[LoopSimplify] Preserve LCSSA when merging exit blocks.

Summary:
This fixes PR26682. Also add LCSSA as a preserved pass to LoopSimplify,
that looks correct to me and allows to write a test for the issue.

Reviewers: chandlerc, bogner, sanjoy

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D21112

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@272224 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 92be720)

Author: Michael Zolotukhin

lib/Transforms/Utils/LoopSimplify.cpp

@@ -691,8 +691,10 @@ static bool simplifyOneLoop(Loop *L, SmallVectorImpl<Loop *> &Worklist,
       }
       DT->eraseNode(ExitingBlock);
 
-      BI->getSuccessor(0)->removePredecessor(ExitingBlock);
-      BI->getSuccessor(1)->removePredecessor(ExitingBlock);
+      BI->getSuccessor(0)->removePredecessor(
+          ExitingBlock, /* DontDeleteUselessPHIs */ PreserveLCSSA);
+      BI->getSuccessor(1)->removePredecessor(
+          ExitingBlock, /* DontDeleteUselessPHIs */ PreserveLCSSA);
       ExitingBlock->eraseFromParent();
     }
   }
@@ -754,6 +756,7 @@ namespace {
       AU.addPreserved<ScalarEvolutionWrapperPass>();
       AU.addPreserved<SCEVAAWrapperPass>();
       AU.addPreserved<DependenceAnalysis>();
+      AU.addPreservedID(LCSSAID);
       AU.addPreservedID(BreakCriticalEdgesID);  // No critical edges added.
     }
 
@@ -789,11 +792,27 @@ bool LoopSimplify::runOnFunction(Function &F) {
   SE = SEWP ? &SEWP->getSE() : nullptr;
   AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
   bool PreserveLCSSA = mustPreserveAnalysisID(LCSSAID);
+#ifndef NDEBUG
+  if (PreserveLCSSA) {
+    assert(DT && "DT not available.");
+    assert(LI && "LI not available.");
+    bool InLCSSA =
+        all_of(*LI, [&](Loop *L) { return L->isRecursivelyLCSSAForm(*DT); });
+    assert(InLCSSA && "Requested to preserve LCSSA, but it's already broken.");
+  }
+#endif
 
   // Simplify each loop nest in the function.
   for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
     Changed |= simplifyLoop(*I, DT, LI, SE, AC, PreserveLCSSA);
 
+#ifndef NDEBUG
+  if (PreserveLCSSA) {
+    bool InLCSSA =
+        all_of(*LI, [&](Loop *L) { return L->isRecursivelyLCSSAForm(*DT); });
+    assert(InLCSSA && "LCSSA is broken after loop-simplify.");
+  }
+#endif
   return Changed;
 }
 

test/Transforms/LoopSimplify/pr26682.ll

@@ -0,0 +1,32 @@
+; RUN: opt < %s -lcssa -loop-simplify -indvars -S | FileCheck %s
+target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-unknown"
+
+@a = external global i32, align 4
+
+; Check that loop-simplify merges two loop exits, but preserves LCSSA form.
+; CHECK-LABEL: @foo
+; CHECK: for:
+; CHECK: %or.cond = and i1 %cmp1, %cmp2
+; CHECK-NOT: for.cond:
+; CHECK: for.end:
+; CHECK: %a.lcssa = phi i32 [ %a, %for ]
+define i32 @foo(i32 %x) {
+entry:
+  br label %for
+
+for:
+  %iv = phi i32 [ 0, %entry ], [ %iv.next, %for.cond ]
+  %cmp1 = icmp eq i32 %x, 0
+  %iv.next = add nuw nsw i32 %iv, 1
+  %a = load i32, i32* @a
+  br i1 %cmp1, label %for.cond, label %for.end
+
+for.cond:
+  %cmp2 = icmp slt i32 %iv.next, 4
+  br i1 %cmp2, label %for, label %for.end
+
+for.end:
+  %a.lcssa = phi i32 [ %a, %for ], [ %a, %for.cond ]
+  ret i32 %a.lcssa
+}