CODEWITHSUNDEEP
c++multithreadinggccthread-local
keelar
keelar

Reputation: 6026

Non-static global variable vs. global static __thread variable

If I define a static __thread variable in global scope, is it equivalent to the regular non-static global variable? In other words, are the following two variables equivalent to each other if they are all in global scope:

int regular_global_int;
static __thread int static_thread_local_int;

If the answer is no, can I know what's the major different between these two and when should I use which one?

Upvotes: 2

Views: 1151

Answers (2)

Richard Pennington
Richard Pennington

Reputation: 19965

I compiled 

int regular_global_int;
__thread int static_thread_local_int;
int main()
{
 regular_global_int = 1;
 static_thread_local_int = 1;
}

at http://ellcc.org/blog/?page_id=340 I had to take off the static to keep the optimizer from getting rid of the static variable. I got

main:                                   # @main
# BB#0:                                 # %entry
    movl    $1, regular_global_int(%rip)
    movl    $1, %fs:static_thread_local_int@TPOFF
    xorl    %eax, %eax
    retq

for the x86_64. The thread local variable is accessed differently to provide the thread local storage.

Upvotes: 1

Brian Bi
Brian Bi

Reputation: 119382

Global variables, and more generally namespace-scope variables, automatically have static storage duration when not declared with a storage class specifier. At namespace scope, static does not mean "static storage duration"; it means the variable has internal linkage. Hence

int x;
static int x;

at namespace scope both declare x with static storage duration but the two declarations are nevertheless not the same as the first declaration gives x external linkage but the second gives it internal linkage.

In the case that you write

static thread_local int x;

the thread_local storage class specifier causes x to have thread-local storage duration (rather than static storage duration) whereas static itself again has its usual meaning at namespace scope. So x is thread-local and it has internal linkage.

Upvotes: 3

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