[llvm] [VPlan] Optimize FindLast of FindIV w/o sentinel. (PR #172569)

[Mel Chen via llvm-commits]

================
@@ -5502,36 +5503,78 @@ void VPlanTransforms::optimizeFindIVReductions(VPlan &Plan,
         CheckSentinel(IVSCEV, UseMax, /*IsSigned=*/true);
     if (!SentinelVal) {
       SentinelVal = CheckSentinel(IVSCEV, UseMax, /*IsSigned=*/false);
-      if (!SentinelVal)
-        continue;
       UseSigned = false;
     }
 
-    auto *RdxResult = cast<VPInstruction>(vputils::findRecipe(
+    // If no sentinel was found, fall back to a boolean AnyOf reduction to track
+    // if the condition was ever true. Requires the IV to not wrap, otherwise we
+    // cannot use min/max.
+    if (!SentinelVal) {
+      auto *AR = cast<SCEVAddRecExpr>(IVSCEV);
+      if (AR->hasNoSignedWrap())
+        UseSigned = true;
+      else if (AR->hasNoUnsignedWrap())
+        UseSigned = false;
+      else
+        continue;
+    }
+
+    VPInstruction *RdxResult = cast<VPInstruction>(vputils::findRecipe(
         BackedgeVal,
         match_fn(m_VPInstruction<VPInstruction::ComputeReductionResult>())));
 
-    // Create the reduction result in the middle block using sentinel directly.
     RecurKind MinMaxKind =
         UseMax ? (UseSigned ? RecurKind::SMax : RecurKind::UMax)
                : (UseSigned ? RecurKind::SMin : RecurKind::UMin);
     VPIRFlags Flags(MinMaxKind, /*IsOrdered=*/false, /*IsInLoop=*/false,
                     FastMathFlags());
-    VPValue *Sentinel = Plan.getConstantInt(*SentinelVal);
     DebugLoc ExitDL = RdxResult->getDebugLoc();
     VPBuilder MiddleBuilder(RdxResult);
     VPValue *ReducedIV =
         MiddleBuilder.createNaryOp(VPInstruction::ComputeReductionResult,
                                    RdxResult->getOperand(0), Flags, ExitDL);
-    auto *Cmp =
-        MiddleBuilder.createICmp(CmpInst::ICMP_NE, ReducedIV, Sentinel, ExitDL);
-    VPInstruction *NewRdxResult = MiddleBuilder.createSelect(
-        Cmp, ReducedIV, PhiR->getStartValue(), ExitDL);
+
+    VPInstruction *NewRdxResult;
+    VPValue *StartVPV = PhiR->getStartValue();
+    if (SentinelVal) {
+      // Sentinel-based approach: reduce IVs with min/max, compare against
+      // sentinel to detect if condition was ever true, select accordingly.
+      VPValue *Sentinel = Plan.getConstantInt(*SentinelVal);
+      auto *Cmp = MiddleBuilder.createICmp(CmpInst::ICMP_NE, ReducedIV,
+                                           Sentinel, ExitDL);
+      NewRdxResult = cast<VPInstruction>(
+          MiddleBuilder.createSelect(Cmp, ReducedIV, StartVPV, ExitDL));
----------------
Mel-Chen wrote:

```suggestion
    VPValue *NewRdxResult;
    VPValue *StartVPV = PhiR->getStartValue();
    if (SentinelVal) {
      // Sentinel-based approach: reduce IVs with min/max, compare against
      // sentinel to detect if condition was ever true, select accordingly.
      VPValue *Sentinel = Plan.getConstantInt(*SentinelVal);
      auto *Cmp = MiddleBuilder.createICmp(CmpInst::ICMP_NE, ReducedIV,
                                           Sentinel, ExitDL);
      NewRdxResult = 
          MiddleBuilder.createSelect(Cmp, ReducedIV, StartVPV, ExitDL);
```

https://github.com/llvm/llvm-project/pull/172569