diff --git a/libc/private/bionic_asm_tls.h b/libc/private/bionic_asm_tls.h
new file mode 100644
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--- /dev/null
+++ b/libc/private/bionic_asm_tls.h
@@ -0,0 +1,119 @@
+/*
+ * Copyright (C) 2018 The Android Open Source Project
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *  * Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ *  * Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in
+ *    the documentation and/or other materials provided with the
+ *    distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+ * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+ * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
+ * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
+ * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+ * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#pragma once
+
+/** WARNING WARNING WARNING
+ **
+ ** This header file is *NOT* part of the public Bionic ABI/API and should not
+ ** be used/included by user-serviceable parts of the system (e.g.
+ ** applications).
+ **
+ ** It is only provided here for the benefit of Android components that need a
+ ** pre-allocated slot for performance reasons (including ART, the OpenGL
+ ** subsystem, and sanitizers).
+ **/
+
+// Bionic TCB / TLS slots:
+//
+//  - TLS_SLOT_SELF: On x86-{32,64}, the kernel makes TLS memory available via
+//    the gs/fs segments. To get the address of a TLS variable, the first slot
+//    of TLS memory (accessed using %gs:0 / %fs:0) holds the address of the
+//    gs/fs segment. This slot is used by:
+//     - OpenGL and compiler-rt
+//     - Accesses of x86 ELF TLS variables
+//
+//  - TLS_SLOT_OPENGL and TLS_SLOT_OPENGL_API: These two aren't used by bionic
+//    itself, but allow the graphics code to access TLS directly rather than
+//    using the pthread API.
+//
+//  - TLS_SLOT_STACK_GUARD: Used for -fstack-protector by:
+//     - Clang targeting Android/arm64
+//     - gcc targeting Linux/x86-{32,64}
+//
+//  - TLS_SLOT_SANITIZER: Lets sanitizers avoid using pthread_getspecific for
+//    finding the current thread state.
+//
+//  - TLS_SLOT_DTV: Pointer to ELF TLS dynamic thread vector.
+//
+//  - TLS_SLOT_ART_THREAD_SELF: Fast storage for Thread::Current() in ART.
+//
+//  - TLS_SLOT_BIONIC_TLS: Optimizes accesses to bionic_tls by one load versus
+//    finding it using __get_thread().
+
+#if defined(__arm__) || defined(__aarch64__)
+
+// The ARM ELF TLS ABI specifies[1] that the thread pointer points at a 2-word
+// TCB followed by the executable's TLS segment. Both the TCB and the
+// executable's segment are aligned according to the segment, so Bionic requires
+// a minimum segment alignment, which effectively reserves an 8-word TCB. The
+// ARM spec allocates the first TCB word to the DTV.
+//
+// [1] "Addenda to, and Errata in, the ABI for the ARM Architecture". Section 3.
+// http://infocenter.arm.com/help/topic/com.arm.doc.ihi0045e/IHI0045E_ABI_addenda.pdf
+
+#define MIN_TLS_SLOT              -1 // update this value when reserving a slot
+#define TLS_SLOT_BIONIC_TLS       -1
+#define TLS_SLOT_DTV              0
+#define TLS_SLOT_THREAD_ID        1
+// Slot 2 is free (was historically used for TLS_SLOT_ERRNO)
+#define TLS_SLOT_OPENGL           3
+#define TLS_SLOT_OPENGL_API       4
+#define TLS_SLOT_STACK_GUARD      5
+#define TLS_SLOT_SANITIZER        6 // was historically used for dlerror
+#define TLS_SLOT_ART_THREAD_SELF  7
+#define TLS_SLOT_TSAN             8 // should be replaced with TLS_SLOT_SANITIZER
+
+// The maximum slot is fixed by the minimum TLS alignment in Bionic executables.
+// It should be changed to 7 once TLS_SLOT_TSAN is removed.
+#define MAX_TLS_SLOT              8
+
+#elif defined(__i386__) || defined(__x86_64__)
+
+// x86 uses variant 2 ELF TLS layout, which places the executable's TLS segment
+// immediately before the thread pointer. New slots are allocated at positive
+// offsets from the thread pointer.
+
+#define MIN_TLS_SLOT              0
+
+#define TLS_SLOT_SELF             0
+#define TLS_SLOT_THREAD_ID        1
+// Slot 2 is free (was historically used for TLS_SLOT_ERRNO)
+#define TLS_SLOT_OPENGL           3
+#define TLS_SLOT_OPENGL_API       4
+#define TLS_SLOT_STACK_GUARD      5
+#define TLS_SLOT_SANITIZER        6 // was historically used for dlerror
+#define TLS_SLOT_ART_THREAD_SELF  7
+#define TLS_SLOT_TSAN             8 // should be replaced with TLS_SLOT_SANITIZER
+#define TLS_SLOT_DTV              9
+#define TLS_SLOT_BIONIC_TLS       10
+#define MAX_TLS_SLOT              10 // update this value when reserving a slot
+
+#endif
+
+#define BIONIC_TLS_SLOTS (MAX_TLS_SLOT - MIN_TLS_SLOT + 1)