[llvm] r227411 - [LPM] Clean up the use of TLS in pretty stack trace and disable it

[Kristof Beyls]

Hi Eric,

 

I’ve experimented a bit with a *very* simple program using a TLS variable.

It seems to me that in binutils 2.25, ld.bfd supports non-PIC compiled code and ld.gold support PIC-compiled code.
But neither supports both PIC and non-PIC compiled TLS variables.

For details of my experiments, see below.

 

$ cat t3.cpp

static volatile thread_local int tls_i;

 

int main() {

  tls_i = 4;

  return tls_i;

}

# Works with g++ 4.9.1:

$ g++ -O3 -std=c++11 -o t3_g++-4.9.1_O3 t3.cpp

# breaks with clang++-3.5 in combination with gold 2.25:

$ clang++-3.5 -O3 -std=c++11 -o t3_clang++-3.5_O3 t3.cpp -Wl,-v

GNU gold (GNU Binutils for Debian 2.25) 1.11

/usr/bin/ld: error: /tmp/t3-4c39c0.o: unsupported reloc 545 against local symbol

/usr/bin/ld: error: /tmp/t3-4c39c0.o: unsupported reloc 548 against local symbol

/tmp/t3-4c39c0.o:t3.cpp:function main: error: cannot relocate invalid reloc 545 in object file

/tmp/t3-4c39c0.o:t3.cpp:function main: error: cannot relocate invalid reloc 548 in object file

clang: error: linker command failed with exit code 1 (use -v to see invocation)

# works with clang++-3.5 and ld.bfd 2.25:

$ clang++-3.5 -O3 -std=c++11 -o t3_clang++-3.5_O3 t3.cpp -Wl,-v -fuse-ld=bfd

GNU ld (GNU Binutils for Debian) 2.25

llvm-test at llvm-farm-juno-5:~/kristof$ ./t3_clang++-3.5_O3; echo $?

4

# SUMMARY: so far, gold doesn’t support some TLS relocations that clang produces.

# Try another example to show the problem with ld.bfd, on relocation 0x20c=524=R_AARCH64_TLSLD_MOVW_DTPREL_G1:

clang++-3.5 -fPIC -O3 -std=c++11 -o t3_clang++-3.5_O3 t3.cpp -Wl,-v -fuse-ld=bfd

GNU ld (GNU Binutils for Debian) 2.25

/usr/bin/ld.bfd: /tmp/t3-3b0353.o: unrecognized relocation (0x20c) in section `.text'

/usr/bin/ld.bfd: final link failed: Bad value

clang: error: linker command failed with exit code 1 (use -v to see invocation)

llvm-test at llvm-farm-juno-5:~/kristof$ clang++-3.5 -fPIC -O3 -std=c++11 -o t3_clang++-3.5_O3 t3.cpp -Wl,-v

GNU gold (GNU Binutils for Debian 2.25) 1.11

 

 

From: Eric Christopher [mailto:echristo at gmail.com] 
Sent: 29 January 2015 19:38
To: Kristof Beyls; Chandler Carruth; Hans Wennborg; jingyu at google.com
Cc: Commit Messages and Patches for LLVM
Subject: Re: [llvm] r227411 - [LPM] Clean up the use of TLS in pretty stack trace and disable it

 

Hi Kristof,

 

I know Jing (on this email) was working on gold tls relocations when I noticed this a while back. I'm pretty sure she implemented them (at least I can link now), but I'm not sure how well it's working. Can you confirm or elaborate more?

 

-eric

 

On Thu Jan 29 2015 at 11:25:23 AM Kristof Beyls <kristof.beyls at arm.com> wrote:

For AArch64, clang produces relocations for TLS variables that ld.bfd or ld.gold can’t handle.

It seems that gcc, ld and gas only support the “small memory model 16Mib TLS address model”,
and AFAIK, it may take a while for these to grow support for other address models.

 

I haven’t had time to come fully up to speed on this, but at the moment, the only workable
solution I can think of is to:

·         Not use thread_local variables in LLVM for the 3.6 release on AArch64, so that clang 3.6
can bootstrap itself on AArch64-linux.

·         On trunk, let clang targeting AArch64-ELF systems only produce the relocations that ld supports.
At first sight that seems possible, but I’m not sure.

 

Does anyone have a better set of suggestions than the above?

 

Thanks,

 

Kristof

 

From: llvm-commits-bounces at cs.uiuc.edu [mailto:llvm-commits-bounces at cs.uiuc.edu] On Behalf Of Chandler Carruth
Sent: 29 January 2015 18:59
To: Hans Wennborg
Cc: Commit Messages and Patches for LLVM
Subject: Re: [llvm] r227411 - [LPM] Clean up the use of TLS in pretty stack trace and disable it

 

I'm still waiting for some of the dust to settle with ppc and arm. I'll have a list before long and can help merge.

On Jan 29, 2015 9:50 AM, "Hans Wennborg" <hans at chromium.org> wrote:

On Wed, Jan 28, 2015 at 5:23 PM, Chandler Carruth <chandlerc at gmail.com> wrote:
> Author: chandlerc
> Date: Wed Jan 28 19:23:04 2015
> New Revision: 227411
>
> URL: http://llvm.org/viewvc/llvm-project?rev=227411 <http://llvm.org/viewvc/llvm-project?rev=227411&view=rev> &view=rev
> Log:
> [LPM] Clean up the use of TLS in pretty stack trace and disable it
> entirely when threads are not enabled. This should allow anyone who
> needs to bootstrap or cope with a host loader without TLS support to
> limp along without threading support.

I assume we want this (and r227412) for the 3.6 branch. Any others?
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