Using Thread Local Storage on riscv

[Anitha Quest]

Hi All, 

I am trying to use Thread Local Storage, understand the usage on riscv 

Below is what I am doing : 

referring to TLS section in below doc

https://github.com/riscv/riscv-elf-psabi-doc/blob/master/riscv-elf.md

1) I have the appropriate attributes for TLS variables 

__thread int TLS_var1 __attribute__((tls_model("local-exec")));

__thread int TLS_var2 __attribute__((tls_model("local-exec")))

posix_thread1(){

TLS_var1++;

..

}

posix_thread1(){

TLS_var2++;

..

}

2) linker script has the following sections 

/* Thread Local Storage sections  */

  .tdata  :

   {

     PROVIDE_HIDDEN (__tdata_start = .);

     *(.tdata .tdata.* .gnu.linkonce.td.*)

   }

  .tbss  : { *(.tbss .tbss.* .gnu.linkonce.tb.*) *(.tcommon) }

3) I am building the code with compiler options 

-mcmodel=medany -static -march=rv64imac -mabi=lp64 -ggdb -Og -ftls-model=local-exec

Questions : 

I was not sure what should $tp (thread pointer) be assigned to ? 

 How to find out the start of thread control block area ?

Please let me know, what I am missing to get a basic program using TLS working. 

Thanks,

Anitha

[Jim Wilson]

On Mon, Jul 5, 2021 at 11:48 PM Anitha Quest <anitha....@gmail.com> wrote:

I am trying to use Thread Local Storage, understand the usage on riscv 

Questions : 

I was not sure what should $tp (thread pointer) be assigned to ? 

 How to find out the start of thread control block area ?

The OS creates the thread control block and sets $tp when starting a process.  If you are using a linux compiler and linux simulator/hardware, then TLS should just work.  If you are using an embedded elf compiler, then you need to write some basic OS support to manage processes and TLS which is probably a fair amount of work.

Jim

[Sober Liu]

You may find example in riscv-test.

To init tp and sp, https://github.com/riscv/riscv-tests/blob/master/benchmarks/common/crt.S#L119

To copy tls data for each hart: https://github.com/riscv/riscv-tests/blob/master/benchmarks/common/syscalls.c#L96

 

 

From: Jim Wilson <ji...@sifive.com>
Sent: 2021年7月7日 7:12
To: Anitha Quest <anitha....@gmail.com>
Cc: RISC-V SW Dev <sw-...@groups.riscv.org>
Subject: Re: [sw-dev] Using Thread Local Storage on riscv

 

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[Anitha Quest]

Thanks for the info Liu and Jim, 

I have a follow on question. 

Of different TLS models mentioned - https://github.com/riscv/riscv-elf-psabi-doc/blob/master/riscv-elf.md#thread-local-storage

Local Exec , Initial Exec ..., what is the default TLS model when no compiler flag is specified ? i am guessing its Local Exec as i using static linking ? 

Let me know, 

Anee

[Tommy Murphy]

Any use?

https://gcc.gnu.org/onlinedocs/gcc/Code-Gen-Options.html

-ftls-model=model

Alter the thread-local storage model to be used (see Thread-Local). The model argument should be one of ‘global-dynamic’, ‘local-dynamic’, ‘initial-exec’ or ‘local-exec’. Note that the choice is subject to optimization: the compiler may use a more efficient model for symbols not visible outside of the translation unit, or if -fpic is not given on the command line.

The default without -fpic is ‘initial-exec’; with -fpic the default is ‘global-dynamic’.

---

From: Anitha Quest <anitha....@gmail.com>
Sent: Thursday, July 22, 2021 6:12:44 PM
To: Sober Liu <sob...@nvidia.com>
Cc: Jim Wilson <ji...@sifive.com>; RISC-V SW Dev <sw-...@groups.riscv.org>

To view this discussion on the web visit https://groups.google.com/a/groups.riscv.org/d/msgid/sw-dev/CACmAOAkUjctk-bz4pigrAR6c6LWbGqw%2BRQMkKXNn9S1e-4%3DS8A%40mail.gmail.com.

[Jim Wilson]

On Thu, Jul 22, 2021 at 10:12 AM Anitha Quest <anitha....@gmail.com> wrote:

Of different TLS models mentioned - https://github.com/riscv/riscv-elf-psabi-doc/blob/master/riscv-elf.md#thread-local-storage

Local Exec , Initial Exec ..., what is the default TLS model when no compiler flag is specified ? i am guessing its Local Exec as i using static linking ? 

The ultimate reference for TLS is

    https://akkadia.org/drepper/tls.pdf

It is very technical but explains how everything works.

GCC defaults to the global exec model, but optimizes to a more efficient tls model if it can.  The code is

    https://gcc.gnu.org/git/?p=gcc.git;a=blob;f=gcc/varasm.c;h=53cf6dea3f367bdd182bc9eb40970991cfd9b668;hb=HEAD#l6669

So if not a shared library and local, then local exec model.  If not a shared library and not local, then initial exec model.  If optimizing and local, then local dynamic.  Otherwise, global dynamic.  And -ftls-model= sets the default which is global dynamic if the option is not used.  The code will then be further optimized at link time if possible.

Jim

[Anitha Quest]

Thanks! Jim

[Tommy Murphy]

Hi Jim

> GCC defaults to the global exec model, but optimizes to a more efficient tls model if it can. 

That's not what the GCC manual says.

Is it incorrect?

For RISC-V specifically?

In general?

> https://gcc.gnu.org/onlinedocs/gcc/Code-Gen-Options.html

>

> -ftls-model=model

> ...

[Jim Wilson]

Everything is in agreement here.

-fpic means compile for a shared library.  So it is global-dynamic for a shared library and initial-exec if not a shared library.

Internally, gcc sets the default tls model to global-dynamic, and then optimizes it to initial-exec when not -fpic.  But exactly how the internal implementation works doesn't matter except to developers.  What the end user sees is that the default for pic is global-dynamic and the default for not-pic is initial-exec.

Jim

[Tommy Murphy]

Hi Jim

I'm confused by this:

Originally you said 

> GCC defaults to the global exec model, but optimizes to a more efficient tls model if it can. 

But the manual says:

> The model argument should be one of ‘global-dynamic’, ‘local-dynamic’, ‘initial-exec’ or ‘local-exec’.

When you said "global exec" did you actually mean "global dynamic"?

Thanks

Regards

Tommy

[Jim Wilson]

On Mon, Jul 26, 2021 at 2:28 PM Tommy Murphy <tommy_...@hotmail.com> wrote:

When you said "global exec" did you actually mean "global dynamic"?

Yes, sorry, global dynamic.

Jim

[Tommy Murphy]

Thanks Jim.