Index: lib/Transforms/Scalar/Reassociate.cpp
===================================================================
--- lib/Transforms/Scalar/Reassociate.cpp
+++ lib/Transforms/Scalar/Reassociate.cpp
@@ -1031,8 +1031,19 @@
     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());
+
+  // We can safely preserve the nuw flag in all cases.  It's also safe to turn a
+  // nuw nsw shl into a nuw nsw mul.  However, nsw in isolation requires special
+  // handling.
+  bool NSW = cast<BinaryOperator>(Shl)->hasNoSignedWrap();
+  bool NUW = cast<BinaryOperator>(Shl)->hasNoUnsignedWrap();
+  if (NUW && NSW)
+    Mul->setHasNoSignedWrap(NSW);
+  Mul->setHasNoUnsignedWrap(NUW);
   return Mul;
 }
 
Index: test/Transforms/Reassociate/wrap-flags.ll
===================================================================
--- /dev/null
+++ test/Transforms/Reassociate/wrap-flags.ll
@@ -0,0 +1,34 @@
+; RUN: opt < %s -reassociate -dce -S | FileCheck %s
+; PR12985
+
+; Verify the nsw flags are preserved when converting shl to mul.
+
+; CHECK-LABEL: @shl_to_mul_nsw(
+; CHECK: %mul = mul i32 %i, -2147483648
+; CHECK: %mul2 = add i32 %mul, 1
+define i32 @shl_to_mul_nsw(i32 %i) {
+entry:
+  %mul = shl nsw i32 %i, 31
+  %mul2 = add i32 %mul, 1
+  ret i32 %mul2
+}
+
+; CHECK-LABEL: @shl_to_mul_nuw(
+; CHECK: %mul = mul nuw i32 %i, 4
+; CHECK: %mul2 = add i32 %mul, 1
+define i32 @shl_to_mul_nuw(i32 %i) {
+entry:
+  %mul = shl nuw i32 %i, 2
+  %mul2 = add i32 %mul, 1
+  ret i32 %mul2
+}
+
+; CHECK-LABEL: @shl_to_mul_nuw_nsw(
+; CHECK: %mul = mul nuw nsw i32 %i, 4
+; CHECK: %mul2 = add i32 %mul, 1
+define i32 @shl_to_mul_nuw_nsw(i32 %i) {
+entry:
+  %mul = shl nuw nsw i32 %i, 2
+  %mul2 = add i32 %mul, 1
+  ret i32 %mul2
+}