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Date: Wed, 17 Oct 2012 09:48:56 +0200
From: musl <b.brezillon.musl@...il.com>
To: musl@...ts.openwall.com
Subject: Re: TLS (thread-local storage) support

On 17/10/2012 03:58, Rich Felker wrote:
> On Wed, Oct 17, 2012 at 03:49:33AM +0200, boris brezillon wrote:
>> 2012/10/17 Rich Felker <dalias@...ifal.cx>:
>>> On Wed, Oct 17, 2012 at 02:08:11AM +0200, boris brezillon wrote:
>>>>>> I agree. This should be made optional. But if we don't compile libc
>>>>>> with fsplit-stack (-fnosplit-stack).
>>>>>> Each call to a libc func from an external func compiled with split
>>>>>> stack may lead to a 64K stack chunk alloc.
>>>>> Where does this allocation take place from? There should simply be a
>>>>> way to inhibit it.
>>>> In the linker (gold linker).
>>> Well gold isn't running at runtime. I assume you mean it _arranges_
>>> for this allocation to take place somehow, and that's what I'm
>>> wondering about whether there's a way to avoid.
>> The easiest way to avoid big stack chunk allocation is to compile musl
>> with -fno-split-stack option.
>> This will not add any overhead to functions (no split stack prolog)
>> And this will add a note to the shared object which tells the linker
>> to avoid __morestack to __morestack_non_split replacement.
> Where is this documented? The GCC manual doesn't mention anything
> about -fno-split-stack having special behavior like that, so for lack
> of any documentation otherwise, it "should" just be the option to turn
> off -fsplit-stack..
You're right, I misunderstood how -fno-split-stack was implemented.
I tried to compile a source file with -fno-split-stack and didn't find any 'no-split-stack' note in the generated object
file.
When I compile it with -fsplit-stack both 'no-split-stack' and 'split-stack' notes are added.


>
> I'm not claiming you're wrong, just that this all seems poorly
> documented.
>
> Rich

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