[LV] Fix the vector code generation for first order recurrence

Summary:
In first order recurrences where phi's are used outside the loop,
we should generate an additional vector.extract of the second last element from
the vectorized phi update.
This is because we require the phi itself (which is the value at the second last
iteration of the vector loop) and not the phi's update within the loop.
Also fix the code gen when we just unroll, but don't vectorize.
Fixes PR32396.

Reviewers: mssimpso, mkuper, anemet

Subscribers: llvm-commits, mzolotukhin

Differential Revision: https://reviews.llvm.org/D31979

llvm-svn: 300238

1 files changed, 3 insertions, 12 deletions

diff --git a/llvm/lib/Transforms/Utils/LoopUtils.cpp b/llvm/lib/Transforms/Utils/LoopUtils.cpp
index 444bc16..175d013a 100644
--- a/llvm/lib/Transforms/Utils/LoopUtils.cpp
+++ b/llvm/lib/Transforms/Utils/LoopUtils.cpp
@@ -553,22 +553,13 @@ bool RecurrenceDescriptor::isFirstOrderRecurrence(PHINode *Phi, Loop *TheLoop,
   if (!Previous || !TheLoop->contains(Previous) || isa<PHINode>(Previous))
     return false;
 
+  // Ensure every user of the phi node is dominated by the previous value.
+  // The dominance requirement ensures the loop vectorizer will not need to
+  // vectorize the initial value prior to the first iteration of the loop.
   for (User *U : Phi->users())
     if (auto *I = dyn_cast<Instruction>(U)) {
-      // Ensure every user of the phi node is dominated by the previous value.
-      // The dominance requirement ensures the loop vectorizer will not need to
-      // vectorize the initial value prior to the first iteration of the loop.
       if (!DT->dominates(Previous, I))
         return false;
-      // When the phi node has users outside the loop, the current logic for
-      // fixFirstOrderRecurrences may generate incorrect code. Specifically, we
-      // extract the last element from the vectorized phi, which would be the
-      // update to the phi before exiting the loop. However, what we want is the
-      // previous phi value before the update (i.e. the second last update
-      // before end of the vectorized loop).
-      // See added test cases in first-order-recurrence.ll
-      if (!TheLoop->contains(I))
-        return false;
     }
 
   return true;