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Date: Sat, 30 Mar 2019 13:57:32 +0100
From: Markus Wichmann <nullplan@....net>
To: musl@...ts.openwall.com
Subject: Re: Does TD point to itself intentionally?

On Sat, Mar 30, 2019 at 07:11:41AM -0400, Frediano Ziglio wrote:
> But "lea" how? It would be a rdfsbase instruction as "standard" registers
> are used for other purposes. But as you said you cannot assume rdfsbase would
> work so it's hard to inline it. Doing that way you can inline that single
> assembly instruction easily.
>
> Frediano

I don't understand the objection. I was talking about replacing
__pthread_self() with:

asm ("lea %%fs:0, %0" : "=r"(self));

In case you are unfamilliar with that instruction: If the %0 were
replaced with %rax, this would assemble to the opcode:

64 40 8d 04 25 00 00 00 00

My god... having written this down, it would apparently be cheaper (code
size wise) to encode

xorl %eax,%eax
leaq %fs:(%rax),%rax

Because in 64-bit mode you need a SIB byte to encode absolute addresses,
and the SIB byte in this mode only does 32-bit displacements. Let's see...

31 C0
64 40 8d 00

Yep. 9 bytes vs. 6 bytes. But now I'm micro-optimizing. Though this
optimization would also be valid for the current implementation.
Something like:

static inline struct pthread *__pthread_self()
{
#ifdef MY_PATCH
#define INST "lea"
#else
#define INST "mov"
#endif
	struct pthread *self = 0;
	__asm__ (INST " %%fs:0,%0" : "+r" (self) );
	return self;
}

My question was more about removing this conceptual hurdle, and making
it more clear that FS indeed points to the thread descriptor, and not a
pointer to the thread descriptor. I know full well we can't remove
"self", nor skip the initialization, since both of these are ABI.

Ciao,
Markus

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