s390: Represent FP/RA saved in register in SFrame

GCC on s390x, when in a leaf function, can be observed to save the
frame pointer (FP) and/or return address (RA) register in a floating-
point registers (FPR) instead of on the stack.  This is declared using
the following CFI directive:

  .cfi_register <fp/ra-regnum>, <fpr-regnum>

SFrame cannot represent the FP and/or RA being saved in another
register.  It does only track the CFA base register (SP/FP), CFA offset
from CFA base register, and FP and RA save area offsets from CFA.

On s390x the FP and/or RA are only saved in another FPR when in a leaf
function.  That is a function that does not call any other function.
Therefore it can ever only be the topmost function in a call chain.
An unwinder by default has access to all registers of the function that
is the topmost on the call stack.  Therefore no further information
is required to restore FP/RA from the FPR.

Represent FP/RA saved in another register on s390x, by encoding the
DWARF register number shifted by one to the left with the least-
significant bit set in the offset as follows:

  offset = (regnum << 1) | 1

The use of the least-significant bit of the offset as indication is
possible, as the stack pointer (SP), the CFA, and any register save
area slots are 8-byte aligned according to the s390x ELF ABI:
- The stack pointer (SP) "shall maintain an 8-byte alignment". [1]
- The CFA is defined as SP at call site +160. [2]
- Pointers and 8-byte integers, such as general register values, must
  be 8-byte aligned. [3]
SFrame FP and RA stack offsets must therefore always be a multiple of
8 on s390x.  Note that for the same reason the DWARF data alignment
factor is -8 on s390x (see DWARF2_CIE_DATA_ALIGNMENT).

Add s390x-specific SFrame (error) tests for FP/RA saved in FPRs in leaf
function.

[1]: s390x ELF ABI, sections "Register Roles" and "Stack Frame
     Allocation", https://github.com/IBM/s390x-abi/releases
[2]: s390x ELF ABI, commit 4e38ad9c8a88 ("Document the CFA"),
     https://github.com/IBM/s390x-abi/commit/4e38ad9c8a88
[3]: s390x ELF ABI, section "Fundamental Types", table "Scalar types",
     https://github.com/IBM/s390x-abi/releases

include/
	* sframe.h (SFRAME_V2_S390X_OFFSET_IS_REGNUM): New s390x-
	specific macro to test whether an SFrame FP/RA offset is a DWARF
	register number.
	(SFRAME_V2_S390X_OFFSET_ENCODE_REGNUM): New s390x-specific macro
	to encode a DWARF register number into an SFrame FP/RA offset.
	(SFRAME_V2_S390X_OFFSET_DECODE_REGNUM): New s390x-specific macro
	to decode an SFrame FP/RA offset into a DWARF register number.
	* sframe-api.h (sframe_fre_get_fp_offset,
	sframe_fre_get_fp_offset): Add comment that for s390x the offset
	may be an encoded register number.

gas/
	* gen-sframe.c (s390_sframe_xlate_do_register): New S390-
	specific function.  Uses SFRAME_V2_S390X_OFFSET_ENCODE_REGNUM to
	represent FP/RA saved in another register on s390x.
	(sframe_xlate_do_register): Invoke s390_sframe_xlate_do_register
	on s390x.

libsframe/
	* sframe.c (sframe_fre_get_fp_offset, sframe_fre_get_fp_offset):
	Add comment that for s390x the offset may be an encoded register
	number.
	* sframe-dump.c (is_sframe_abi_arch_s390x): New helper to test
	whether ABI/arch is s390x.
	(dump_sframe_func_with_fres): Use
	SFRAME_V2_S390X_OFFSET_IS_REGNUM and
	SFRAME_V2_S390X_OFFSET_DECODE_REGNUM to dump FP/RA saved in
	another register on s390x.
	* doc/sframe-spec.texi (s390x): Document s390x-specific
	representation of FP/RA saved in another register.

gas/testsuite/
	* gas/cfi-sframe/cfi-sframe.exp: Update s390x-specific SFrame
	(error) tests.
	* gas/cfi-sframe/cfi-sframe-s390x-fpra-register-err-2.s: Rename
	to ...
	* gas/cfi-sframe/cfi-sframe-s390x-fpra-register-err-2.d:
	Likewise.
	* gas/cfi-sframe/cfi-sframe-s390x-fpra-register-1.s: This.  Test
	no longer triggers a warning, as SFrame can represent FP and RA
	saved in registers.
	* gas/cfi-sframe/cfi-sframe-s390x-fpra-register-1.d: Likewise.
	* gas/cfi-sframe/cfi-sframe-s390x-fpra-register-err-1.d: Test
	now triggers a different warning, as SFrame can represent FP and
	RA saved in registers, but not FP without RA saved in register.

Signed-off-by: Jens Remus <jremus@linux.ibm.com>

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