Augment the arm_asm.h header file to simplify function prologues and
epilogues whilst adding support for PACBTI enablement via macros for
hand-written assembly functions. For PACBTI, both prologues/epilogues
as well as cfi-related directives are automatically amended
accordingly, depending on the compile-time mbranch-protection argument
values.
It defines the following preprocessor macros:
* HAVE_PAC_LEAF: Indicates whether pac-signing has been requested for
leaf functions.
* PAC_LEAF_PUSH_IP: Whether leaf functions should push the pac code
to the stack irrespective of whether the ip register is clobbered in
the function or not.
* STACK_ALIGN_ENFORCE: Whether a dummy register should be added to
the push list as necessary in the prologue to ensure stack
alignment preservation at the start of assembly function. The
epilogue behavior is likewise affected by this flag, ensuring any
pushed dummy registers also get popped on function return.
It also defines the following assembler macros:
* prologue: In addition to pushing any callee-saved registers onto
the stack, it generates any requested pacbti instructions.
Pushed registers are specified via the optional `first', `last',
`push_ip' and `push_lr' macro argument parameters.
when a single register number is provided, it pushes that
register. When two register numbers are provided, they specify a
rage to save. If push_ip and/or push_lr are non-zero, the
respective registers are also saved. Stack alignment is requested
via the `align` argument, which defaults to the value of
STACK_ALIGN_ENFORCE, unless manually overridden.
For example:
prologue push_ip=1 -> push {ip}
prologue push_ip=1, align8=1 -> push {r2, ip}
prologue push_ip=1, push_lr=1 -> push {ip, lr}
prologue 1 -> push {r1}
prologue 1, align8=1 -> push {r0, r1}
prologue 1 push_ip=1 -> push {r1, ip}
prologue 1 4 -> push {r1-r4}
prologue 1 4 push_ip=1 -> push {r1-r4, ip}
* epilogue: pops registers off the stack and emits pac key signing
instruction, if requested. The `first', `last', `push_ip',
`push_lr' and `align' function as per the prologue macro,
generating pop instead of push instructions.
Stack alignment is enforced via the following helper macro
call-chain:
{prologue|epilogue} ->_align8 -> _preprocess_reglist ->
_preprocess_reglist1 -> {_prologue|_epilogue}
Finally, the necessary cfi directives for adding debug information
to prologue and epilogue are generated via the following macros:
* cfisavelist - prologue macro helper function, generating
necessary .cfi_offset directives associated with push instruction.
Therefore, the net effect of calling `prologue 1 2 push_ip=1' is
to generate the following:
push {r1-r2, ip}
.cfi_adjust_cfa_offset 12
.cfi_offset 143, -4
.cfi_offset 2, -8
.cfi_offset 1, -12
* cfirestorelist - epilogue macro helper function, emitting
.cfi_restore instructions prior to resetting the cfa offset. As
such, calling `epilogue 1 2 push_ip=1' will produce:
pop {r1-r2, ip}
.cfi_register 143, 12
.cfi_restore 2
.cfi_restore 1
.cfi_def_cfa_offset 0
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