Currently, the GNU/Linux ABI does not really specify whether the thread pointer (the address of the TCB) may change at a function boundary.
Traditionally, GCC assumes that the ABI allows caching addresses of thread-local variables across function calls. Such caching varies in aggressiveness between targets, probably due to differences in the choice of -mtls-dialect=gnu and -mtls-dialect=gnu2 as the default for the targets. (Caching with -mtls-dialect=gnu2 appears to be more aggressive.) In addition to that, glibc defines errno as this: extern int *__errno_location (void) __attribute__ ((__const__)); #define errno (*__errno_location ()) And the const attribute has the side effect of caching the address of errno within the same stack frame. With stackful coroutines, such address caching is only valid if coroutines are only ever resumed on the same thread on which they were suspended. (The C++ coroutine implementation is not stackful and is not affected by this at the ABI level.) Historically, I think we took the position that cross-thread resumption is undefined. But the ABIs aren't crystal-clear on this matter. One important piece of software for GNU is QEMU (not just for GNU/Linux, Hurd development also benefits from virtualization). QEMU uses stackful coroutines extensively. There are some hard-to-change code areas where resumption happens across threads unfortunately. These increasingly cause problems with more inlining, inter-procedural analysis, and a general push towards LTO (which is also needed for some security hardening features). Should the GNU toolchain offer something to help out the QEMU developers? Maybe GCC could offer an option to disable the caching for all TLS models. glibc could detect that mode based on a new preprocessor macro and adjust its __errno_location declaration and similar function declarations. There will be a performance impact of this, of course, but it would make the QEMU usage well-defined (at the lowest levels). If this is a programming model that should be supported, then restoring some of the optimizations would be possible, by annotating context-switching functions and TLS-address-dependent functions. But I think QEMU would immediately benefit from just the simple approach that disables address caching of TLS variables. Thanks, Florian
