diff --git a/lib/Transforms/Scalar/Reassociate.cpp b/lib/Transforms/Scalar/Reassociate.cpp
index 9825a590847..e0ef8cfe46b 100644
--- a/lib/Transforms/Scalar/Reassociate.cpp
+++ b/lib/Transforms/Scalar/Reassociate.cpp
@@ -1938,6 +1938,12 @@ Instruction *ReassociatePass::canonicalizeNegConstExpr(Instruction *I) {
   if (!User->isCommutative() && User->getOperand(1) != I)
     return nullptr;
 
+  // Don't canonicalize x + (-Constant * y) -> x - (Constant * y), if the
+  // resulting subtract will be broken up later.  This can get us into an
+  // infinite loop during reassociation.
+  if (UserOpcode == Instruction::FAdd && ShouldBreakUpSubtract(User))
+    return nullptr;
+
   // Change the sign of the constant.
   APFloat Val = CF->getValueAPF();
   Val.changeSign();
diff --git a/test/Transforms/Reassociate/canonicalize-neg-const.ll b/test/Transforms/Reassociate/canonicalize-neg-const.ll
index 465460cb53b..7cb2c3a10e2 100644
--- a/test/Transforms/Reassociate/canonicalize-neg-const.ll
+++ b/test/Transforms/Reassociate/canonicalize-neg-const.ll
@@ -154,3 +154,25 @@ define i4 @test13(i4 %x) {
   %add = add i4 %mul, 3
   ret i4 %add
 }
+
+; This tests used to cause an infinite loop where we would loop between
+; canonicalizing the negated constant (i.e., (X + Y*-5.0) -> (X - Y*5.0)) and
+; breaking up a subtract (i.e., (X - Y*5.0) -> X + (0 - Y*5.0)). To break the
+; cycle, we don't canonicalize the negative constant if we're going to later
+; break up the subtract.
+;
+; Check to make sure we don't canonicalize
+;   (%pow2*-5.0 + %sub) -> (%sub - %pow2*5.0)
+; as we would later break up this subtract causing a cycle.
+;
+; CHECK-LABEL: @pr34078
+; CHECK: %mul5.neg = fmul fast double %pow2, -5.000000e-01
+; CHECK: %sub1 = fadd fast double %mul5.neg, %sub
+define double @pr34078(double %A) {
+  %sub = fsub fast double 1.000000e+00, %A
+  %pow2 = fmul double %A, %A
+  %mul5 = fmul fast double %pow2, 5.000000e-01
+  %sub1 = fsub fast double %sub, %mul5
+  %add = fadd fast double %sub1, %sub1
+  ret double %add
+}