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From: Florian Weimer <fweimer@redhat.com>
To: Stefan Hajnoczi <stefanha@gmail.com>
Cc: Richard Henderson <richard.henderson@linaro.org>,
	qemu-devel <qemu-devel@nongnu.org>,
	Stefan Hajnoczi <stefanha@redhat.com>,
	Serge Guelton <sguelton@redhat.com>
Subject: Re: Portable inline asm to get address of TLS variable
Date: Wed, 16 Feb 2022 21:46:02 +0100	[thread overview]
Message-ID: <871r02zgj9.fsf@oldenburg.str.redhat.com> (raw)
In-Reply-To: <CAJSP0QUOXwwBzXpBNhGb_uuxM8AqP2mOD_7tSSWoEhErdVnHNw@mail.gmail.com> (Stefan Hajnoczi's message of "Wed, 16 Feb 2022 20:33:39 +0000")

* Stefan Hajnoczi:

> I'm basically asking whether the &tls_var input operand is treated as
> volatile and part of the inline assembly or whether it's just regular
> C code that the compiler may optimize with the surrounding function?

&tls_var is evaluated outside of the inline assembly, any compiler
barrier will come after that.  It's subject to CSE (or whatever it's
called.  Three asm statements in a row

  asm volatile("" : "=r"(dst_ptr) : "0"(&tls_var));
  asm volatile("" : "=r"(dst_ptr) : "0"(&tls_var));
  asm volatile("" : "=r"(dst_ptr) : "0"(&tls_var));

result in

	movq	tls_var@gottpoff(%rip), %rax
	addq	%fs:0, %rax
	movq	%rax, %rdx
	movq	%rax, %rdx

which is probably not what you want.

Thanks,
Florian



  reply	other threads:[~2022-02-16 20:53 UTC|newest]

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-02-16 17:46 Portable inline asm to get address of TLS variable Stefan Hajnoczi
2022-02-16 18:13 ` Florian Weimer
2022-02-16 20:28   ` Stefan Hajnoczi
2022-02-16 20:33     ` Stefan Hajnoczi
2022-02-16 20:46       ` Florian Weimer [this message]
2022-02-17  9:30         ` Stefan Hajnoczi
2022-02-16 20:40     ` Florian Weimer
2022-02-17  9:28       ` Stefan Hajnoczi
2022-02-17 11:40         ` Paolo Bonzini
2022-02-17 15:02           ` Serge Guelton
2022-02-17 15:11             ` Stefan Hajnoczi
2022-02-17 15:51             ` Paolo Bonzini
2022-02-17 14:59         ` Serge Guelton
2022-03-01 11:54         ` Florian Weimer
2022-03-01 13:39           ` Stefan Hajnoczi
2022-04-19 11:32             ` Florian Weimer
2022-04-19 18:38               ` Thomas Rodgers
2022-04-20 14:12               ` Stefan Hajnoczi
2022-02-16 22:28 ` Paolo Bonzini

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