Linux's thread local storage implementation

__thread Foo foo;

How is "foo" actually resolved? Does the compiler silently replace every instance of "foo" with a function call? Is "foo" stored somewhere relative to the bottom of the stack, and 开发者_如何学Pythonthe compiler stores this as "hey, for each thread, have this space near the bottom of the stack, and foo is stored as 'offset x from bottom of stack'"?

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It's a little complicated (this document explains it in great detail), but it's basically neither. Instead the compiler puts a special .tdata section in the executable, which contains all the thread-local variables. At runtime, a new data section for each thread is created with a copy of the data in the (read-only) .tdata section, and when threads are switched at runtime, the section is also switched automatically.

The end result is that __thread variables are just as fast as regular variables, and they don't take up extra stack space, either.