[LoopInterchange] Add simplifyLCSSAPhis: remove phi from non-exit bb

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>
@@ -1837,6 +1838,38 @@ static void moveLCSSAPhis(BasicBlock *InnerExit, BasicBlock *InnerHeader,
   for (PHINode *P : LcssaInnerLatch)
     P->moveBefore(InnerExit->getFirstNonPHIIt());
 
+  // This deals with a corner case of LCSSA phi nodes in the outer loop latch
+  // block: the loop was in LCSSA form, some transformations can come along
+  // (e.g. unswitch) and create an empty block:
+  //
+  //   BB4:
+  //     br label %BB5
+  //   BB5:
+  //     %old.cond.lcssa = phi i16 [ %cond, %BB4 ]
+  //     br outer.header
+  //
+  // Interchange then brings it in LCSSA form again and we get:
+  //
+  //   BB4:
+  //     %new.cond.lcssa = phi i16 [ %cond, %BB3 ]
+  //     br label %BB5
+  //   BB5:
+  //     %old.cond.lcssa = phi i16 [ %new.cond.lcssa, %BB4 ]
+  //
+  // Which means that we have a chain of LCSSA phi nodes from %new.cond.lcssa
+  // to %old.cond.lcssa. The problem is that interchange can reoder blocks BB4
+  // and BB5 placing the use before the def if we don't check this. The
+  // observation is that %old.cond.lcssa is unused, so instead of moving and
+  // renaming these phi nodes, just delete it if it's trivially dead. If it
+  // isn't trivially dead, it is handled above. The loop should still be in
+  // LCSSA form, and if it isn't, formLCSSARecursively is called after the
+  // interchange rewrite.
+  SmallVector<PHINode *, 8> LcssaOuterLatch(
+      llvm::make_pointer_range(OuterLatch->phis()));
+  for (PHINode *P : LcssaOuterLatch)
+     if (isInstructionTriviallyDead(P))
+       P->eraseFromParent();
+
   // Deal with LCSSA PHI nodes in the loop nest exit block. For PHIs that have
   // incoming values defined in the outer loop, we have to add a new PHI
   // in the inner loop latch, which became the exit block of the outer loop,

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

@@ -0,0 +1,75 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --prefix-filecheck-ir-name TEST --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 BB4 and BB5, where BB4 is the exit
+; block of the inner loop and BB5 the latch of the outer loop, correctly
+; deal with the phi-node use-def chain %new.cond.lcssa -> %old.cond.lcssa.
+
+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 %[[BB2_PREHEADER:.*]]
+; CHECK:       [[BB1_PREHEADER:.*]]:
+; CHECK-NEXT:    br label %[[TESTBB1:.*]]
+; CHECK:       [[TESTBB1]]:
+; CHECK-NEXT:    [[I:%.*]] = phi i64 [ [[I_NEXT:%.*]], %[[BB5:.*]] ], [ 1, %[[BB1_PREHEADER]] ]
+; CHECK-NEXT:    br label %[[BB2_SPLIT:.*]]
+; CHECK:       [[BB2_PREHEADER]]:
+; CHECK-NEXT:    br label %[[TESTBB2:.*]]
+; CHECK:       [[TESTBB2]]:
+; CHECK-NEXT:    [[J:%.*]] = phi i16 [ [[TMP1:%.*]], %[[BB3_SPLIT:.*]] ], [ 0, %[[BB2_PREHEADER]] ]
+; CHECK-NEXT:    br label %[[BB1_PREHEADER]]
+; CHECK:       [[BB2_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 %[[TESTBB3:.*]]
+; CHECK:       [[TESTBB3]]:
+; CHECK-NEXT:    [[J_NEXT:%.*]] = add i16 [[J]], 1
+; CHECK-NEXT:    br label %[[TESTBB4:.*]]
+; CHECK:       [[BB3_SPLIT]]:
+; CHECK-NEXT:    [[NEW_COND_LCSSA:%.*]] = phi i16 [ [[COND]], %[[BB5]] ]
+; CHECK-NEXT:    [[TMP1]] = add i16 [[J]], 1
+; CHECK-NEXT:    br i1 true, label %[[EXIT:.*]], label %[[TESTBB2]]
+; CHECK:       [[TESTBB4]]:
+; CHECK-NEXT:    br label %[[BB5]]
+; CHECK:       [[BB5]]:
+; CHECK-NEXT:    [[I_NEXT]] = add i64 [[I]], 1
+; CHECK-NEXT:    [[CMP286_US:%.*]] = icmp ugt i64 [[I]], 0
+; CHECK-NEXT:    br i1 [[CMP286_US]], label %[[TESTBB1]], label %[[BB3_SPLIT]]
+; CHECK:       [[EXIT]]:
+; CHECK-NEXT:    ret i16 0
+;
+entry:
+  br label %BB1
+
+BB1:
+  %i = phi i64 [ 1, %entry ], [ %i.next, %BB5 ]
+  br label %BB2
+
+BB2:
+  %j = phi i16 [ 0, %BB1 ], [ %j.next, %BB3 ]
+  %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 %BB3
+
+BB3:
+  %j.next = add i16 %j, 1
+  br i1 true, label %BB4, label %BB2
+
+BB4:
+  %new.cond.lcssa = phi i16 [ %cond, %BB3 ]
+  br label %BB5
+
+BB5:
+  %old.cond.lcssa = phi i16 [ %new.cond.lcssa, %BB4 ]
+  %i.next = add i64 %i, 1
+  %cmp286.us = icmp ugt i64 %i, 0
+  br i1 %cmp286.us, label %BB1, label %exit
+
+exit:
+  ret i16 0
+}