utils: Fix segfault in flattencfg

v2: continue iterating through the rest of the bb
    use for loop

v3: initialize FlattenCFG pass in ScalarOps
    add test

v4: split off initializing flattencfg to a separate patch
    add comment

Signed-off-by: Jan Vesely <[email protected]>
llvm-svn: 215574

Author: jvesely

llvm/lib/Transforms/Utils/FlattenCFG.cpp

@@ -238,19 +238,20 @@ bool FlattenCFGOpt::FlattenParallelAndOr(BasicBlock *BB, IRBuilder<> &Builder,
     // Do branch inversion.
     BasicBlock *CurrBlock = LastCondBlock;
     bool EverChanged = false;
-    while (1) {
+    for (;CurrBlock != FirstCondBlock;
+          CurrBlock = CurrBlock->getSinglePredecessor()) {
       BranchInst *BI = dyn_cast<BranchInst>(CurrBlock->getTerminator());
       CmpInst *CI = dyn_cast<CmpInst>(BI->getCondition());
+      if (!CI)
+        continue;
+
       CmpInst::Predicate Predicate = CI->getPredicate();
       // Canonicalize icmp_ne -> icmp_eq, fcmp_one -> fcmp_oeq
       if ((Predicate == CmpInst::ICMP_NE) || (Predicate == CmpInst::FCMP_ONE)) {
         CI->setPredicate(ICmpInst::getInversePredicate(Predicate));
         BI->swapSuccessors();
         EverChanged = true;
       }
-      if (CurrBlock == FirstCondBlock)
-        break;
-      CurrBlock = CurrBlock->getSinglePredecessor();
     }
     return EverChanged;
   }

llvm/test/Transforms/Util/flattencfg.ll

@@ -0,0 +1,26 @@
+; RUN: opt -flattencfg -S < %s | FileCheck %s
+
+
+; This test checks whether the pass completes without a crash.
+; The code is not transformed in any way
+;
+; CHECK-LABEL: @test_not_crash
+define void @test_not_crash(i32 %in_a) #0 {
+entry:
+  %cmp0 = icmp eq i32 %in_a, -1
+  %cmp1 = icmp ne i32 %in_a, 0
+  %cond0 = and i1 %cmp0, %cmp1
+  br i1 %cond0, label %b0, label %b1
+
+b0:                                ; preds = %entry
+  %cmp2 = icmp eq i32 %in_a, 0
+  %cmp3 = icmp ne i32 %in_a, 1
+  %cond1 = or i1 %cmp2, %cmp3
+  br i1 %cond1, label %exit, label %b1
+
+b1:                                       ; preds = %entry, %b0
+  br label %exit
+
+exit:                               ; preds = %entry, %b0, %b1
+  ret void
+}