkList.push_back(FactOrCheck::getConditionFact(
+        DTN, CmpInst::ICMP_UGE, StartValue, PN,
+        ConditionTy(CmpInst::ICMP_ULE, B, StartValue)));
+    // 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::getConditionFact(
+        DTN, CmpInst::ICMP_UGT, PN, B,
+        ConditionTy(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.

llvm/test/Transforms/ConstraintElimination/monotonic-int-phis-decrement.ll

@@ -14,8 +14,7 @@ define void @add_rec_decreasing_cond_true_constant(i8 noundef %len) {
 ; CHECK-NEXT:    [[CMP2_NOT:%.*]] = icmp eq i8 [[K_0]], 0
 ; CHECK-NEXT:    br i1 [[CMP2_NOT]], label [[EXIT:%.*]], label [[LOOP_LATCH]]
 ; CHECK:       loop.latch:
-; CHECK-NEXT:    [[CMP_NOT_I:%.*]] = icmp ult i8 [[K_0]], 5
-; CHECK-NEXT:    call void @use(i1 [[CMP_NOT_I]])
+; CHECK-NEXT:    call void @use(i1 true)
 ; CHECK-NEXT:    [[K_DEC]] = add i8 [[K_0]], -1
 ; CHECK-NEXT:    br label [[LOOP_HEADER]]
 ; CHECK:       exit:
@@ -87,8 +86,7 @@ define void @add_rec_decreasing_cond_true_start_signed_positive(i8 noundef %star
 ; CHECK-NEXT:    [[CMP2_NOT:%.*]] = icmp eq i8 [[K_0]], 0
 ; CHECK-NEXT:    br i1 [[CMP2_NOT]], label [[EXIT:%.*]], label [[LOOP_LATCH]]
 ; CHECK:       loop.latch:
-; CHECK-NEXT:    [[CMP_NOT_I:%.*]] = icmp ult i8 [[K_0]], [[START]]
-; CHECK-NEXT:    call void @use(i1 [[CMP_NOT_I]])
+; CHECK-NEXT:    call void @use(i1 true)
 ; CHECK-NEXT:    [[K_DEC]] = add i8 [[K_0]], -1
 ; CHECK-NEXT:    br label [[LOOP_HEADER]]
 ; CHECK:       exit:

llvm/test/Transforms/ConstraintElimination/monotonic-int-phis-nested-loops.ll

@@ -117,8 +117,7 @@ define void @inner_add_rec_decreasing(i32 noundef %len) {
 ; CHECK-NEXT:    [[CMP2_NOT:%.*]] = icmp eq i32 [[K_0]], 0
 ; CHECK-NEXT:    br i1 [[CMP2_NOT]], label [[OUTER_LATCH]], label [[INNER_LATCH]]
 ; CHECK:       inner.latch:
-; CHECK-NEXT:    [[CMP_NOT_I:%.*]] = icmp ult i32 [[K_0]], [[LEN]]
-; CHECK-NEXT:    call void @use(i1 [[CMP_NOT_I]])
+; CHECK-NEXT:    call void @use(i1 true)
 ; CHECK-NEXT:    [[K_DEC]] = add i32 [[K_0]], -1
 ; CHECK-NEXT:    br label [[INNER_HEADER]]
 ; CHECK:       outer.latch:

llvm/test/Transforms/PhaseOrdering/loop-access-checks.ll

@@ -270,7 +270,7 @@ declare void @abort()
 
 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 @@ define void @monkey(ptr noundef %arr, i32 noundef %len) {
 ; 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