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Re: [PATCH v3] MIPS: Changed current_thread_info() to an equivalent supp

To: Daniel Sanders <daniel.sanders@imgtec.com>
Subject: Re: [PATCH v3] MIPS: Changed current_thread_info() to an equivalent supported by both clang and GCC
From: Behan Webster <behanw@converseincode.com>
Date: Tue, 24 Feb 2015 11:29:59 -0800
Cc: Ralf Baechle <ralf@linux-mips.org>, Paul Burton <paul.burton@imgtec.com>, Markos Chandras <markos.chandras@imgtec.com>, James Hogan <james.hogan@imgtec.com>, linux-mips@linux-mips.org, Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>, David Daney <ddaney.cavm@gmail.com>
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On 02/24/15 07:02, Daniel Sanders wrote:
> Without this, a 'break' instruction is executed very early in the boot and
> the boot hangs.
>
> The problem is that clang doesn't honour named registers on local variables
> and silently treats them as normal uninitialized variables. However, it
> does honour them on global variables.
>
> Signed-off-by: Daniel Sanders <daniel.sanders@imgtec.com>
> Cc: Ralf Baechle <ralf@linux-mips.org>
> Cc: Paul Burton <paul.burton@imgtec.com>
> Cc: Markos Chandras <markos.chandras@imgtec.com>
> Cc: James Hogan <james.hogan@imgtec.com>
> Cc: linux-mips@linux-mips.org
> Cc: Behan Webster <behanw@converseincode.com>
> Cc: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
> Cc: David Daney <ddaney.cavm@gmail.com>
> ---
> v2 of this patch has been updated following David Daney's request to preserve
> the name of the original named register local.
>
> v3 of this patch just rebases to master and adds some background discussion 
> from
> the previous threads.
>
> For reference, a similar patch for ARM's stack pointer has already been 
> merged:
>   0abc08b ARM: 8170/1: Add global named register current_stack_pointer for ARM
>
> LLVM is unlikely to support uninitialized reads of named register locals in 
> the
> foreseeable future. There are some significant implementation difficulties and
> there were objections based on the future direction of LLVM. The thread is at
> http://lists.cs.uiuc.edu/pipermail/llvmdev/2014-March/071555.html. I've linked
> to the bit where the issues started to be discussed rather than the start of
> the thread.
>
> Difficulty and objections aside, it's also a very large amount of work to
> support a single (as far as I know) user of named register locals, especially
> when the kernel has already accepted patches to switch named register locals 
> to
> named register globals in the arm and arm64/aarch64 arches.
>
>  arch/mips/include/asm/thread_info.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/mips/include/asm/thread_info.h 
> b/arch/mips/include/asm/thread_info.h
> index 55ed660..2f0dba3 100644
> --- a/arch/mips/include/asm/thread_info.h
> +++ b/arch/mips/include/asm/thread_info.h
> @@ -55,10 +55,10 @@ struct thread_info {
>  #define init_stack           (init_thread_union.stack)
>  
>  /* How to get the thread information struct from C.  */
> +register struct thread_info *__current_thread_info __asm__("$28");
> +
>  static inline struct thread_info *current_thread_info(void)
>  {
> -     register struct thread_info *__current_thread_info __asm__("$28");
> -
>       return __current_thread_info;
>  }
>  

Acked-by: Behan Webster <behanw@converseincode.com>

-- 
Behan Webster
behanw@converseincode.com


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