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> > Thread-local is way too magical for me. > There's a perfectly cromulent register just begging to be used; [...] what magic are you afraid of here? Most of

by cesarb 2y ago

> > Thread-local is way too magical for me. > There's a perfectly cromulent register just begging to be used; [...] what magic are you afraid of here? Most of the magic is not when using the thread-local variable, but when allocating it. When you declare a "static __thread char *p", how do you know that for instance this is located at the 123th word of the per-thread area? What if that declaration is on a dynamic library, which was loaded late (dlopen) into the process? What about threads which were started before that dynamic library was loaded, and therefore did not have enough space in their per-thread area for that thread-local variable, when they call into code which references it? What happens if the thread-local variable has an initializer? The documentation at https://gcc.gnu.org/onlinedocs/gcc/Thread-Local.html https://gcc.gnu.org/onlinedocs/gcc/Thread-Local.html links to a 81-page document describing four TLS access models, and that's just for Unix-style ELF; Windows platforms have their own complexities (which IIRC includes a per-process maximum of 64 or 1088 TLS slots, with slots above the first 64 being handled in a slightly different way).

maccard 2y ago
The initialisation model in c++ is totally and utterly broken and indecipherable. That doesn’t stop me from doing vector<int> foo = {1,2, 3};
intelVISA 2y ago
Avoiding thread locals due to dynamic libraries being bad is justified but still doesn't feel like the right tradeoff.
AshamedCaptain 2y ago
When you declare a `static char *p;', how do you even know in which address of memory it is going to end up ?? How do you know what will happen if another compilation unit declares another variable of the same name? Another static library? Another dynamic library? What about initialization, what about other constructors that may read memory before main() runs? What about injected threads that are started before that? Madness, I tell you, absolute and utter madness.
geocar 2y ago
> When you declare a "static __thread char p", how do you know that for instance this is located at the 123th word of the per-thread area? Believe it or not, it's exactly the same way it knows that "static char p" is located at the 123rd word of the data section: The linker does it! > What if that declaration is on a dynamic library, which was loaded late (dlopen) into the process? Same thing, except the linker is dynamic. > What about threads which were started before that dynamic library was loaded, and therefore did not have enough space in their per-thread area for that thread-local variable, when they call into code which references it? What happens if the thread-local variable has an initializer? Seriously? I mean, you probably do have enough space because address space is huge, but dlopen should move TLS. > links to a 81-page document describing four TLS access models, and that's just for Unix-style ELF Just because someone can write 81-pages about something doesn't mean that it takes 81-pages to understand something.

by cesarb 2y ago

maccard 2y ago

The initialisation model in c++ is totally and utterly broken and indecipherable. That doesn’t stop me from doing vector<int> foo = {1,2, 3};

intelVISA 2y ago

Avoiding thread locals due to dynamic libraries being bad is justified but still doesn't feel like the right tradeoff.

AshamedCaptain 2y ago

When you declare a `static char *p;', how do you even know in which address of memory it is going to end up ?? How do you know what will happen if another compilation unit declares another variable of the same name? Another static library? Another dynamic library? What about initialization, what about other constructors that may read memory before main() runs? What about injected threads that are started before that? Madness, I tell you, absolute and utter madness.

geocar 2y ago

> When you declare a "static __thread char p", how do you know that for instance this is located at the 123th word of the per-thread area? Believe it or not, it's exactly the same way it knows that "static char p" is located at the 123rd word of the data section: The linker does it! > What if that declaration is on a dynamic library, which was loaded late (dlopen) into the process? Same thing, except the linker is dynamic. > What about threads which were started before that dynamic library was loaded, and therefore did not have enough space in their per-thread area for that thread-local variable, when they call into code which references it? What happens if the thread-local variable has an initializer? Seriously? I mean, you probably do have enough space because address space is huge, but dlopen should move TLS. > links to a 81-page document describing four TLS access models, and that's just for Unix-style ELF Just because someone can write 81-pages about something doesn't mean that it takes 81-pages to understand something.