Re: [RFC PATCH] Read pseudo registers from frame instead of regcache

[Simon Marchi <simon dot marchi at polymtl dot ca>]

On 2018-05-28 15:13, Simon Marchi wrote:
> Ok, I was assuming that it was never possible for the debug info to
> describe how pseudo registers are saved, only raw registers.  Do you
> have an architecture in mind where it's possible to have a pseudo
> register mentioned in the unwind information?  GNU as doesn't accept
> at all "ymm0" or "ymm0h" as an argument to .cfi_offset, so I don't
> know how ymm0 would be represented if we wanted to save it (despite it
> not being callee saved according to the ABI).

I looked at ARMv7, and one case where this might happen is the VFP 
registers.  The dX registers (64-bits) are the raw ones and the sX 
(32-bits) are the pseudo ones (where two sX registers make up one dX 
register).  The GNU assembler accepts using s0 with .cfi_offset (for 
backwards compatibility, I guess?), so we end up with it in the unwind 
info.  However, this is an obsolete way of doing it according to the 
"DWARF for the ARM Architecture" document [1].  They recommend to 
describe the sX registers as bit pieces of the dX registers.  That 
recommendation would be in line with the "only raw registers in unwind 
info" assumption.  But it means that we can encounter this in the wild.

Simon

[1] 
http://infocenter.arm.com/help/topic/com.arm.doc.ihi0040b/IHI0040B_aadwarf.pdf