Re: [Libunwind-devel] [PATCH] restore "x0" and "x1" registers

[C Bergström]

I'm in the middle man for this discussion and apologize about the delay..

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On Thu, Sep 25, 2014 at 9:59 PM, Yvan Roux <address@hidden> wrote:

Hi (sorry I was on a trip)

>> This patch is needed for us in AARCH64 - Restore "x0" and "x1" registers
>> because they contain exception pointer and selector.

According to the unw_resume() description for local unwinding,
restoring the machine state when no signal is involved only restore
the callee-saved registers (r19..r28 for AArch64 + the link register
r30).

Can you give me more input here why it is needed ?

Thanks,
Yvan

unw_resume man page contains the following:

Most platforms reserve some registers to pass arguments to exception handlers (e.g., IA-64 uses r15-r18 for this purpose). These registers are normally treated like "scratch" registers. However, if libunwind is used to set an exception argument register to a particular value (e.g., via
unw_set_reg()), then unw_resume() will install this value as the contents of the register. In other words, the exception handling arguments are installed even in cases where normally only the "preserved" registers are restored.