[llvm] [LoopVectorize] Vectorize select-cmp reduction pattern for increasing integer induction variable (PR #67812)

[Mel Chen via llvm-commits]

================
@@ -658,6 +660,93 @@ RecurrenceDescriptor::isAnyOfPattern(Loop *Loop, PHINode *OrigPhi,
                                                      : RecurKind::FAnyOf);
 }
 
+// We are looking for loops that do something like this:
+//   int r = 0;
+//   for (int i = 0; i < n; i++) {
+//     if (src[i] > 3)
+//       r = i;
+//   }
+// The reduction value (r) is derived from either the values of an increasing
+// induction variable (i) sequence, or from the start value (0).
+// The LLVM IR generated for such loops would be as follows:
+//   for.body:
+//     %r = phi i32 [ %spec.select, %for.body ], [ 0, %entry ]
+//     %i = phi i32 [ %inc, %for.body ], [ 0, %entry ]
+//     ...
+//     %cmp = icmp sgt i32 %5, 3
+//     %spec.select = select i1 %cmp, i32 %i, i32 %r
+//     %inc = add nsw i32 %i, 1
+//     ...
+// Since 'i' is an increasing induction variable, the reduction value after the
+// loop will be the maximum value of 'i' that the condition (src[i] > 3) is
+// satisfied, or the start value (0 in the example above). When the start value
+// of the increasing induction variable 'i' is greater than the minimum value of
+// the data type, we can use the minimum value of the data type as a sentinel
+// value to replace the start value. This allows us to perform a single
+// reduction max operation to obtain the final reduction result.
+// TODO: It is possible to solve the case where the start value is the minimum
+// value of the data type or a non-constant value by using mask and multiple
+// reduction operations.
+RecurrenceDescriptor::InstDesc
+RecurrenceDescriptor::isFindLastIVPattern(PHINode *OrigPhi, Instruction *I,
+                                          ScalarEvolution &SE) {
+  // TODO: Support the vectorization of FindLastIV when the reduction phi is
+  // used by more than one select instruction. This vectorization is only
+  // performed when the SCEV of each increasing induction variable used by the
+  // select instructions is identical.
+  if (!OrigPhi->hasOneUse())
+    return InstDesc(false, I);
+
+  // TODO: Match selects with multi-use cmp conditions.
+  Value *NonRdxPhi = nullptr;
+  if (!match(I, m_CombineOr(m_Select(m_OneUse(m_Cmp()), m_Value(NonRdxPhi),
+                                     m_Specific(OrigPhi)),
+                            m_Select(m_OneUse(m_Cmp()), m_Specific(OrigPhi),
+                                     m_Value(NonRdxPhi)))))
+    return InstDesc(false, I);
+
+  auto IsIncreasingLoopInduction = [&](Value *V) {
+    Type *Ty = V->getType();
+    if (!SE.isSCEVable(Ty))
+      return false;
+
+    auto *AR = dyn_cast<SCEVAddRecExpr>(SE.getSCEV(V));
+    if (!AR)
----------------
Mel-Chen wrote:

You’ve raised a great point! 
Although this issue hasn’t surfaced due to the recognition order of AnyOf and FindLastIV, I agree that adding a loop check is safer.
ee12efbfe6987ff1ad5de829f3c695e7fa70ebad
And please take a look the pre-commit test for nested loop: #118519

https://github.com/llvm/llvm-project/pull/67812