[llvm] [ConstraintElim] Support arbitrary incoming values for indcutions. (PR #68032)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

<details>
<summary>Changes</summary>

Support arbitray incoming values for AddRecs by getting the loop predecessor and checking if its SCEV matches the AddRec start.

This is done after the existing check, which can help to catch cases where the expression gets simplified by SCEV to either an IR constant or existing value which can be used instead.

---
Full diff: https://github.com/llvm/llvm-project/pull/68032.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/Scalar/ConstraintElimination.cpp (+6-4) 
- (modified) llvm/test/Transforms/ConstraintElimination/monotonic-int-phis-nested-loops.ll (+1-2) 


``````````diff
diff --git a/llvm/lib/Transforms/Scalar/ConstraintElimination.cpp b/llvm/lib/Transforms/Scalar/ConstraintElimination.cpp
index bca37da5ff10f2b..312e9f19b116940 100644
--- a/llvm/lib/Transforms/Scalar/ConstraintElimination.cpp
+++ b/llvm/lib/Transforms/Scalar/ConstraintElimination.cpp
@@ -859,7 +859,8 @@ void State::addInfoForInductions(BasicBlock &BB) {
     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();
@@ -868,9 +869,10 @@ void State::addInfoForInductions(BasicBlock &BB) {
     StartValue = C->getValue();
   else if (auto *U = dyn_cast<SCEVUnknown>(StartSCEV))
     StartValue = U->getValue();
-
-  if (!StartValue)
-    return;
+  else {
+    StartValue = PN->getIncomingValueForBlock(LoopPred);
+    assert(SE.getSCEV(StartValue) == AR->getStart() && "inconsistent start value);
+  }
 
   DomTreeNode *DTN = DT.getNode(InLoopSucc);
   auto Inc = SE.getMonotonicPredicateType(AR, CmpInst::ICMP_UGT);
diff --git a/llvm/test/Transforms/ConstraintElimination/monotonic-int-phis-nested-loops.ll b/llvm/test/Transforms/ConstraintElimination/monotonic-int-phis-nested-loops.ll
index 4835d48c57b6327..e5d101f7fdea106 100644
--- a/llvm/test/Transforms/ConstraintElimination/monotonic-int-phis-nested-loops.ll
+++ b/llvm/test/Transforms/ConstraintElimination/monotonic-int-phis-nested-loops.ll
@@ -17,8 +17,7 @@ define void @start_value_of_inner_add_rec_is_add_rec_condition_can_be_simplified
 ; CHECK-NEXT:    [[CMP2_NOT:%.*]] = icmp eq i32 [[K_0]], [[LEN]]
 ; 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_INC]] = add i32 [[K_0]], 1
 ; CHECK-NEXT:    br label [[INNER_HEADER]]
 ; CHECK:       outer.latch:

``````````

</details>


https://github.com/llvm/llvm-project/pull/68032