Re: [musl] [PATCH] riscv: Fix setjmp assembly when compiling for ilp32f/lp64f.

[Rich Felker <dalias@libc.org>]

On Wed, Jul 24, 2024 at 12:47:14AM +0200, Alex Rønne Petersen wrote:
> On Tue, Jul 23, 2024 at 11:22 PM Szabolcs Nagy <nsz@port70.net> wrote:
> >
> > * Alex Rønne Petersen <alex@alexrp.com> [2024-06-29 04:04:34 +0200]:
> > > To keep things simple, I just changed the instruction mnemonics appropriately,
> > > rather than adding complexity by changing the buffer size/offsets based on ABI.
> > >
> > > Signed-off-by: Alex Rønne Petersen <alex@alexrp.com>
> >
> > fwiw this looks good to me.
> >
> > the only weirdness is that the math code uses __riscv_flen
> > and this code __riscv_float_abi*. i don't know if there
> > is semantic difference.
> 
> `__riscv_flen` tells you the width of the FP registers on the target
> CPU. This is semantically distinct from `__riscv_float_abi`. For
> example, while it would probably be a bit silly, there's no particular
> reason why I couldn't target the LP64F ABI on an RV64IMAFDC machine.
> In that case, no code needs to concern itself with the upper bits of
> the FP registers.
> 
> I took a quick peek at some of the `__riscv_flen` checks in musl. They
> look ok. They're checking the capabilities of the machine for the
> purposes of performing a computation; they're not making ABI
> decisions. In my silly example above, if I tell the compiler to do so
> with `-march=rv64...d`, it would theoretically be fine for the
> compiler to generate double-precision float instructions for
> computations as long as values are passed/returned according to LP64F
> rules.

If you're building code for -sf or -sp ABI, but could run on a machine
with larger floating point register file, it's possible that the user
could have libc built not to use fp registers at all or only 32-bit
registers (respectively), but the calling application could be built
for and running on a machine with 64-bit registers. In this case we
need to understand what the ABI says. Are the 64-bit register, if
present, call-saved in lower ABI levels where they don't participate
in the calling convention? If so, no #ifdef is sufficient and there
must be a runtime hwcap check here to determine which form of
save/restore to do, like on arm and powerpc.

Rich