[LoopInterchange] Form LCSSA phis for values in orig outer loop header.

Values defined in the outer loop header could be used in the inner loop
latch. In that case, we need to create LCSSA phis for them, because after
interchanging they will be defined in the new inner loop and used in the
new outer loop.

Author: fhahn

llvm/lib/Transforms/Scalar/LoopInterchange.cpp

@@ -27,6 +27,7 @@
 #include "llvm/IR/DiagnosticInfo.h"
 #include "llvm/IR/Dominators.h"
 #include "llvm/IR/Function.h"
+#include "llvm/IR/IRBuilder.h"
 #include "llvm/IR/InstrTypes.h"
 #include "llvm/IR/Instruction.h"
 #include "llvm/IR/Instructions.h"
@@ -1594,6 +1595,17 @@ bool LoopInterchangeTransform::adjustLoopBranches() {
   InnerLoopHeader->replacePhiUsesWith(OuterLoopPreHeader, InnerLoopPreHeader);
   InnerLoopHeader->replacePhiUsesWith(OuterLoopLatch, InnerLoopLatch);
 
+  // Values defined in the outer loop header could be used in the inner loop
+  // latch. In that case, we need to create LCSSA phis for them, because after
+  // interchanging they will be defined in the new inner loop and used in the
+  // new outer loop.
+  IRBuilder<> Builder(OuterLoopHeader->getContext());
+  SmallVector<Instruction *, 4> MayNeedLCSSAPhis;
+  for (Instruction &I :
+       make_range(OuterLoopHeader->begin(), std::prev(OuterLoopHeader->end())))
+    MayNeedLCSSAPhis.push_back(&I);
+  formLCSSAForInstructions(MayNeedLCSSAPhis, *DT, *LI, SE, Builder);
+
   return true;
 }
 

llvm/test/Transforms/LoopInterchange/lcssa-preheader.ll

@@ -101,3 +101,65 @@ outer.crit_edge:              ; preds = %outer.latch
 for.cond.cleanup:                                 ; preds = %outer.crit_edge, %entry
   ret void
 }
+
+@global = external local_unnamed_addr global [4 x [4 x [2 x i16]]] align 16
+
+; %N.ext is defined in the outer loop header and used in the inner loop. After
+; interchanging, it will be defined in the new inner loop and used in the new;
+; outer latch, so we need to create a new LCSSA phi node for it.
+
+define void @test2(i32 %N) {
+; CHECK-LABEL: @test2(
+; CHECK-NEXT:  bb:
+; CHECK-NEXT:    br label [[INNER_PREHEADER:%.*]]
+; CHECK:       outer.header.preheader:
+; CHECK-NEXT:    br label [[OUTER_HEADER:%.*]]
+; CHECK:       outer.header:
+; CHECK-NEXT:    [[OUTER_IV:%.*]] = phi i64 [ [[OUTER_IV_NEXT:%.*]], [[OUTER_LATCH:%.*]] ], [ 0, [[OUTER_HEADER_PREHEADER:%.*]] ]
+; CHECK-NEXT:    [[N_EXT:%.*]] = sext i32 [[N:%.*]] to i64
+; CHECK-NEXT:    br label [[INNER_SPLIT1:%.*]]
+; CHECK:       inner.preheader:
+; CHECK-NEXT:    br label [[INNER:%.*]]
+; CHECK:       inner:
+; CHECK-NEXT:    [[INNER_IV:%.*]] = phi i64 [ [[TMP0:%.*]], [[INNER_SPLIT:%.*]] ], [ 0, [[INNER_PREHEADER]] ]
+; CHECK-NEXT:    br label [[OUTER_HEADER_PREHEADER]]
+; CHECK:       inner.split1:
+; CHECK-NEXT:    [[TMP8:%.*]] = getelementptr inbounds [4 x [4 x [2 x i16]]], [4 x [4 x [2 x i16]]]* @global, i64 0, i64 [[INNER_IV]], i64 [[OUTER_IV]], i64 0
+; CHECK-NEXT:    [[INNER_IV_NEXT:%.*]] = add nsw i64 [[INNER_IV]], 1
+; CHECK-NEXT:    [[C_1:%.*]] = icmp ne i64 [[INNER_IV_NEXT]], [[N_EXT]]
+; CHECK-NEXT:    br label [[OUTER_LATCH]]
+; CHECK:       inner.split:
+; CHECK-NEXT:    [[N_EXT_LCSSA:%.*]] = phi i64 [ [[N_EXT]], [[OUTER_LATCH]] ]
+; CHECK-NEXT:    [[TMP0]] = add nsw i64 [[INNER_IV]], 1
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp ne i64 [[TMP0]], [[N_EXT_LCSSA]]
+; CHECK-NEXT:    br i1 [[TMP1]], label [[INNER]], label [[EXIT:%.*]]
+; CHECK:       outer.latch:
+; CHECK-NEXT:    [[OUTER_IV_NEXT]] = add nsw i64 [[OUTER_IV]], 1
+; CHECK-NEXT:    [[C_2:%.*]] = icmp ne i64 [[OUTER_IV]], [[N_EXT]]
+; CHECK-NEXT:    br i1 [[C_2]], label [[OUTER_HEADER]], label [[INNER_SPLIT]]
+; CHECK:       exit:
+; CHECK-NEXT:    ret void
+;
+bb:
+  br label %outer.header
+
+outer.header:                                              ; preds = %bb11, %bb2
+  %outer.iv = phi i64 [ 0, %bb ], [ %outer.iv.next, %outer.latch ]
+  %N.ext = sext i32 %N to i64
+  br label %inner
+
+inner:                                              ; preds = %bb6, %bb4
+  %inner.iv = phi i64 [ 0, %outer.header ], [ %inner.iv.next, %inner ]
+  %tmp8 = getelementptr inbounds [4 x [4 x [2 x i16]]], [4 x [4 x [2 x i16]]]* @global, i64 0, i64 %inner.iv, i64 %outer.iv, i64 0
+  %inner.iv.next = add nsw i64 %inner.iv, 1
+  %c.1 = icmp ne i64 %inner.iv.next, %N.ext
+  br i1 %c.1, label %inner, label %outer.latch
+
+outer.latch:                                             ; preds = %bb6
+  %outer.iv.next = add nsw i64 %outer.iv, 1
+  %c.2 = icmp ne i64 %outer.iv, %N.ext
+  br i1 %c.2 , label %outer.header, label %exit
+
+exit:                                             ; preds = %bb11
+  ret void
+}