[RFC PATCH 0/1] sframe: Represent FP without RA on stack (bitmap)

[Jens Remus]

This patch series adds support in SFrame to represent the frame pointer
(FP) without the return address (RA) being saved on the stack (and/or on
s390x in another register).

This is the second of two proposed alternatives:
1. The alternative patch series uses a dummy padding offset (invalid
   offset from CFA value of zero) as RA offset to represent FP without
   RA on saved on the stack.
2. This patch series changes the SFrame FRE count field into
   a bitmap, to convey which offsets follow the FRE.

Note that it currently applies on top of my v3 patch series series that
adds initial support to generate .sframe from CFI directives on s390x,
although it is independent of that.

A SFrame FRE currently has a 4-bit field containing the offset count
that follow the FRE. While this could account for up to 15 offsets (or
16, when excluding the mandatory CFA offset from CFA base register), it
cannot represent which of these offsets actually follow.

Redefining the 4-bit count field into a 4-bit offset bitmap allows to
track up to 4 offsets (or 5, when excluding the mandatory CFA offset
from CFA base register).

The main downside of this approach is that this is potentially a major
change to the SFrame V2 format, which may require a bump to V3. The
benefits are that (1) it does not add any dummy padding offsets, which
would unnecessarily add bloat to the SFrame information, and that (2)
it does not change any of the external SFrame API.
Using a lookup table the bitmap can easily be translated into an offset
count. Similar any logic that checks the presence of an offset can
easily be implemented using a bit test.

Note that there is a minor implementation issue with regards to the
internal API methods (callbacks), due to the change in SFrame format
changing a method argument from count to bitmap.
Additionally this initial implementation lacks better naming of the
tracked register IDs and any update of the SFrame format specification.

Thanks and regards,
Jens


Jens Remus (1):
  sframe: Represent FP without RA on stack

 gas/gen-sframe.c   | 66 ++++++++++++++++++++++++++++------------------
 include/sframe.h   | 13 ++++++---
 libsframe/sframe.c | 51 +++++++++++++++++++++++++++--------
 3 files changed, 90 insertions(+), 40 deletions(-)

-- 
2.40.1