Fix fast TLS support on Mac.

src/platform-tls-mac.h

Index: src/platform-tls-mac.h
diff --git a/src/platform-tls-mac.h b/src/platform-tls-mac.h
index 86a3347d0f710825808a231031a4f81851b15ffa..3543f5ec8e416d85997ef62b644cc70feea6160f 100644
--- a/src/platform-tls-mac.h
+++ b/src/platform-tls-mac.h
@@ -37,20 +37,21 @@ namespace internal {
 
 #define V8_FAST_TLS_SUPPORTED 1
 
+extern intptr_t kMacTlsBaseOffset;
+
 INLINE(intptr_t InternalGetExistingThreadLocal(intptr_t index));
 
 inline intptr_t InternalGetExistingThreadLocal(intptr_t index) {
-  // The constants below are taken from pthreads.s from the XNU kernel
-  // sources archive at www.opensource.apple.com.
+  intptr_t offset = kMacTlsBaseOffset + kPointerSize * index;
   intptr_t result;
 #if defined(V8_HOST_ARCH_IA32)
-  asm("movl %%gs:0x48(,%1,4), %0;"
+  asm("movl %%gs:(%1), %0;"

[Mark Mentovai, 2011/04/01 14:28:01]

It seems like you should be able to move the arithmetic used to compute |offset|
on line 45 right here into the addressing mode. You can treat kMacTlsBaseOffset
as the base, index as the index, and 4 (or 8) as the width.

[Vitaly Repeshko, 2011/04/01 15:45:18]

Given that we have to load the base from a static variable I thought it wouldn't
be worth the extra complexity in the inline assembly, but after checking what
gcc produces, it seems to be worth it. So I changed the code here.

       :"=r"(result)  // Output must be a writable register.
-      :"0"(index));  // Input is the same as output.
+      :"0"(offset));  // Input is the same as output.
 #else
-  asm("movq %%gs:0x60(,%1,8), %0;"
+  asm("movq %%gs:(%1), %0;"
       :"=r"(result)
-      :"0"(index));
+      :"0"(offset));
 #endif
   return result;
 }