[LoopSimplify] Rebuild LCSSA for the inner loop after separating nested loops.

Summary:
This hopefully fixes PR28825. The problem now was that a value from the
original loop was used in a subloop, which became a sibling after separation.
While a subloop doesn't need an lcssa phi node, a sibling does, and that's
where we broke LCSSA. The most natural way to fix this now is to simply call
formLCSSA on the original loop: it'll do what we've been doing before plus
it'll cover situations described above.

I think we don't need to run formLCSSARecursively here, and we have an assert
to verify this (I've tried testing it on LLVM testsuite + SPECs). I'd be happy
to be corrected here though.

I also changed a run line in the test from '-lcssa -loop-unroll' to
'-lcssa -loop-simplify -indvars', because it exercises LCSSA
preservation to the same extent, but also makes less unrelated
transformation on the CFG, which makes it easier to verify.

Reviewers: chandlerc, sanjoy, silvas

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D23288

llvm-svn: 278173

llvm/lib/Transforms/Utils/LoopSimplify.cpp

@@ -361,39 +361,11 @@ static Loop *separateNestedLoop(Loop *L, BasicBlock *Preheader,
     // Fix LCSSA form for L. Some values, which previously were only used inside
     // L, can now be used in NewOuter loop. We need to insert phi-nodes for them
     // in corresponding exit blocks.
+    // We don't need to form LCSSA recursively, because there cannot be uses
+    // inside a newly created loop of defs from inner loops as those would
+    // already be a use of an LCSSA phi node.
+    formLCSSA(*L, *DT, LI, SE);
 
-    // Go through all instructions in OuterLoopBlocks and check if they are
-    // using operands from the inner loop. In this case we'll need to fix LCSSA
-    // for these instructions.
-    SmallSetVector<Instruction *, 8> WorklistSet;
-    for (BasicBlock *OuterBB: OuterLoopBlocks) {
-      for (Instruction &I : *OuterBB) {
-        for (Value *Op : I.operands()) {
-          Instruction *OpI = dyn_cast<Instruction>(Op);
-          if (!OpI || !L->contains(OpI))
-            continue;
-          WorklistSet.insert(OpI);
-        }
-      }
-    }
-    // We also need to check exit blocks of the outer loop - it might be using
-    // values from what now became an inner loop.
-    SmallVector<BasicBlock*, 8> ExitBlocks;
-    NewOuter->getExitBlocks(ExitBlocks);
-    for (BasicBlock *ExitBB: ExitBlocks) {
-      for (Instruction &I : *ExitBB) {
-        for (Value *Op : I.operands()) {
-          Instruction *OpI = dyn_cast<Instruction>(Op);
-          if (!OpI || !L->contains(OpI))
-            continue;
-          WorklistSet.insert(OpI);
-        }
-      }
-    }
-
-    SmallVector<Instruction *, 8> Worklist(WorklistSet.begin(),
-                                           WorklistSet.end());
-    formLCSSAForInstructions(Worklist, *DT, *LI);
     assert(NewOuter->isRecursivelyLCSSAForm(*DT) &&
            "LCSSA is broken after separating nested loops!");
   }

llvm/test/Transforms/LoopSimplify/pr28272.ll

@@ -1,4 +1,4 @@
-; RUN: opt < %s -lcssa -loop-unroll -S | FileCheck %s
+; RUN: opt < %s -lcssa -loop-simplify -indvars -S | FileCheck %s
 target triple = "x86_64-unknown-linux-gnu"
 
 ; PR28272, PR28825
@@ -106,3 +106,34 @@ bb_end:
   %x = getelementptr i32, i32* %b
   br label %bb_end
 }
+
+; When LoopSimplify separates nested loops, it might break LCSSA form: values
+; from the original loop might occur in a loop, which is now a sibling of the
+; original loop (before separating it was a subloop of the original loop, and
+; thus didn't require an lcssa phi nodes).
+; CHECK-LABEL: @foo4
+define void @foo4() {
+bb1:
+  br label %bb2
+
+; CHECK: bb2.loopexit:
+bb2.loopexit:                                     ; preds = %bb3
+  %i.ph = phi i32 [ 0, %bb3 ]
+  br label %bb2
+
+; CHECK: bb2.outer:
+; CHECK: bb2:
+bb2:                                              ; preds = %bb2.loopexit, %bb2, %bb1
+  %i = phi i32 [ 0, %bb1 ], [ %i, %bb2 ], [ %i.ph, %bb2.loopexit ]
+  %x = load i32, i32* undef, align 8
+  br i1 undef, label %bb2, label %bb3.preheader
+
+; CHECK: bb3.preheader:
+bb3.preheader:                                    ; preds = %bb2
+; CHECK: %x.lcssa = phi i32 [ %x, %bb2 ]
+  br label %bb3
+
+bb3:                                              ; preds = %bb3.preheader, %bb3
+  %y = add i32 2, %x
+  br i1 true, label %bb2.loopexit, label %bb3
+}