[llvm] [VPlan] Optimize FindLast of (binop %IV, live-in) by sinking. (PR #183911)

[via llvm-commits]

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@@ -5732,27 +5771,35 @@ void VPlanTransforms::optimizeFindIVReductions(VPlan &Plan,
     // If there's a header mask, the backedge select will not be the find-last
     // select.
     VPValue *BackedgeVal = PhiR->getBackedgeValue();
-    VPValue *CondSelect = BackedgeVal;
-    if (HeaderMask &&
-        !match(BackedgeVal, m_Select(m_Specific(HeaderMask),
-                                     m_VPValue(CondSelect), m_Specific(PhiR))))
+    VPValue *FindLastSelect = BackedgeVal;
+    if (HeaderMask && !match(BackedgeVal, m_Select(m_Specific(HeaderMask),
+                                                   m_VPValue(FindLastSelect),
+                                                   m_Specific(PhiR))))
       llvm_unreachable("expected header mask select");
 
-    // Get the IV from the conditional select of the reduction phi.
-    // The conditional select should be a select between the phi and the IV.
-    VPValue *Cond, *TrueVal, *FalseVal;
-    if (!match(CondSelect, m_Select(m_VPValue(Cond), m_VPValue(TrueVal),
-                                    m_VPValue(FalseVal))))
+    // The conditional select of the reduction phi should be a select between
+    // the phi and an IV-based expression. Get that expression.
+    VPValue *Cond, *FindLastExpression;
+    if (!match(FindLastSelect, m_Select(m_VPValue(Cond), m_Specific(PhiR),
+                                        m_VPValue(FindLastExpression))) &&
+        !match(FindLastSelect,
+               m_Select(m_VPValue(Cond), m_VPValue(FindLastExpression),
+                        m_Specific(PhiR))))
       continue;
 
-    // The non-phi operand of the select is the IV.
-    assert(is_contained(CondSelect->getDefiningRecipe()->operands(), PhiR));
-    VPValue *IV = TrueVal == PhiR ? FalseVal : TrueVal;
-
-    const SCEV *IVSCEV = vputils::getSCEVExprForVPValue(IV, PSE, &L);
+    // Check if FindLastExpression is a simple expression of a widened IV. If
+    // so, we can track the underlying IV and instead sink the expression.
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ayalz wrote:

```suggestion
    // so, we can track the underlying IV instead and sink the expression.
```

https://github.com/llvm/llvm-project/pull/183911