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

[Florian Hahn via llvm-commits]

================
@@ -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];
+    //
+    // 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, ...
+    //
----------------
fhahn wrote:

Added, thanks!

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