GCC threading models

HOWTO choose a threading model with GCC

The __thread marker available with recent revisions of the C standard is very convenient, but comes at a price. The lookup procedure for thread-local variables can be expensive. Some efficiency can be gained by choosing an appropriate thread model. But I have to choose one so infrequently that I always have to go read a long document every time to remind myself how to choose one. Instead, here are some succint rules for choosing the value x in __attribute__((tls_model(x))) when using gcc. The most efficient models are listed first.

local-exec

The variable is built into the executable (versus a dynamically loaded object), and is only accessed from code that is built into the executable.

initial-exec

The variable is built into the executable.

local-dynamic

The variable is static.

global-dynamic

There are no restrictions on the variable.