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Date: Wed, 12 Feb 2025 19:57:50 +0100
To: cygwin@cygwin.com
Subject: Re: __builtin_thread_pointer
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From: Corinna Vinschen via Cygwin <cygwin@cygwin.com>
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Cc: Corinna Vinschen <corinna-cygwin@cygwin.com>
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On Feb 11 23:52, Dimitry Andric via Cygwin wrote:
> Yes, that is the RISCV or SH specific method. On x86_64, you access it via the fs register, i.e. gcc (on Linux at least :) compiles:
> 
>   void *p = __builtin_thread_pointer();
> 
> to something like:
> 
>   	movq	%fs:0, %rax
> 	movq	%rax, -8(%rbp)
> 
> If somebody knows how to build the Cygwin gcc package from scratch,
> they could inspect the config.log file, to see what that says about
> TLS detection.

https://cygwin.com/pipermail/cygwin/2025-February/257300.html

As I wrote it's different on Windows and there's no good equivalent.

On Windows, %fs points to the TEB on 32 bit x86, while it's %gs on
x86_64.

The TEB is not the same thing as the TCB in NPTL, which is what
__builtin_thread_pointer() returns.

I don't think we can implement this in a way which is even marginally
similar...


Corinna

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