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Date: Tue, 24 Feb 2015 06:42:27 -1000
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Subject: Re: [Qemu-devel] [PATCH] target-tilegx: Execute _start and reach to
 __libc_start_main successfully
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To: Chen Gang S <gang.chen@sunrus.com.cn>, Peter Maydell <peter.maydell@linaro.org>, Chris Metcalf <cmetcalf@ezchip.com>
Cc: "walt@tilera.com" <walt@tilera.com>, Riku Voipio <riku.voipio@iki.fi>, qemu-devel <qemu-devel@nongnu.org>

On 02/24/2015 05:39 AM, Chen Gang S wrote:
> After read through target-arm/translate-a64.c, I guess, the main reason
> is: the zero register (r31) shares with the sp register (also r31).
> 
>  - So it uses cpu_reg() and cpu_reg_sp() for them.
> 
>  - For each zero register access, it will new a tcg temporary variable
>    for it, and release it after finish decoding one insn (so it will not
>    overwrite sp register.).
> 
> For tilegx, zero register (r63) does not share with other registers (sp
> is r54), so we needn't use wrap functions for it.

Perhaps aarch64 is confusing for you.  But Alpha also has zero registers, and
also uses wrapper functions.  See load_gpr and dest_gpr.

The very most important reason to use a wrapper, and thus a tcg temporary that
keeps getting re-initialized to zero, is that the tcg optimizer gets to see
that zero and optimize the code accordingly.


r~