This instruction, if aligned on a 4-byte boundary, defines a valid target
("landing pad") for an indirect call or jump. Since this instruction is a
HINT, it is safe to assemble even if not included in the target
instruction set architecture.
The necessary alignment is already provided by the `func` macro. However
this still lacks the ELF attribute to indicate that the zicfilp is supported
in simple mode. This is left for future work as the ELF specification is not
ratified as of yet.
This will also nonobviously require the assembler to support zicfilp,
insofar as the `tail` pseudo-instruction shall clobber T2 (instead of T1) as
its temporary register.
Currently the start of the byte range for each function is aligned to
4 bytes. But this can lead to situations whence the function is preceded
by a 2-byte C.NOP at the aligned 4-byte boundary. Then the first actual
instruction and the function symbol are only aligned on 2 bytes.
This forcefully disables compression for the alignment and the symbol,
thus ensuring that there is no padding before the function.
configure checks that the assembler supports the B extension (or rather
its constituents) anyway. These macros were dodging sanity checks for
unsupported instructions and nothing else.
The RISC-V B bit manipulation extension was ratified only two months ago.
But it is strictly equivalent to the union of the zba, zbb and zbs
extensions which were defined almost 3 years earlier. Rather than require
new assembler, we can just match the extension name manually and translate
it into its constituent parts.
vtype_vli computes the VTYPE value with the optimal LMUL for a given
element width, tail and mask policies and a run-time vector length.
vtype_ivli does the same, but with the compile-time constant vector
length.
vwtypei and vntypei can be used to widen or narrow a VTYPE value for
use in mixed-width vector-optimised functions.
On most cases, the vector type (VTYPE) for the RISC-V Vector extension
is supplied as an immediate value, with either of the VSETVLI or
VSETIVLI instructions. There is however a third instruction VSETVL
which takes the vector type from a general purpose register. That is so
the type can be selected at run-time.
This introduces a macro to load a (valid) vector type into a register.
The syntax follows that of VSETVLI and VSETIVLI, with element size,
group multiplier, then tail and mask policies.
Rémi Denis-Courmont