diff --git a/libc/private/bionic_tls.h b/libc/private/bionic_tls.h
index 4749cee..90914c3 100644
--- a/libc/private/bionic_tls.h
+++ b/libc/private/bionic_tls.h
@@ -26,8 +26,7 @@
  * SUCH DAMAGE.
  */
 
-#ifndef __BIONIC_PRIVATE_BIONIC_TLS_H_
-#define __BIONIC_PRIVATE_BIONIC_TLS_H_
+#pragma once
 
 #include <locale.h>
 #include <mntent.h>
@@ -35,73 +34,48 @@
 #include <sys/cdefs.h>
 #include <sys/param.h>
 
+#include "bionic_asm_tls.h"
 #include "bionic_macros.h"
 #include "__get_tls.h"
 #include "grp_pwd.h"
 
-__BEGIN_DECLS
-
 /** 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 the system dynamic
- ** linker and the OpenGL sub-system (which needs to access the
- ** pre-allocated slot directly for performance reason).
+ ** 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).
  **/
 
-// Well-known TLS slots. What data goes in which slot is arbitrary unless otherwise noted.
-enum {
-  TLS_SLOT_SELF = 0, // The kernel requires this specific slot for x86.
-  TLS_SLOT_THREAD_ID,
+class pthread_internal_t;
 
-  // TLS slot 2 was used for errno but is now free.
+// This struct is small, so the linker can allocate a temporary copy on its
+// stack. It can't be combined with pthread_internal_t because:
+//  - native bridge requires pthread_internal_t to have the same layout across
+//    architectures, and
+//  - On x86, this struct would have to be placed at the front of
+//    pthread_internal_t, moving fields like `tid`.
+//  - We'd like to avoid having a temporary pthread_internal_t object that
+//    needs to be transferred once the final size of static TLS is known.
+struct bionic_tcb {
+  void* raw_slots_storage[BIONIC_TLS_SLOTS];
 
-  // 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_OPENGL_API = 3,
-  TLS_SLOT_OPENGL = 4,
+  // Return a reference to a slot given its TP-relative TLS_SLOT_xxx index.
+  // The thread pointer (i.e. __get_tls()) points at &tls_slot(0).
+  void*& tls_slot(size_t tpindex) {
+    return raw_slots_storage[tpindex - MIN_TLS_SLOT];
+  }
 
-  TLS_SLOT_STACK_GUARD = 5, // GCC requires this specific slot for x86.
+  // Initialize the main thread's final object using its bootstrap object.
+  void copy_from_bootstrap(const bionic_tcb* boot) {
+    // Copy everything. Problematic slots will be reinitialized.
+    *this = *boot;
+  }
 
-  // Lets sanitizers avoid using pthread_getspecific for finding the current
-  // thread state. (Slot 6 was historically used for dlerror instead.)
-  TLS_SLOT_SANITIZER = 6,
-
-  // Fast storage for Thread::Current() in ART.
-  TLS_SLOT_ART_THREAD_SELF = 7,
-
-  // Lets TSAN avoid using pthread_getspecific for finding the current thread
-  // state.
-  TLS_SLOT_TSAN = 8,
-
-  BIONIC_TLS_SLOTS // Must come last!
+  pthread_internal_t* thread() {
+    return static_cast<pthread_internal_t*>(tls_slot(TLS_SLOT_THREAD_ID));
+  }
 };
 
-// ~3 pages.
-struct bionic_tls {
-  locale_t locale;
-
-  char basename_buf[MAXPATHLEN];
-  char dirname_buf[MAXPATHLEN];
-
-  mntent mntent_buf;
-  char mntent_strings[BUFSIZ];
-
-  char ptsname_buf[32];
-  char ttyname_buf[64];
-
-  char strerror_buf[NL_TEXTMAX];
-  char strsignal_buf[NL_TEXTMAX];
-
-  group_state_t group;
-  passwd_state_t passwd;
-};
-
-#define BIONIC_TLS_SIZE (__BIONIC_ALIGN(sizeof(bionic_tls), PAGE_SIZE))
-
 /*
  * Bionic uses some pthread keys internally. All pthread keys used internally
  * should be created in constructors, except for keys that may be used in or
@@ -126,12 +100,42 @@
  */
 #define BIONIC_PTHREAD_KEY_COUNT (BIONIC_PTHREAD_KEY_RESERVED_COUNT + PTHREAD_KEYS_MAX)
 
-__END_DECLS
+class pthread_key_data_t {
+ public:
+  uintptr_t seq; // Use uintptr_t just for alignment, as we use pointer below.
+  void* data;
+};
 
-#if defined(__cplusplus)
+// ~3 pages. This struct is allocated as static TLS memory (i.e. at a fixed
+// offset from the thread pointer).
+struct bionic_tls {
+  pthread_key_data_t key_data[BIONIC_PTHREAD_KEY_COUNT];
+
+  locale_t locale;
+
+  char basename_buf[MAXPATHLEN];
+  char dirname_buf[MAXPATHLEN];
+
+  mntent mntent_buf;
+  char mntent_strings[BUFSIZ];
+
+  char ptsname_buf[32];
+  char ttyname_buf[64];
+
+  char strerror_buf[NL_TEXTMAX];
+  char strsignal_buf[NL_TEXTMAX];
+
+  group_state_t group;
+  passwd_state_t passwd;
+
+  // Initialize the main thread's final object using its bootstrap object.
+  void copy_from_bootstrap(const bionic_tls* boot __attribute__((unused))) {
+    // Nothing in bionic_tls needs to be preserved in the transition to the
+    // final TLS objects, so don't copy anything.
+  }
+};
+
 class KernelArgumentBlock;
-extern void __libc_init_main_thread_early(KernelArgumentBlock& args);
-extern void __libc_init_main_thread_late();
-#endif
-
-#endif /* __BIONIC_PRIVATE_BIONIC_TLS_H_ */
+extern "C" void __libc_init_main_thread_early(const KernelArgumentBlock& args, bionic_tcb* temp_tcb);
+extern "C" void __libc_init_main_thread_late();
+extern "C" void __libc_init_main_thread_final();