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sh needs runtime-selected atomic backends since there are a number of
supported models that use non-forwards-compatible (non-smp-compatible)
atomic mechanisms. previously, the code paths for this were highly
inefficient since they involved C function calls with multiple
branches in the callee and heavy spills in the caller. the new code
performs calls the runtime-selected asm fragment from inline asm with
extremely minimal clobbers, rather than using a function call.
for the sh4a case where the atomic mechanism is known and there is no
forward-compatibility issue, the movli.l and movco.l instructions are
provided as a_ll and a_sc, allowing the new shared atomic.h to
generate efficient inline versions of all the basic atomic operations
without needing a cas loop.
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previously, only archs that needed to do stack cleanup defined a
__cp_cancel label for acting on cancellation in their syscall asm, and
a default definition was provided by a weak alias to __cancel, the C
function. this resulted in wrong codegen for arm on gcc versions
affected by pr 68178 and possibly similar issues (like pr 66609) on
other archs, and also created an inconsistency where the __cp_begin
and __cp_end labels were treated as const data but __cp_cancel was
treated as a function. this in turn caused incorrect code generation
on archs where function pointers point to function descriptors rather
than code (for now, only sh/fdpic).
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clone calls back to a function pointer provided by the caller, which
will actually be a pointer to a function descriptor on fdpic. the
obvious solution is to have a separate version of clone for fdpic, but
I have taken a simpler approach to go around the problem. instead of
calling the pointed-to function from asm, a direct call is made to an
internal C function which then calls the pointed-to function. this
lets the C compiler generate the appropriate calling convention for an
indirect call with no need for ABI-specific assembly.
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nominally the low bits of the trap number on sh are the number of
syscall arguments, but they have never been used by the kernel, and
some code making syscalls does not even know the number of arguments
and needs to pass an arbitrary high number anyway.
sh3/sh4 traditionally used the trap range 16-31 for syscalls, but part
of this range overlapped with hardware exceptions/interrupts on sh2
hardware, so an incompatible range 32-47 was chosen for sh2.
using trap number 31 everywhere, since it's in the existing sh3/sh4
range and does not conflict with sh2 hardware, is a proposed
unification of the kernel syscall convention that will allow binaries
to be shared between sh2 and sh3/sh4. if this is not accepted into the
kernel, we can refit the sh2 target with runtime selection mechanisms
for the trap number, but doing so would be invasive and would entail
non-trivial overhead.
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due to the way the interrupt and syscall trap mechanism works,
userspace on sh2 must never set the stack pointer to an invalid value.
thus, the approach used on most archs, where __unmapself executes with
no stack for the interval between SYS_munmap and SYS_exit, is not
viable on sh2.
in order not to pessimize sh3/sh4, the sh asm version of __unmapself
is not removed. instead it's renamed and redirected through code that
calls either the generic (safe) __unmapself or the sh3/sh4 asm,
depending on compile-time and run-time conditions.
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the sh2 target is being considered an ISA subset of sh3/sh4, in the
sense that binaries built for sh2 are intended to be usable on later
cpu models/kernels with mmu support. so rather than hard-coding
sh2-specific atomics, the runtime atomic selection mechanisms that was
already in place has been extended to add sh2 atomics.
at this time, the sh2 atomics are not SMP-compatible; since the ISA
lacks actual atomic operations, the new code instead masks interrupts
for the duration of the atomic operation, producing an atomic result
on single-core. this is only possible because the kernel/hardware does
not impose protections against userspace doing so. additional changes
will be needed to support future SMP systems.
care has been taken to avoid producing significant additional code
size in the case where it's known at compile-time that the target is
not sh2 and does not need sh2-specific code.
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even hidden functions need @PLT symbol references; otherwise an
absolute address is produced instead of a PC-relative one.
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this caused the dynamic linker/startup code to abort when r0 happened
to contain a negative value.
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in a few places, non-hidden symbols were referenced from asm in ways
that assumed ld-time binding. while these is no semantic reason these
symbols need to be hidden, fixing the references without making them
hidden was going to be ugly, and hidden reduces some bloat anyway.
in the asm files, .global/.hidden directives have been moved to the
top to unclutter the actual code.
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linux, gcc, etc. all use "sh" as the name for the superh arch. there
was already some inconsistency internally in musl: the dynamic linker
was searching for "ld-musl-sh.path" as its path file despite its own
name being "ld-musl-superh.so.1". there was some sentiment in both
directions as to how to resolve the inconsistency, but overall "sh"
was favored.
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