Index: lib/Transforms/Utils/SimplifyCFG.cpp
===================================================================
--- lib/Transforms/Utils/SimplifyCFG.cpp
+++ lib/Transforms/Utils/SimplifyCFG.cpp
@@ -2378,16 +2378,33 @@
   // Do not perform this transformation if it would require
   // insertion of a large number of select instructions. For targets
   // without predication/cmovs, this is a big pessimization.
-  BasicBlock *CommonDest = PBI->getSuccessor(PBIOp);
 
+  // Also do not perform this transformation if any phi node in the common
+  // destination block can trap when reached by BB or PBB (PR17073). In that
+  // case, it would be unsafe to hoist the operation into a select instruction.
+
+  BasicBlock *CommonDest = PBI->getSuccessor(PBIOp);
   unsigned NumPhis = 0;
   for (BasicBlock::iterator II = CommonDest->begin();
-       isa<PHINode>(II); ++II, ++NumPhis)
+       isa<PHINode>(II); ++II, ++NumPhis) {
     if (NumPhis > 2) // Disable this xform.
       return false;
 
+    PHINode *PN = cast<PHINode>(II);
+    Value *BIV = PN->getIncomingValueForBlock(BB);
+    if (ConstantExpr *CE = dyn_cast<ConstantExpr>(BIV))
+      if (CE->canTrap())
+        return false;
+
+    unsigned PBBIdx = PN->getBasicBlockIndex(PBI->getParent());
+    Value *PBIV = PN->getIncomingValue(PBBIdx);
+    if (ConstantExpr *CE = dyn_cast<ConstantExpr>(PBIV))
+      if (CE->canTrap())
+        return false;
+  }
+
   // Finally, if everything is ok, fold the branches to logical ops.
-  BasicBlock *OtherDest  = BI->getSuccessor(BIOp ^ 1);
+  BasicBlock *OtherDest = BI->getSuccessor(BIOp ^ 1);
 
   DEBUG(dbgs() << "FOLDING BRs:" << *PBI->getParent()
                << "AND: " << *BI->getParent());
Index: test/Transforms/SimplifyCFG/PR17073.ll
===================================================================
--- test/Transforms/SimplifyCFG/PR17073.ll
+++ test/Transforms/SimplifyCFG/PR17073.ll
@@ -0,0 +1,73 @@
+; RUN: opt < %s -simplifycfg -S | FileCheck %s
+
+; In PR17073 ( http://llvm.org/pr17073 ), we illegally hoisted an operation that can trap.
+; The first test confirms that we don't do that when the trapping op is reached by the current BB (block1).
+; The second test confirms that we don't do that when the trapping op is reached by the previous BB (entry).
+; The third test confirms that we can still do this optimization for an operation (add) that doesn't trap.
+; The tests must be complicated enough to prevent previous SimplifyCFG actions from optimizing away
+; the instructions that we're checking for.
+
+target datalayout = "e-m:o-p:32:32-f64:32:64-f80:128-n8:16:32-S128"
+target triple = "i386-apple-macosx10.9.0"
+
+@a = common global i32 0, align 4
+@b = common global i8 0, align 1
+
+; CHECK-LABEL: can_trap1 
+; CHECK-NOT: or i1 %tobool, icmp eq (i32* bitcast (i8* @b to i32*), i32* @a)
+; CHECK-NOT: select i1 %tobool, i32* null, i32* select (i1 icmp eq (i64 urem (i64 2, i64 zext (i1 icmp eq (i32* bitcast (i8* @b to i32*), i32* @a) to i64)), i64 0), i32* null, i32* @a) 
+define i32* @can_trap1() {
+entry:
+  %0 = load i32* @a, align 4
+  %tobool = icmp eq i32 %0, 0
+  br i1 %tobool, label %exit, label %block1
+
+block1:
+  br i1 icmp eq (i32* bitcast (i8* @b to i32*), i32* @a), label %exit, label %block2
+
+block2:
+  br label %exit
+
+exit:
+  %storemerge = phi i32* [ null, %entry ],[ null, %block2 ], [ select (i1 icmp eq (i64 urem (i64 2, i64 zext (i1 icmp eq (i32* bitcast (i8* @b to i32*), i32* @a) to i64)), i64 0), i32* null, i32* @a), %block1 ]
+  ret i32* %storemerge
+}
+
+; CHECK-LABEL: can_trap2 
+; CHECK-NOT: or i1 %tobool, icmp eq (i32* bitcast (i8* @b to i32*), i32* @a)
+; CHECK-NOT: select i1 %tobool, i32* select (i1 icmp eq (i64 urem (i64 2, i64 zext (i1 icmp eq (i32* bitcast (i8* @b to i32*), i32* @a) to i64)), i64 0), i32* null, i32* @a), i32* null
+define i32* @can_trap2() {
+entry:
+  %0 = load i32* @a, align 4
+  %tobool = icmp eq i32 %0, 0
+  br i1 %tobool, label %exit, label %block1
+
+block1:
+  br i1 icmp eq (i32* bitcast (i8* @b to i32*), i32* @a), label %exit, label %block2
+
+block2:
+  br label %exit
+
+exit:
+  %storemerge = phi i32* [ select (i1 icmp eq (i64 urem (i64 2, i64 zext (i1 icmp eq (i32* bitcast (i8* @b to i32*), i32* @a) to i64)), i64 0), i32* null, i32* @a), %entry ],[ null, %block2 ], [ null, %block1 ]
+  ret i32* %storemerge
+}
+
+; CHECK-LABEL: cannot_trap 
+; CHECK: select i1 icmp eq (i32* bitcast (i8* @b to i32*), i32* @a), i32* select (i1 icmp eq (i64 add (i64 zext (i1 icmp eq (i32* bitcast (i8* @b to i32*), i32* @a) to i64), i64 2), i64 0), i32* null, i32* @a), i32* null
+define i32* @cannot_trap() {
+entry:
+  %0 = load i32* @a, align 4
+  %tobool = icmp eq i32 %0, 0
+  br i1 %tobool, label %exit, label %block1
+
+block1:
+  br i1 icmp eq (i32* bitcast (i8* @b to i32*), i32* @a), label %exit, label %block2
+
+block2:
+  br label %exit
+
+exit:
+  %storemerge = phi i32* [ null, %entry ],[ null, %block2 ], [ select (i1 icmp eq (i64 add (i64 2, i64 zext (i1 icmp eq (i32* bitcast (i8* @b to i32*), i32* @a) to i64)), i64 0), i32* null, i32* @a), %block1 ]
+  ret i32* %storemerge
+}