[LoopInterchange] Also look at lcssa phis in outer loop latch block

llvm/lib/Transforms/Scalar/LoopInterchange.cpp

@@ -44,6 +44,7 @@
 #include "llvm/Transforms/Scalar/LoopPassManager.h"
 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
 #include "llvm/Transforms/Utils/LoopUtils.h"
+#include "llvm/Transforms/Utils/Local.h"
 #include <cassert>
 #include <utility>
 #include <vector>
@@ -1872,6 +1873,50 @@ static void moveLCSSAPhis(BasicBlock *InnerExit, BasicBlock *InnerHeader,
   InnerLatch->replacePhiUsesWith(InnerLatch, OuterLatch);
 }
 
+// This deals with a corner case when a LCSSA phi node appears in a non-exit
+// block: the outer loop latch block does not need to be exit block of the
+// inner loop. Consider a loop that was in LCSSA form, but then some
+// transformation like loop-unswitch comes along and creates an empty block,
+// where BB5 in this example is the outer loop latch block:
+//
+//   BB4:
+//     br label %BB5
+//   BB5:
+//     %old.cond.lcssa = phi i16 [ %cond, %BB4 ]
+//     br outer.header
+//
+// Interchange then brings it in LCSSA form again resulting in this chain of
+// single-input phi nodes:
+//
+//   BB4:
+//     %new.cond.lcssa = phi i16 [ %cond, %BB3 ]
+//     br label %BB5
+//   BB5:
+//     %old.cond.lcssa = phi i16 [ %new.cond.lcssa, %BB4 ]
+//
+// The problem is that interchange can reoder blocks BB4 and BB5 placing the
+// use before the def if we don't check this.
+//
+static void simplifyLCSSAPhis(Loop *OuterLoop, Loop *InnerLoop) {
+  BasicBlock *InnerLoopExit = InnerLoop->getExitBlock();
+  BasicBlock *OuterLoopLatch = OuterLoop->getLoopLatch();
+
+  // Do not modify lcssa phis where they actually belong, i.e. in exit blocks.
+  if (OuterLoopLatch == InnerLoopExit)
+    return;
+
+  // Collect and remove phis in non-exit blocks if they have 1 input.
+  SmallVector<PHINode *, 8> Phis(
+      llvm::make_pointer_range(OuterLoopLatch->phis()));
+  for (PHINode *Phi : Phis) {
+    assert(Phi->getNumIncomingValues() == 1 && "Single input phi expected");
+    LLVM_DEBUG(dbgs() << "Removing 1-input phi in non-exit block: " << *Phi
+                      << "\n");
+    Phi->replaceAllUsesWith(Phi->getIncomingValue(0));
+    Phi->eraseFromParent();
+  }
+}
+
 bool LoopInterchangeTransform::adjustLoopBranches() {
   LLVM_DEBUG(dbgs() << "adjustLoopBranches called\n");
   std::vector<DominatorTree::UpdateType> DTUpdates;
@@ -1882,6 +1927,9 @@ bool LoopInterchangeTransform::adjustLoopBranches() {
   assert(OuterLoopPreHeader != OuterLoop->getHeader() &&
          InnerLoopPreHeader != InnerLoop->getHeader() && OuterLoopPreHeader &&
          InnerLoopPreHeader && "Guaranteed by loop-simplify form");
+
+  simplifyLCSSAPhis(OuterLoop, InnerLoop);
+
   // Ensure that both preheaders do not contain PHI nodes and have single
   // predecessors. This allows us to move them easily. We use
   // InsertPreHeaderForLoop to create an 'extra' preheader, if the existing

llvm/test/Transforms/LoopInterchange/lcssa-phi-outer-latch.ll

@@ -0,0 +1,78 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 6
+; RUN: opt < %s -passes=loop-interchange -cache-line-size=64 -verify-dom-info -verify-loop-info -verify-scev -verify-loop-lcssa -S | FileCheck %s
+
+; This test is checking that blocks outer.body and outer.latch, where outer.body is the exit
+; block of the inner loop and outer.latch the latch of the outer loop, correctly
+; deal with the phi-node use-def chain %new.cond.lcssa -> %old.cond.lcssa. What we expect
+; here is that block outer.latch does not contain a phi node, because it is a single input
+; phi in a non-exit block.
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+
+define i16 @main() {
+; CHECK-LABEL: define i16 @main() {
+; CHECK-NEXT:  [[ENTRY:.*:]]
+; CHECK-NEXT:    br label %[[INNER_HEADER_PREHEADER:.*]]
+; CHECK:       [[OUTER_HEADER_PREHEADER:.*]]:
+; CHECK-NEXT:    br label %[[OUTER_HEADER:.*]]
+; CHECK:       [[OUTER_HEADER]]:
+; CHECK-NEXT:    [[I:%.*]] = phi i64 [ [[I_NEXT:%.*]], %[[OUTER_LATCH:.*]] ], [ 1, %[[OUTER_HEADER_PREHEADER]] ]
+; CHECK-NEXT:    br label %[[INNER_HEADER_SPLIT:.*]]
+; CHECK:       [[INNER_HEADER_PREHEADER]]:
+; CHECK-NEXT:    br label %[[INNER_HEADER:.*]]
+; CHECK:       [[INNER_HEADER]]:
+; CHECK-NEXT:    [[J:%.*]] = phi i16 [ [[TMP1:%.*]], %[[INNER_LATCH_SPLIT:.*]] ], [ 0, %[[INNER_HEADER_PREHEADER]] ]
+; CHECK-NEXT:    br label %[[OUTER_HEADER_PREHEADER]]
+; CHECK:       [[INNER_HEADER_SPLIT]]:
+; CHECK-NEXT:    [[ARRAYIDX_US_US:%.*]] = getelementptr i16, ptr null, i16 [[J]]
+; CHECK-NEXT:    [[TMP0:%.*]] = load i16, ptr [[ARRAYIDX_US_US]], align 1
+; CHECK-NEXT:    [[COND:%.*]] = select i1 false, i16 0, i16 0
+; CHECK-NEXT:    br label %[[INNER_LATCH:.*]]
+; CHECK:       [[INNER_LATCH]]:
+; CHECK-NEXT:    [[J_NEXT:%.*]] = add i16 [[J]], 1
+; CHECK-NEXT:    br label %[[OUTER_BODY:.*]]
+; CHECK:       [[INNER_LATCH_SPLIT]]:
+; CHECK-NEXT:    [[NEW_COND_LCSSA:%.*]] = phi i16 [ [[COND]], %[[OUTER_LATCH]] ]
+; CHECK-NEXT:    [[TMP1]] = add i16 [[J]], 1
+; CHECK-NEXT:    br i1 true, label %[[EXIT:.*]], label %[[INNER_HEADER]]
+; CHECK:       [[OUTER_BODY]]:
+; CHECK-NEXT:    br label %[[OUTER_LATCH]]
+; CHECK:       [[OUTER_LATCH]]:
+; CHECK-NEXT:    [[I_NEXT]] = add i64 [[I]], 1
+; CHECK-NEXT:    [[CMP286_US:%.*]] = icmp ugt i64 [[I]], 0
+; CHECK-NEXT:    br i1 [[CMP286_US]], label %[[OUTER_HEADER]], label %[[INNER_LATCH_SPLIT]]
+; CHECK:       [[EXIT]]:
+; CHECK-NEXT:    [[OLD_COND_LCSSA_LCSSA:%.*]] = phi i16 [ [[NEW_COND_LCSSA]], %[[INNER_LATCH_SPLIT]] ]
+; CHECK-NEXT:    ret i16 [[OLD_COND_LCSSA_LCSSA]]
+;
+entry:
+  br label %outer.header
+
+outer.header:
+  %i = phi i64 [ 1, %entry ], [ %i.next, %outer.latch ]
+  br label %inner.header
+
+inner.header:
+  %j = phi i16 [ 0, %outer.header ], [ %j.next, %inner.latch ]
+  %arrayidx.us.us = getelementptr i16, ptr null, i16 %j
+  %0 = load i16, ptr %arrayidx.us.us, align 1
+  %cond = select i1 false, i16 0, i16 0
+  br label %inner.latch
+
+inner.latch:
+  %j.next = add i16 %j, 1
+  br i1 true, label %outer.body, label %inner.header
+
+outer.body:
+  %new.cond.lcssa = phi i16 [ %cond, %inner.latch ]
+  br label %outer.latch
+
+outer.latch:
+  %old.cond.lcssa = phi i16 [ %new.cond.lcssa, %outer.body ]
+  %i.next = add i64 %i, 1
+  %cmp286.us = icmp ugt i64 %i, 0
+  br i1 %cmp286.us, label %outer.header, label %exit
+
+exit:
+  ret i16 %old.cond.lcssa
+}