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

[via llvm-commits]

================
@@ -847,6 +847,55 @@ 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];
+    //
+    // There is a first-order recurrence on "a". For this loop, the shorthand
+    // scalar IR looks like:
+    //
+    //   scalar.ph:
+    //     s_init = a[-1]
+    //     br scalar.body
+    //
+    //   scalar.body:
+    //     i = phi [0, scalar.ph], [i+1, scalar.body]
+    //     s1 = phi [s_init, scalar.ph], [s2, scalar.body]
+    //     s2 = a[i]
+    //     b[i] = s2 - s1
+    //     br cond, scalar.body, ...
+    //
+    // In this example, s1 is a recurrence because it's value depends on the
+    // previous iteration. In the first phase of vectorization, we created a
+    // vector phi v1 for s1. We now complete the vectorization and produce the
+    // shorthand vector IR shown below (for VF = 4, UF = 1).
+    //
+    //   vector.ph:
+    //     v_init = vector(..., ..., ..., a[-1])
+    //     br vector.body
+    //
+    //   vector.body
+    //     i = phi [0, vector.ph], [i+4, vector.body]
+    //     v1 = phi [v_init, vector.ph], [v2, vector.body]
+    //     v2 = a[i, i+1, i+2, i+3];
+    //     v3 = vector(v1(3), v2(0, 1, 2))
+    //     b[i, i+1, i+2, i+3] = v2 - v3
+    //     br cond, vector.body, middle.block
+    //
+    //   middle.block:
+    //     x = v2(3)
+    //     br scalar.ph
+    //
+    //   scalar.ph:
+    //     s_init = phi [x, middle.block], [a[-1], otherwise]
+    //     br scalar.body
+    //
+    // After execution completes the vector loop, we extract the next value of
+    // the recurrence (x) to use as the initial value in the scalar loop. This
+    // is modeled by ExtractRecurrenceResume.
----------------
ayalz wrote:

```suggestion
    // is modeled by ExtractFromEnd.
```

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