[LoopInterchange] Check lcssa phis in the inner latch in scenarios of multi-level...

We already know that we need to check whether lcssa
phis are supported in inner loop exit block or in
outer loop exit block, and we have logic to check
them already. Presumably the inner loop latch does
not have lcssa phis and there is no code that deals
with lcssa phis in the inner loop latch. However,
that assumption is not true, when we have loops
with more than two-level nesting. This patch adds
checks for lcssa phis in the inner latch.

Reviewed By: Whitney

Differential Revision: https://reviews.llvm.org/D102300

diff --git a/llvm/lib/Transforms/Scalar/LoopInterchange.cpp b/llvm/lib/Transforms/Scalar/LoopInterchange.cpp
index 745bc0e..34545f3 100644 (file)
--- a/llvm/lib/Transforms/Scalar/LoopInterchange.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopInterchange.cpp
@@ -1022,6 +1022,40 @@ static bool areOuterLoopExitPHIsSupported(Loop *OuterLoop, Loop *InnerLoop) {
   return true;
 }
 
+// In case of multi-level nested loops, it may occur that lcssa phis exist in
+// the latch of InnerLoop, i.e., when defs of the incoming values are further
+// inside the loopnest. Sometimes those incoming values are not available
+// after interchange, since the original inner latch will become the new outer
+// latch which may have predecessor paths that do not include those incoming
+// values.
+// TODO: Handle transformation of lcssa phis in the InnerLoop latch in case of
+// multi-level loop nests.
+static bool areInnerLoopLatchPHIsSupported(Loop *OuterLoop, Loop *InnerLoop) {
+  if (InnerLoop->getSubLoops().empty())
+    return true;
+  // If the original outer latch has only one predecessor, then values defined
+  // further inside the looploop, e.g., in the innermost loop, will be available
+  // at the new outer latch after interchange.
+  if (OuterLoop->getLoopLatch()->getUniquePredecessor() != nullptr)
+    return true;
+
+  // The outer latch has more than one predecessors, i.e., the inner
+  // exit and the inner header.
+  // PHI nodes in the inner latch are lcssa phis where the incoming values
+  // are defined further inside the loopnest. Check if those phis are used
+  // in the original inner latch. If that is the case then bail out since
+  // those incoming values may not be available at the new outer latch.
+  BasicBlock *InnerLoopLatch = InnerLoop->getLoopLatch();
+  for (PHINode &PHI : InnerLoopLatch->phis()) {
+    for (auto *U : PHI.users()) {
+      Instruction *UI = cast<Instruction>(U);
+      if (InnerLoopLatch == UI->getParent())
+        return false;
+    }
+  }
+  return true;
+}
+
 bool LoopInterchangeLegality::canInterchangeLoops(unsigned InnerLoopId,
                                                   unsigned OuterLoopId,
                                                   CharMatrix &DepMatrix) {
@@ -1057,6 +1091,18 @@ bool LoopInterchangeLegality::canInterchangeLoops(unsigned InnerLoopId,
         return false;
       }
 
+  if (!areInnerLoopLatchPHIsSupported(OuterLoop, InnerLoop)) {
+    LLVM_DEBUG(dbgs() << "Found unsupported PHI nodes in inner loop latch.\n");
+    ORE->emit([&]() {
+      return OptimizationRemarkMissed(DEBUG_TYPE, "UnsupportedInnerLatchPHI",
+                                      InnerLoop->getStartLoc(),
+                                      InnerLoop->getHeader())
+             << "Cannot interchange loops because unsupported PHI nodes found "
+                "in inner loop latch.";
+    });
+    return false;
+  }
+
   // TODO: The loops could not be interchanged due to current limitations in the
   // transform module.
   if (currentLimitations()) {

diff --git a/llvm/test/Transforms/LoopInterchange/innermost-latch-uses-values-in-middle-header.ll b/llvm/test/Transforms/LoopInterchange/innermost-latch-uses-values-in-middle-header.ll
new file mode 100644 (file)
index 0000000..65f1290
--- /dev/null
+++ b/llvm/test/Transforms/LoopInterchange/innermost-latch-uses-values-in-middle-header.ll
@@ -0,0 +1,58 @@
+; REQUIRES: asserts
+; RUN: opt < %s -basic-aa -loop-interchange -verify-dom-info -verify-loop-info \
+; RUN:     -S -debug 2>&1 | FileCheck %s
+
+@a = common global i32 0, align 4
+@d = common dso_local local_unnamed_addr global [1 x [6 x i32]] zeroinitializer, align 4
+
+;; After interchanging the innermost and the middle loop, we should not continue
+;; doing interchange for the (new) middle loop and the outermost loop, because of
+;; values defined in the new innermost loop not available in the exiting block of
+;; the entire loop nest.
+; CHECK: Loops are legal to interchange
+; CHECK: Loops interchanged.
+; CHECK: Found unsupported PHI nodes in inner loop latch.
+; CHECK: Not interchanging loops. Cannot prove legality.
+define void @innermost_latch_uses_values_in_middle_header() {
+entry:
+  %0 = load i32, i32* @a, align 4
+  %b = add i32 80, 1
+  br label %outermost.header
+
+outermost.header:                      ; preds = %outermost.latch, %entry
+  %indvar.outermost = phi i32 [ 10, %entry ], [ %indvar.outermost.next, %outermost.latch ]
+  %tobool71.i = icmp eq i32 %0, 0
+  br i1 %tobool71.i, label %middle.header, label %outermost.latch
+
+middle.header:                            ; preds = %middle.latch, %outermost.header
+  %indvar.middle = phi i64 [ 4, %outermost.header ], [ %indvar.middle.next, %middle.latch ]
+  %indvar.middle.wide = zext i32 %b to i64 ; a def in the middle header
+  br label %innermost.header
+
+innermost.header:                                         ; preds = %middle.header, %innermost.latch
+  %indvar.innermost = phi i64 [ %indvar.innermost.next, %innermost.latch ], [ 4, %middle.header ]
+  br label %innermost.body
+
+innermost.body:                                      ; preds = %innermost.header
+  %arrayidx9.i = getelementptr inbounds [1 x [6 x i32]], [1 x [6 x i32]]* @d, i64 0, i64 %indvar.innermost, i64 %indvar.middle
+  store i32 0, i32* %arrayidx9.i, align 4
+  br label %innermost.latch
+
+innermost.latch:                             ; preds = %innermost.body
+  %indvar.innermost.next = add nsw i64 %indvar.innermost, 1
+  %tobool5.i = icmp eq i64 %indvar.innermost.next, %indvar.middle.wide ; corresponding use in the innermost latch
+  br i1 %tobool5.i, label %middle.latch, label %innermost.header
+
+middle.latch:                                      ; preds = %innermost.latch
+  %indvar.middle.next = add nsw i64 %indvar.middle, -1
+  %tobool2.i = icmp eq i64 %indvar.middle.next, 0
+  br i1 %tobool2.i, label %outermost.latch, label %middle.header
+
+outermost.latch:                                      ; preds = %middle.latch, %outermost.header
+  %indvar.outermost.next = add nsw i32 %indvar.outermost, -5
+  %tobool.i = icmp eq i32 %indvar.outermost.next, 0
+  br i1 %tobool.i, label %outermost.exit, label %outermost.header
+
+outermost.exit:                                           ; preds = %outermost.latch
+  ret void
+}