The operand sepcifiers in a_cas and a_cas_p for riscv64 were incorrect:
there's a backwards branch in the routine, so despite tmp being written
at the end of the assembly fragment it cannot be allocated in one of the
input registers because the input values may be needed for another trip
around the loop.
For code that follows the guaranteed forward progress requirements, the
backwards branch is rarely taken: SiFive's hardware only fails a store
conditional on execptional cases (ie, instruction cache misses inside
the loop), and until recently a bug in QEMU allowed back-to-back
store conditionals to succeed. The bug has been fixed in the latest
QEMU release, but it turns out that the fix caused this latent bug in
musl to manifest.
the "A" constraint is simply for an address expression that's a single
register, but it's not yet supported by clang, and has no advantage
here over just using a register operand for the address. the latter is
actually preferable in the a_cas_p case because it avoids aliasing an
lvalue onto the memory.
most egregious problem was the lack of memory clobber and lack of
volatile asm; this made the atomics memory barriers but not compiler
barriers. use of "+r" rather than "=r" for a clobbered temp was also
wrong, since the initial value is indeterminate.
Author: Alex Suykov <alex.suykov@gmail.com>
Author: Aric Belsito <lluixhi@gmail.com>
Author: Drew DeVault <sir@cmpwn.com>
Author: Michael Clark <mjc@sifive.com>
Author: Michael Forney <mforney@mforney.org>
Author: Stefan O'Rear <sorear2@gmail.com>
This port has involved the work of many people over several years. I
have tried to ensure that everyone with substantial contributions has
been credited above; if any omissions are found they will be noted
later in an update to the authors/contributors list in the COPYRIGHT
file.
The version committed here comes from the riscv/riscv-musl repo's
commit 3fe7e2c75df78eef42dcdc352a55757729f451e2, with minor changes by
me for issues found during final review:
- a_ll/a_sc atomics are removed (according to the ISA spec, lr/sc
are not safe to use in separate inline asm fragments)
- a_cas[_p] is fixed to be a memory barrier
- the call from the _start assembly into the C part of crt1/ldso is
changed to allow for the possibility that the linker does not place
them nearby each other.
- DTP_OFFSET is defined correctly so that local-dynamic TLS works
- reloc.h LDSO_ARCH logic is simplified and made explicit.
- unused, non-functional crti/n asm files are removed.
- an empty .sdata section is added to crt1 so that the
__global_pointer reference is resolvable.
- indentation style errors in some asm files are fixed.
Palmer Dabbelt
Rich Felker