[llvm] [VPlan] Model FOR resume value extraction in VPlan. (PR #93396)

[Florian Hahn via llvm-commits]

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@@ -847,14 +847,84 @@ bool VPlanTransforms::adjustFixedOrderRecurrences(VPlan &Plan,
     // all users.
     RecurSplice->setOperand(0, FOR);
 
+    // This is the second phase of vectorizing first-order recurrences. An
+    // overview of the transformation is described below. Suppose we have the
+    // following loop.
+    //
+    //   for (int i = 0; i < n; ++i)
+    //     b[i] = a[i] - a[i - 1];
+    //
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fhahn wrote:

Merged with the example loop, thanks!

https://github.com/llvm/llvm-project/pull/93396