Re: CVS commit: src/sys/lib/libunwind

[Rin Okuyama <rokuyama.rk%gmail.com@localhost>]

On 2021/05/31 23:35, Joerg Sonnenberger wrote:
> On Mon, May 31, 2021 at 12:12:24PM +0000, Rin Okuyama wrote:
> > Bump LAST_REGISTER and LAST_RESTORE_REG to REGNO_ARM32_S31 for arm.
>
> This is not desirable as it significantly increases the memory use.
> The goal here is to actually alias the single and double register in the
> space. That's what the existing was doing.

Existing code did not.

With LAST_REGISTER = REGNO_ARM32_D31, libunwind gave up unwinding frames
when it encounters s0-s31; see parseInstructions():

https://nxr.netbsd.org/xref/src/sys/lib/libunwind/DwarfParser.hpp#parseInstructions

And with LAST_RESTORE_REG = REGNO_ARM32_D31, it did not copy-back contents
of s0-s31 at all; see stepWithDwarf():

https://nxr.netbsd.org/xref/src/sys/lib/libunwind/DwarfInstructions.hpp#136

IMO, in order to support VFP-register aliasing, we need to heavily modify
MI codes. This adds complexities to the code, and only ARM would receive
benefits from them.

Existing code was not acceptable for me; for example, GDB aborted *every
time* exceptions were raised. I won't revert this change at the moment.
Please provide working alternative if you don't like mine.

Thanks,
rin

> Joerg