diff --git a/clang/lib/CodeGen/CGBuiltin.cpp b/clang/lib/CodeGen/CGBuiltin.cpp
--- a/clang/lib/CodeGen/CGBuiltin.cpp
+++ b/clang/lib/CodeGen/CGBuiltin.cpp
@@ -13113,6 +13113,8 @@
     return Builder.CreateBitCast(Sext, FPVecTy);
   };
 
+  IRBuilder<>::FastMathFlagGuard FMFGuard(Builder);
+
   switch (BuiltinID) {
   default: return nullptr;
   case X86::BI_mm_prefetch: {
diff --git a/clang/test/CodeGen/builtins-x86-reduce.c b/clang/test/CodeGen/builtins-x86-reduce.c
new file mode 100644
--- /dev/null
+++ b/clang/test/CodeGen/builtins-x86-reduce.c
@@ -0,0 +1,11 @@
+// RUN: %clang_cc1 %s -triple x86_64-unknown-unknown -target-feature +avx512f -emit-llvm -o - | FileCheck %s
+
+typedef double double8 __attribute__((ext_vector_type(8)));
+
+double foo(double8 a, double b) {
+  return __builtin_ia32_reduce_fmax_pd512(a) + b;
+}
+
+// CHECK: fadd
+// CHECK-NOT: nnan
+// CHECK-SAME: double