On 29.01.2021 20:15, Victor Stinner wrote:
Hi Mark,
I tried to use C11 _Thread_local (Thread Local Storage, TLS) only with GCC and clang and I got issues on macOS: https://github.com/python/cpython/pull/23976
My PR uses __thread if _Thread_local is not available.
I don't think that MSC (Visual Studio) implements C11 _Thread_local, but it provides __declspec(thread). I tried it and I got issues with DLL. I didn't dig into this issue. MSC loves C++ but seems stuck at C99 (I'm not even sure if it's fully implemented).
According to https://docs.microsoft.com/en-us/cpp/build/reference/std-specify-language-st... and https://docs.microsoft.com/en-us/cpp/overview/install-c17-support?view=msvc-... , they only partially support C99, and C11 and C17 support is available in the Insider Preview version. The preview version supports all the required but no optional features (including optional features that were required in C99) and suggest to use features test macros. They also say that the 2 standard modes are functonally equivalent except the version macro since C17 is a bugfix release. AFAICS, this means that global C99 requirement is out of the question; C11/C17 can be considered when that MSVC version is released -- but alternative solutions are still needed for code chunks using optional features.
It seems like declaring a TLS in libpython and using it from an extension module (another dynamic library) is causing issues depending on the linker. It "should" work on macOS, but it doesn't.
See https://bugs.python.org/issue40522 for the produced machine code on x86-64. In short, it's usually a single MOV using the FS register (two MOV in the worst case).
Victor
On Thu, Jan 28, 2021 at 5:29 PM Mark Shannon <mark@hotpy.org> wrote:
Hi everyone,
PEP 7 says that C code should conform to C89 with a subset of C99 allowed. It's 2021 and all the major compilers support C11 (ignoring the optional parts).
C11 has support for thread locals, static asserts, and anonymous structs and unions. All useful features.
Is there a good reason not to start using C11 now?
Cheers, Mark.
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-- Regards, Ivan