[llvm-dev] -msave-args backend support for x86_64

[Joerg Sonnenberger via llvm-dev]

On Thu, Apr 27, 2017 at 10:53:18PM +1000, David Gwynne via llvm-dev wrote:
> that assumes that shipping dwarf is reasonable and handling dwarf is
> simple. in my situation im trying to make an in kernel debugger more
> useful. the debugger is the thing that happens when the kernel crashes.
> adding dwarf to the kernel increases its size by more than a factor of
> 4 (44.7M vs 10.6M), and adding code for dwarf handling would increase
> that size further.

This doesn't need full debug data, just .eh_frame when emitting the
proper markers. From a debugger perspective it means unwinding into the
caller's frame and asking for the value of the registers at the call
site. The advantage is that code generation can pick different options
based on the individual requirements, i.e. it no longer has to be a
strict binary choice but can accomodate different use cases ("I will use
a debugger, so skip redundant instructions if it is still in a
register").

Joerg