Always initialize thread local storage

We had previously not enabled TLS in MODE=tiny in order to keep the
smallest example programs (e.g. life.com) just 16kb in size. But it
was error prone doing that, so now we just always enable it because
this change uses hacks to ensure it won't increase life.com's size.

This change also fixes a bug on NetBSD, where signal handlers would
break thread local storage if SA_SIGINFO was being used. This looks
like it might be a bug in NetBSD, but it's got a simple workaround.

libc/runtime/enable_tls.c

@@ -16,12 +16,16 @@
 │ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/assert.h"
 #include "libc/bits/bits.h"
+#include "libc/bits/weaken.h"
 #include "libc/calls/calls.h"
 #include "libc/calls/strace.internal.h"
 #include "libc/calls/syscall-sysv.internal.h"
 #include "libc/dce.h"
 #include "libc/errno.h"
+#include "libc/intrin/kprintf.h"
+#include "libc/log/libfatal.internal.h"
 #include "libc/macros.internal.h"
 #include "libc/nexgen32e/threaded.h"
 #include "libc/nt/thread.h"
@@ -52,12 +56,35 @@ __msabi extern typeof(TlsAlloc) *const __imp_TlsAlloc;
 
 extern unsigned char __tls_mov_nt_rax[];
 extern unsigned char __tls_add_nt_rax[];
+_Alignas(long) static char __static_tls[5008];
 
 /**
  * Enables thread local storage.
+ *
+ * This function is always called by the core runtime to guarantee TLS
+ * is always available to your program. You must build your code using
+ * -mno-tls-direct-seg-refs if you want to use _Thread_local.
+ *
+ * You can use __get_tls() to get the linear address of your tib. When
+ * accessing TLS via privileged code you must use __get_tls_privileged
+ * because we need code morphing to support The New Technology and XNU
+ *
+ * On XNU and The New Technology, this function imposes 1ms of latency
+ * during startup for larger binaries like Python.
+ *
+ * If you don't want TLS and you're sure you're not using it, then you
+ * can disable it as follows:
+ *
+ *     int main() {
+ *       __tls_enabled = false;
+ *       // do stuff
+ *     }
+ *
+ * This is useful if you want to wrestle back control of %fs using the
+ * arch_prctl() function. However, such programs might not be portable
+ * and your `errno` variable also won't be thread safe anymore.
  */
 privileged void __enable_tls(void) {
-  if (__tls_enabled) return;
   STRACE("__enable_tls()");
 
   // allocate tls memory for main process
@@ -74,20 +101,42 @@ privileged void __enable_tls(void) {
   size_t siz;
   cthread_t tib;
   char *mem, *tls;
-  siz = ROUNDUP(_TLSZ + _TIBZ, FRAMESIZE);
-  mem = _mapanon(siz);
+  siz = ROUNDUP(_TLSZ + _TIBZ, alignof(__static_tls));
+  if (siz <= sizeof(__static_tls)) {
+    // if tls requirement is small then use the static tls block
+    // which helps avoid a system call for appes with little tls
+    // this is crucial to keeping life.com 16 kilobytes in size!
+    _Static_assert(alignof(__static_tls) >= alignof(cthread_t));
+    mem = __static_tls;
+  } else {
+    // if this binary needs a hefty tls block then we'll bank on
+    // malloc() being linked, which links _mapanon().  otherwise
+    // if you exceed this, you need to STATIC_YOINK("_mapanon").
+    // please note that it's probably too early to call calloc()
+    assert(weaken(_mapanon));
+    siz = ROUNDUP(siz, FRAMESIZE);
+    mem = weaken(_mapanon)(siz);
+    assert(mem);
+  }
   tib = (cthread_t)(mem + siz - _TIBZ);
   tls = mem + siz - _TIBZ - _TLSZ;
   tib->self = tib;
   tib->self2 = tib;
   tib->err = __errno;
-  tib->tid = sys_gettid();
-  memmove(tls, _tdata_start, _TLDZ);
+  if (IsLinux()) {
+    // gnu/systemd guarantees pid==tid for the main thread so we can
+    // avoid issuing a superfluous system call at startup in program
+    tib->tid = __pid;
+  } else {
+    tib->tid = sys_gettid();
+  }
+  __repmovsb(tls, _tdata_start, _TLDZ);
 
   // ask the operating system to change the x86 segment register
   int ax, dx;
   if (IsWindows()) {
     __tls_index = __imp_TlsAlloc();
+    assert(0 <= __tls_index && __tls_index < 64);
     asm("mov\t%1,%%gs:%0" : "=m"(*((long *)0x1480 + __tls_index)) : "r"(tib));
   } else if (IsFreebsd()) {
     asm volatile("syscall"
@@ -95,9 +144,12 @@ privileged void __enable_tls(void) {
                  : "0"(__NR_sysarch), "D"(AMD64_SET_FSBASE), "S"(tib)
                  : "rcx", "r11", "memory", "cc");
   } else if (IsNetbsd()) {
+    // netbsd has sysarch(X86_SET_FSBASE) but we can't use that because
+    // signal handlers will cause it to be reset due to net setting the
+    // _mc_tlsbase field in struct mcontext_netbsd.
     asm volatile("syscall"
                  : "=a"(ax), "=d"(dx)
-                 : "0"(__NR_sysarch), "D"(X86_SET_FSBASE), "S"(tib)
+                 : "0"(__NR__lwp_setprivate), "D"(tib)
                  : "rcx", "r11", "memory", "cc");
   } else if (IsXnu()) {
     asm volatile("syscall"
@@ -179,7 +231,7 @@ privileged void __enable_tls(void) {
       }
 
       // we're checking for the following expression:
-      //   0144 == p[0] &&           // fs
+      //   0144 == p[0] &&           // %fs
       //   0110 == p[1] &&           // rex.w (64-bit operand size)
       //   (0213 == p[2] ||          // mov reg/mem → reg (word-sized)
       //   0003 == p[2]) &&          // add reg/mem → reg (word-sized)
@@ -195,7 +247,7 @@ privileged void __enable_tls(void) {
           !p[8]) {
 
         // now change the code
-        p[0] = 0145;  // this changes gs segment to fs segment
+        p[0] = 0145;                       // change %fs to %gs
         p[5] = (dis & 0x000000ff) >> 000;  // displacement
         p[6] = (dis & 0x0000ff00) >> 010;  // displacement
         p[7] = (dis & 0x00ff0000) >> 020;  // displacement