Index: lib/CodeGen/SelectionDAG/DAGCombiner.cpp
===================================================================
--- lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -10943,13 +10943,9 @@
     }
   }
 
-  // fold (fsub A, (fneg B)) -> (fadd A, B)
-  if (isNegatibleForFree(N1, LegalOperations, TLI, &Options))
-    return DAG.getNode(ISD::FADD, DL, VT, N0,
-                       GetNegatedExpression(N1, DAG, LegalOperations), Flags);
-
   // If 'unsafe math' is enabled, fold lots of things.
-  if (Options.UnsafeFPMath) {
+  if (Options.UnsafeFPMath || 
+      (Flags.hasAllowReassociation() && Flags.hasNoSignedZeros())) {
     // (fsub x, (fadd x, y)) -> (fneg y)
     // (fsub x, (fadd y, x)) -> (fneg y)
     if (N1.getOpcode() == ISD::FADD) {
@@ -10964,6 +10960,11 @@
     }
   }
 
+  // fold (fsub A, (fneg B)) -> (fadd A, B)
+  if (isNegatibleForFree(N1, LegalOperations, TLI, &Options))
+    return DAG.getNode(ISD::FADD, DL, VT, N0,
+                       GetNegatedExpression(N1, DAG, LegalOperations), Flags);
+
   // FSUB -> FMA combines:
   if (SDValue Fused = visitFSUBForFMACombine(N)) {
     AddToWorklist(Fused.getNode());
Index: test/CodeGen/X86/fp-fold.ll
===================================================================
--- test/CodeGen/X86/fp-fold.ll
+++ test/CodeGen/X86/fp-fold.ll
@@ -88,6 +88,17 @@
   ret float %r
 }
 
+define float @fsub_neg_y(float %x, float %y) {
+; ANY-LABEL: fsub_neg_y:
+; ANY:       # %bb.0:
+; ANY-NEXT:    mulss {{.*}}(%rip), %xmm0
+; ANY-NEXT:    retq
+  %mul = fmul float %x, 5.000000e+00
+  %add = fadd float %mul, 1.0
+  %r = fsub nsz reassoc float 1.0, %add 
+  ret float %r
+}
+
 define float @fsub_negzero(float %x) {
 ; STRICT-LABEL: fsub_negzero:
 ; STRICT:       # %bb.0:
@@ -153,3 +164,5 @@
   %r = fmul reassoc float %mul, 4.0
   ret float %r
 }
+
+declare float @llvm.fabs.f32(float) #1
Index: test/Transforms/InstCombine/fsub.ll
===================================================================
--- test/Transforms/InstCombine/fsub.ll
+++ test/Transforms/InstCombine/fsub.ll
@@ -60,6 +60,19 @@
   ret float %t2
 }
 
+; With nsz and reassoc: Y - (X + Y) --> -X 
+define float @sub_add_neg_x(float %x, float %y) {
+; CHECK-LABEL: @sub_add_neg_x(
+; CHECK-NEXT:    [[TMP1:%.*]] = fmul float [[X:%.*]], 5.000000e+00
+; CHECK-NEXT:    [[T2:%.*]] = fsub reassoc nsz float -0.000000e+00, [[TMP1]]
+; CHECK-NEXT:    ret float [[T2]]
+;
+  %mul = fmul float %x, 5.000000e+00
+  %add = fadd float %mul, 1.0
+  %r = fsub nsz reassoc float 1.0, %add
+  ret float %r
+}
+
 ; Same as above: if 'Z' is not -0.0, swap fsub operands and convert to fadd.
 
 define float @sub_sub_known_not_negzero(float %x, float %y) {