Index: lib/Transforms/Scalar/Reassociate.cpp
===================================================================
--- lib/Transforms/Scalar/Reassociate.cpp
+++ lib/Transforms/Scalar/Reassociate.cpp
@@ -1016,6 +1016,17 @@
   Sub->replaceAllUsesWith(New);
   New->setDebugLoc(Sub->getDebugLoc());
 
+  // Preserve NSW/NUW flags.
+  if (Sub->getOpcode() == Instruction::Sub) {
+    bool NSW = cast<BinaryOperator>(Sub)->hasNoSignedWrap();
+    bool NUW = cast<BinaryOperator>(Sub)->hasNoUnsignedWrap();
+    New->setHasNoSignedWrap(NSW);
+    New->setHasNoUnsignedWrap(NUW);
+    if (BinaryOperator *NegBO = dyn_cast<BinaryOperator>(NegVal)) {
+      NegBO->setHasNoSignedWrap(NSW);
+      NegBO->setHasNoUnsignedWrap(NUW);
+    }
+  }
   DEBUG(dbgs() << "Negated: " << *New << '\n');
   return New;
 }
@@ -1031,8 +1042,14 @@
     BinaryOperator::CreateMul(Shl->getOperand(0), MulCst, "", Shl);
   Shl->setOperand(0, UndefValue::get(Shl->getType())); // Drop use of op.
   Mul->takeName(Shl);
+
+  // Everyone now refers to the mul instruction.
   Shl->replaceAllUsesWith(Mul);
   Mul->setDebugLoc(Shl->getDebugLoc());
+
+  // Preserve NSW/NUW flags.
+  Mul->setHasNoSignedWrap(cast<BinaryOperator>(Shl)->hasNoSignedWrap());
+  Mul->setHasNoUnsignedWrap(cast<BinaryOperator>(Shl)->hasNoUnsignedWrap());
   return Mul;
 }
 
Index: test/Transforms/Reassociate/wrap-flags.ll
===================================================================
--- /dev/null
+++ test/Transforms/Reassociate/wrap-flags.ll
@@ -0,0 +1,27 @@
+; RUN: opt < %s -reassociate -dce -S | FileCheck %s
+; PR12985
+
+; Verify the nsw flag is preserved when converting shl to mul.
+; CHECK-LABEL: @shl_to_mul(
+; CHECK: %mul = mul nsw i32 %i, 4
+; CHECK: %mul2 = add nsw i32 %mul, 1
+
+define i32 @shl_to_mul(i32 %i) {
+entry:
+  %mul = shl nsw i32 %i, 2
+  %mul2 = add nsw i32 %mul, 1
+  ret i32 %mul2
+}
+
+; Verify the nsw flag is preserved when converting (X-Y)->(X+(0-Y)).
+; CHECK-LABEL: @breakup_sub(
+; CHECK: %y.neg = sub nsw i32 0, %y
+; CHECK: %sub = add nsw i32 %y.neg, %x
+; CHECK: %add = add nsw i32 %sub, %z
+
+define i32 @breakup_sub(i32 %x, i32 %y, i32 %z) {
+entry:
+  %sub = sub nsw i32 %x, %y
+  %add = add nsw i32 %sub, %z
+  ret i32 %add
+}