Index: lib/Headers/x86intrin.h
===================================================================
--- lib/Headers/x86intrin.h
+++ lib/Headers/x86intrin.h
@@ -34,6 +34,25 @@
 
 #if !defined(_MSC_VER) || __has_feature(modules) || defined(__BMI__)
 #include <bmiintrin.h>
+#else
+/* The TZCNT is special in that it's encoded in a backward-compatible way and
+ * behaves as BSF on non-BMI targets, hence some compilers provide it even
+ * if the target doesn't support BMI instructions.  LLVM won't emit the TZCNT
+ * instruction if BMI isn't available, but to not break code assuming
+ * unconditional availability of the tzcnt instruction, provide the intrinsic
+ * here (with a BMI-independent implementation). */
+#define _tzcnt_u16(a)     (__tzcnt_u16((a)))
+#define _tzcnt_u32(a)     (__tzcnt_u32((a)))
+static __inline__ unsigned short __attribute__((__always_inline__, __nodebug__))
+__tzcnt_u16(unsigned short __X) { return __X ? __builtin_ctzs(__X) : 16; }
+static __inline__ unsigned int __attribute__((__always_inline__, __nodebug__))
+__tzcnt_u32(unsigned int __X) { return __X ? __builtin_ctz(__X) : 32; }
+#ifdef __x86_64__
+#define _tzcnt_u64(a)     (__tzcnt_u64((a)))
+static __inline__ unsigned long long
+__attribute__((__always_inline__, __nodebug__))
+__tzcnt_u64(unsigned long long __X) { return __X ? __builtin_ctzll(__X) : 64; }
+#endif /* __x86_64__ */
 #endif
 
 #if !defined(_MSC_VER) || __has_feature(modules) || defined(__BMI2__)
Index: test/Headers/tzcnt.c
===================================================================
--- test/Headers/tzcnt.c
+++ test/Headers/tzcnt.c
@@ -0,0 +1,23 @@
+// RUN: %clang_cc1 -triple i386-pc-win32 -target-cpu broadwell \
+// RUN:     -fms-extensions -fms-compatibility -fms-compatibility-version=17.00 \
+// RUN:     -ffreestanding -emit-obj -o /dev/null -Werror \
+// RUN:     -isystem %S/Inputs/include %s
+
+// RUN: %clang_cc1 -triple x86_64-pc-win32  \
+// RUN:     -fms-extensions -fms-compatibility -fms-compatibility-version=17.00 \
+// RUN:     -ffreestanding -emit-obj -o /dev/null -Werror \
+// RUN:     -isystem %S/Inputs/include %s
+//
+// REQUIRES: x86-registered-target
+
+typedef __SIZE_TYPE__ size_t;
+#include <intrin.h>
+
+void f() {
+  // Check that _tzcnt is available even without BMI; ffmpeg relies on this.
+  __tzcnt_u16(0);
+  __tzcnt_u32(0);
+#ifdef __x86_64__
+  __tzcnt_u64(0);
+#endif
+}