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

[via llvm-commits]

================
@@ -3469,78 +3469,18 @@ void InnerLoopVectorizer::fixVectorizedLoop(VPTransformState &State,
                                VF.getKnownMinValue() * UF);
 }
 
-void InnerLoopVectorizer::fixFixedOrderRecurrence(
-    VPFirstOrderRecurrencePHIRecipe *PhiR, VPTransformState &State) {
-  // 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.
-
+void InnerLoopVectorizer::fixFixedOrderRecurrence(VPLiveOut *LO,
+                                                  VPTransformState &State) {
   // Extract the last vector element in the middle block. This will be the
   // initial value for the recurrence when jumping to the scalar loop.
-  VPValue *PreviousDef = PhiR->getBackedgeValue();
-  Value *Incoming = State.get(PreviousDef, UF - 1);
-  auto *ExtractForScalar = Incoming;
-  auto *IdxTy = Builder.getInt32Ty();
-  Value *RuntimeVF = nullptr;
-  if (VF.isVector()) {
-    auto *One = ConstantInt::get(IdxTy, 1);
-    Builder.SetInsertPoint(LoopMiddleBlock->getTerminator());
-    RuntimeVF = getRuntimeVF(Builder, IdxTy, VF);
-    auto *LastIdx = Builder.CreateSub(RuntimeVF, One);
-    ExtractForScalar =
-        Builder.CreateExtractElement(Incoming, LastIdx, "vector.recur.extract");
-  }
+  Value *ExtractForScalar = State.get(LO->getOperand(0), UF - 1, true);
 
   // Fix the initial value of the original recurrence in the scalar loop.
   Builder.SetInsertPoint(LoopScalarPreHeader, LoopScalarPreHeader->begin());
-  PHINode *Phi = cast<PHINode>(PhiR->getUnderlyingValue());
+  PHINode *Phi = LO->getPhi();
   auto *Start = Builder.CreatePHI(Phi->getType(), 2, "scalar.recur.init");
-  auto *ScalarInit = PhiR->getStartValue()->getLiveInIRValue();
+  auto *ScalarInit =
+      LO->getPhi()->getIncomingValueForBlock(LoopScalarPreHeader);
----------------
ayalz wrote:

```suggestion
  auto *ScalarInit = Phi->getIncomingValueForBlock(LoopScalarPreHeader);
```

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