[compiler-rt] Don't change g_tls_size after initialization.

[timshen]

Summary

g_tls_size is not supposed to be changed after initialization. It's not
atomic, not guarded by a lock, nor thread_local. But it's read by
multiple threads.

The reason why it's mutated is mips and powerpc64 specific. We can
implement the same funcitonality without mutating g_tls_size.

I'm not sure how to write a test for this. Please advice. Thanks!

[kcc]

All this code is under defined(mips) || defined(powerpc64)
which means that I don't really know it, sorry.

Please get LGTM from someone else who knows (ore pretends to know) Power and MIPS.

A test would be very very welcome, something with lots of threads, TLS, DSOs, and such.

[echristo]

LGTM.

-eric

[timshen]

I actually don't know how to write a test that stably reproduce the failure. Let me submit it first to see if it breaks anything.

compiler-rt/lib/sanitizer_common/sanitizer_linux_libcdep.cc

 static uptr TlsPreTcbSize() {
 # if defined(__mips__)
   const uptr kTcbHead = 16; // sizeof (tcbhead_t)
 # elif defined(__powerpc64__)
   const uptr kTcbHead = 88; // sizeof (tcbhead_t)
 # endif
   const uptr kTlsAlign = 16;
   const uptr kTlsPreTcbSize =
-    (ThreadDescriptorSize() + kTcbHead + kTlsAlign - 1) & ~(kTlsAlign - 1);
-  InitTlsSize();
-  g_tls_size = (g_tls_size + kTlsPreTcbSize + kTlsAlign -1) & ~(kTlsAlign - 1);
+      RoundUpTo(ThreadDescriptorSize() + kTcbHead, kTlsAlign);
   return kTlsPreTcbSize;
 }
 #endif
 
 uptr ThreadSelf() {
   uptr descr_addr;
 # if defined(__i386__)
   asm("mov %%gs:%c1,%0" : "=r"(descr_addr) : "i"(kThreadSelfOffset));

 #endif
 
 #if !SANITIZER_GO
 uptr GetTlsSize() {
 #if SANITIZER_FREEBSD || SANITIZER_ANDROID
   uptr addr, size;
   GetTls(&addr, &size);
   return size;
+#elif defined(__mips__) || defined(__powerpc64__)
+  return RoundUpTo(g_tls_size + TlsPreTcbSize(), 16);
 #else
   return g_tls_size;
 #endif
 }
 #endif
 
 void GetThreadStackAndTls(bool main, uptr *stk_addr, uptr *stk_size,
                           uptr *tls_addr, uptr *tls_size) {