Re: [PATCH v2] RISC-V: Implement TLS Descriptors.

[Tatsuyuki Ishi <ishitatsuyuki@gmail.com>]

> On Nov 16, 2023, at 14:33, Fangrui Song <maskray@google.com> wrote:
> 
> On Wed, Nov 15, 2023 at 9:23 PM Jeff Law <jeffreyalaw@gmail.com> wrote:
>> 
>> 
>> 
>> On 11/15/23 18:51, Tatsuyuki Ishi wrote:
>>>> On Nov 16, 2023, at 10:07, Jeff Law <jeffreyalaw@gmail.com> wrote:
>> 
>>> 
>>> Based on what I have read in the AArch64 backend, there are two ways to
>>> do this: introduce a custom calling convention, or put in a RTX insn
>>> that covers the whole sequence. Ideally we should do the first, but then
>>> there’s the label issue and it’s quite a bit more complicated. So I’m
>>> sticking with this for now.
>> As I said, I think we're OK here.  We can always revamp as we get
>> experience with the implementation -- I don't think any of the stuff
>> we're talking about is an ABI change, they're just implementation details.
>> 
>>> 
>>> Sorry for all the delay on this. My progress has been (and still)
>>> blocked on supporting relaxation of TLSDESC in binutils (turns out you
>>> can’t run static binaries without relaxing it first). But that doesn’t
>>> seem exactly easy to do either, because relaxation that involves GOT
>>> elimination isn’t something we have in the RISC-V backend.
>> Note that binutils is due for another release in the next month or two.
>> It'd certainly be helpful to have any issues there resolved in time for
>> that release.
>> 
>>> 
>>> I’ll try to send a new version of this patch and get this unblocked on
>>> GCC side first.
>> Sounds good.  We can always guard its use behind a feature test for GAS
>> support.
>> 
>> Jeff
> 
> Agreed.
> 
> 
> Tatsuyuki, could you also add some tests? For example
> 
> // end of https://maskray.me/blog/2021-02-14-all-about-thread-local-storage
> __thread int tls0;
> extern __thread int tls1;
> int foo() { return ++tls0 + ++tls1; }
> static __thread int tls2, tls3;
> int bar() { return ++tls2 + ++tls3; }
> 
> I have used this to check rtld and linker behavior. I think we need
> some `scan-assembler`.
> To make it a runnable test, some assembler feature check may be
> needed. Perhaps Jeff can make some suggestion or contribute code!
> 

I believe there’s existing platform-generic TLS coverage in gcc/testsuite/gcc.dg/torture/tls. GCC's test suite seems pretty sparse, but a lot more testing is done by glibc’s testsuite (which is also where I found the static TLS relaxation issue).

Tatsuyuki.

> 
> -- 
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