[indvars] Always fallback to truncation if AddRec widening fails (#70967)

The current code structure results in cases where if a) we can't clone
the IV user (because it's not in our whitelist) or b) can't prove the
SCEV expressions are identical, we'd sometimes leave both the original
unwiddened IV and the partially widdened IV in code. Instead, just
truncate thw wide IV to the use - same as what we'd do if we couldn't
find an addrec to start with.

Noticed this while playing with changing how we produce addrecs. The
current structure results in a very tight interlock between SCEVs
internal capabilities and indvars code.

1 files changed, 56 insertions, 51 deletions

diff --git a/llvm/lib/Transforms/Utils/SimplifyIndVar.cpp b/llvm/lib/Transforms/Utils/SimplifyIndVar.cpp
index f325698..0b40d44 100644
--- a/llvm/lib/Transforms/Utils/SimplifyIndVar.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyIndVar.cpp
@@ -1805,65 +1805,70 @@ Instruction *WidenIV::widenIVUse(WidenIV::NarrowIVDefUse DU, SCEVExpander &Rewri
     return nullptr;
   }
 
-  // Does this user itself evaluate to a recurrence after widening?
-  WidenedRecTy WideAddRec = getExtendedOperandRecurrence(DU);
-  if (!WideAddRec.first)
-    WideAddRec = getWideRecurrence(DU);
-
-  assert((WideAddRec.first == nullptr) ==
-         (WideAddRec.second == ExtendKind::Unknown));
-  if (!WideAddRec.first) {
-    // If use is a loop condition, try to promote the condition instead of
-    // truncating the IV first.
-    if (widenLoopCompare(DU))
+  auto tryAddRecExpansion = [&]() -> Instruction* {
+    // Does this user itself evaluate to a recurrence after widening?
+    WidenedRecTy WideAddRec = getExtendedOperandRecurrence(DU);
+    if (!WideAddRec.first)
+      WideAddRec = getWideRecurrence(DU);
+    assert((WideAddRec.first == nullptr) ==
+           (WideAddRec.second == ExtendKind::Unknown));
+    if (!WideAddRec.first)
       return nullptr;
 
-    // We are here about to generate a truncate instruction that may hurt
-    // performance because the scalar evolution expression computed earlier
-    // in WideAddRec.first does not indicate a polynomial induction expression.
-    // In that case, look at the operands of the use instruction to determine
-    // if we can still widen the use instead of truncating its operand.
-    if (widenWithVariantUse(DU))
+    // Reuse the IV increment that SCEVExpander created as long as it dominates
+    // NarrowUse.
+    Instruction *WideUse = nullptr;
+    if (WideAddRec.first == WideIncExpr &&
+        Rewriter.hoistIVInc(WideInc, DU.NarrowUse))
+      WideUse = WideInc;
+    else {
+      WideUse = cloneIVUser(DU, WideAddRec.first);
+      if (!WideUse)
+        return nullptr;
+    }
+    // Evaluation of WideAddRec ensured that the narrow expression could be
+    // extended outside the loop without overflow. This suggests that the wide use
+    // evaluates to the same expression as the extended narrow use, but doesn't
+    // absolutely guarantee it. Hence the following failsafe check. In rare cases
+    // where it fails, we simply throw away the newly created wide use.
+    if (WideAddRec.first != SE->getSCEV(WideUse)) {
+      LLVM_DEBUG(dbgs() << "Wide use expression mismatch: " << *WideUse << ": "
+                 << *SE->getSCEV(WideUse) << " != " << *WideAddRec.first
+                 << "\n");
+      DeadInsts.emplace_back(WideUse);
       return nullptr;
+    };
 
-    // This user does not evaluate to a recurrence after widening, so don't
-    // follow it. Instead insert a Trunc to kill off the original use,
-    // eventually isolating the original narrow IV so it can be removed.
-    truncateIVUse(DU, DT, LI);
-    return nullptr;
-  }
+    // if we reached this point then we are going to replace
+    // DU.NarrowUse with WideUse. Reattach DbgValue then.
+    replaceAllDbgUsesWith(*DU.NarrowUse, *WideUse, *WideUse, *DT);
 
-  // Reuse the IV increment that SCEVExpander created as long as it dominates
-  // NarrowUse.
-  Instruction *WideUse = nullptr;
-  if (WideAddRec.first == WideIncExpr &&
-      Rewriter.hoistIVInc(WideInc, DU.NarrowUse))
-    WideUse = WideInc;
-  else {
-    WideUse = cloneIVUser(DU, WideAddRec.first);
-    if (!WideUse)
-      return nullptr;
-  }
-  // Evaluation of WideAddRec ensured that the narrow expression could be
-  // extended outside the loop without overflow. This suggests that the wide use
-  // evaluates to the same expression as the extended narrow use, but doesn't
-  // absolutely guarantee it. Hence the following failsafe check. In rare cases
-  // where it fails, we simply throw away the newly created wide use.
-  if (WideAddRec.first != SE->getSCEV(WideUse)) {
-    LLVM_DEBUG(dbgs() << "Wide use expression mismatch: " << *WideUse << ": "
-                      << *SE->getSCEV(WideUse) << " != " << *WideAddRec.first
-                      << "\n");
-    DeadInsts.emplace_back(WideUse);
+    ExtendKindMap[DU.NarrowUse] = WideAddRec.second;
+    // Returning WideUse pushes it on the worklist.
+    return WideUse;
+  };
+
+  if (auto *I = tryAddRecExpansion())
+    return I;
+
+  // If use is a loop condition, try to promote the condition instead of
+  // truncating the IV first.
+  if (widenLoopCompare(DU))
     return nullptr;
-  }
 
-  // if we reached this point then we are going to replace
-  // DU.NarrowUse with WideUse. Reattach DbgValue then.
-  replaceAllDbgUsesWith(*DU.NarrowUse, *WideUse, *WideUse, *DT);
+  // We are here about to generate a truncate instruction that may hurt
+  // performance because the scalar evolution expression computed earlier
+  // in WideAddRec.first does not indicate a polynomial induction expression.
+  // In that case, look at the operands of the use instruction to determine
+  // if we can still widen the use instead of truncating its operand.
+  if (widenWithVariantUse(DU))
+    return nullptr;
 
-  ExtendKindMap[DU.NarrowUse] = WideAddRec.second;
-  // Returning WideUse pushes it on the worklist.
-  return WideUse;
+  // This user does not evaluate to a recurrence after widening, so don't
+  // follow it. Instead insert a Trunc to kill off the original use,
+  // eventually isolating the original narrow IV so it can be removed.
+  truncateIVUse(DU, DT, LI);
+  return nullptr;
 }
 
 /// Add eligible users of NarrowDef to NarrowIVUsers.