Nothing is left from the original fdlibm header nor from the bsd
modifications to it other than some internal api declarations.

Comments are dropped that may be copyrightable content.

Code generation for SET_HIGH_WORD slightly changes, but it only affects
pow, otherwise the generated code is unchanged.

This makes it easier to build musl math code with a compiler that
does not support complex types (tcc) and in general more sensible
factorization of the internal headers.

FP_FAST_FMA can be defined if "the fma function generally executes about
as fast as, or faster than, a multiply and an add of double operands",
which can only be true if the fma call is inlined as an instruction.

gcc sets __FP_FAST_FMA if __builtin_fma is inlined as an instruction,
but that does not mean an fma call will be inlined (e.g. it is defined
with -fno-builtin-fma), other compilers (clang) don't even have such
macro, but this is the closest we can get.

(even if the libc fma implementation is a single instruction, the extern
call overhead is already too big when the macro is used to decide between
x*y+z and fma(x,y,z) so it cannot be based on libc only, defining the
macro unconditionally on targets which have fma in the base isa is also
incorrect: the compiler might not inline fma anyway.)

this solution works with gcc unless fma inlining is explicitly turned off.

POSIX: "[If] either O_TTY_INIT is set in oflag or O_TTY_INIT has the
value zero, open() shall set any non-standard termios structure
terminal parameters to a state that provides conforming behavior."

The Linux kernel tty drivers always perform initialisation on their
devices to set known good termios values during the open(2) call.  This
means that setting O_TTY_INIT to zero is conforming.

the weak version of __syscall_cp_c was using a tail call to __syscall
to avoid duplicating the 6-argument syscall code inline in small
static-linked programs, but now that __syscall no longer exists, the
inline expansion is no longer duplication.

the syscall.h machinery suppported up to 7 syscall arguments, only via
an external __syscall function, but we presently have no syscall call
points that actually make use of that many, and the kernel only
defines 7-argument calling conventions for arm, powerpc (32-bit), and
sh. if it turns out we need them in the future, they can easily be
added.

n32 and n64 ABIs add new argument registers vs o32, so that passing on
the stack is not necessary, so it's not clear why the 5- and
6-argument versions were special-cased to begin with; it seems to have
been pattern-copying from arch/mips (o32).

i've treated the new argument registers like the first 4 in terms of
clobber status (non-clobbered). hopefully this is correct.

the OABI passes these on the stack, using the convention that their
position on the stack is as if the first four arguments (in registers)
also had stack slots. originally this was deemed too awkward to do
inline, falling back to external __syscall, but it's not that bad and
now that external __syscall is being removed, it's necessary.

the inline syscall code is copied directly from powerpc64. the extent
of register clobber specifiers may be excessive on both; if that turns
out to be the case it can be fixed later.

it was never demonstrated to me that this workaround was needed, and
seems likely that, if there ever was any clang version for which it
was needed, it's old enough to be unusably buggy in other ways. if it
turns out some compilers actually can't do the register allocation
right, we'll need to replace this with inline shuffling code, since
the external __syscall dependency is being removed.

this is the first part of a series of patches intended to make
__syscall fully self-contained in the object file produced using
syscall.h, which will make it possible for crt1 code to perform
syscalls.

the (confusingly named) i386 __vsyscall mechanism, which this commit
removes, was introduced before the presence of a valid thread pointer
was mandatory; back then the thread pointer was setup lazily only if
threads were used. the intent was to be able to perform syscalls using
the kernel's fast entry point in the VDSO, which can use the sysenter
(Intel) or syscall (AMD) instruction instead of int $128, but without
inlining an access to the __syscall global at the point of each
syscall, which would incur a significant size cost from PIC setup
everywhere. the mechanism also shuffled registers/calling convention
around to avoid spills of call-saved registers, and to avoid
allocating ebx or ebp via asm constraints, since there are plenty of
broken-but-supported compiler versions which are incapable of
allocating ebx with -fPIC or ebp with -fno-omit-frame-pointer.

the new mechanism preserves the properties of avoiding spills and
avoiding allocation of ebx/ebp in constraints, but does it inline,
using some fairly simple register shuffling, and uses a field of the
thread structure rather than global data for the vdso-provided syscall
code address.

for now, the external __syscall function is refactored not to use the
old __vsyscall so it can be kept, but the intent is to remove it too.

this is a workaround to avoid a crashing regression on qemu-user when
dynamic TLS is installed at dlopen time. the sigaction syscall should
not be able to fail, but it does fail for implementation-internal
signals under qemu user-level emulation if the host libc qemu is
running under reserves the same signals for implementation-internal
use, since qemu makes no provision to redirect/emulate them. after
sigaction fails, the subsequent tkill would terminate the process
abnormally as the default action.

no provision to account for membarrier failing is made in the dynamic
linker code that installs new TLS. at the formal level, the missing
barrier in this case is incorrect, and perhaps we should fail the
dlopen operation, but in practice all the archs we support (and
probably all real-world archs except alpha, which isn't yet supported)
should give the right behavior with no barrier at all as a consequence
of consume-order properties.

in the long term, this workaround should be supplemented or replaced
by something better -- a different fallback approach to ensuring
memory consistency, or dynamic allocation of implementation-internal
signals. the latter is appealing in that it would allow cancellation
to work under qemu-user too, and would even allow many levels of
nested emulation.

This parameter was incorrectly declared to be a pointer to a function
accepting zero parameters.  The intent of makecontext is that it is
possible to pass integer parameters to the function, so this should
have been a pointer to a function accepting an unspecified set of
parameters.

Mark atanhi, atanlo, and aT in atanl.c as static, as they're not
intended to be part of the public API.

These are already static in the LDBL_MANT_DIG == 64 code, so this
patch is just making the LDBL_MANT_DIG == 113 code do the same thing.

The result is the same but takes less code.
Note that __execvpe calls getenv which calls __strchrnul so even
using static output the size of the executable won't grow.

commit 54ca6779 inadvertently
introduced bitwise and where logical and was intended. since the
right-hand operand is always 0 or -1 whenever the left-hand operand is
nonzero, the behavior happened to be equivalent.

priority inheritance is a feature to mitigate priority inversion
situations, where a execution of a medium-priority thread can
unboundedly block forward progress of a high-priority thread when a
lock it needs is held by a low-priority thread.

the natural way to do priority inheritance would be with a simple
futex flag to donate the calling thread's priority to a target thread
while it waits on the futex. unfortunately, linux does not offer such
an interface, but instead insists on implementing the whole locking
protocol in kernelspace with special futex commands that exist solely
for the purpose of doing PI mutexes. this would require the entire
"trylock" logic to be duplicated in the timedlock code path for PI
mutexes, since, once the previous lock holder releases the lock and
the futex call returns, the lock is already held by the caller.
obviously such code duplication is undesirable.

instead, I've made the PI timedlock success path set the mutex lock
count to -1, which can be thought of as "not yet complete", since a
lock count of 0 is "locked, with no recursive references". a simple
branch in a non-hot path of pthread_mutex_trylock can then see and act
on this state, skipping past the code that would check and take the
lock to the same code path that runs after the lock is obtained for a
non-PI mutex.

because we're forced to let the kernel perform the actual lock and
unlock operations whenever the mutex is contended, we have to patch
things up when it does the wrong thing:

1. the lock operation is not aware of whether the mutex is
   error-checking, so it will always fail with EDEADLK rather than
   deadlocking.

2. the lock operation is not aware of whether the mutex is robust, so
   it will successfully obtain mutexes in the owner-died state even if
   they're non-robust, whereas this operation should deadlock.

3. the unlock operation always sets the lock value to zero, whereas
   for robust mutexes, we want to set it to a special value indicating
   that the mutex obtained after its owner died was unlocked without
   marking it consistent, so that future operations all fail with
   ENOTRECOVERABLE.

the first of these is easy to solve, just by performing a futex wait
on a dummy futex address to simulate deadlock or ETIMEDOUT as
appropriate. but problems 2 and 3 interact in a nasty way. to solve
problem 2, we need to back out the spurious success. but if waiters
are present -- which we can't just ignore, because even if we don't
want to wake them, the calling thread is incorrectly inheriting their
priorities -- this requires using the kernel's unlock operation, which
will zero the lock value, thereby losing the "owner died with lock
held" state.

to solve these problems, we overload the mutex's waiters field, which
is unused for PI mutexes since they don't call the normal futex wait
functions, as an indicator that the PI mutex is permanently
non-lockable. originally I wanted to use the count field, but there is
one code path that needs to access this flag without synchronization:
trylock's CAS failure path needs to be able to decide whether to fail
with EBUSY or ENOTRECOVERABLE, the waiters field is already treated as
a relaxed-order atomic in our memory model, so this works out nicely.

there was no point in masking off the pshared bit when first loading
the type, since every subsequent access involves a mask anyway. not
masking it may avoid a subsequent load to check the pshared flag, and
it's just simpler.

commit 84d061d5 wrongly moved the
access to the global next_key outside of the scope of the lock. the
error manifested as spurious failure to find an available key slot
under concurrent calls to pthread_key_create, since the stopping
condition could be met after only a small number of slots were
examined.

commit d6c855ca caused this
"regression", though the behavior was undefined before, overlooking
that f->shend=0 was being used as a sentinel for "EOF" status (actual
EOF or hitting the scanf field width) of the stream helper (shgetc)
functions.

obviously the shgetc macro could be adjusted to check for a null
pointer in addition to the != comparison, but it's the hot path, and
adding extra code/branches to it begins to defeat the purpose.

so instead of setting shend to a null pointer to block further reads,
which no longer works, set it to the current position (rpos). this
makes the shgetc macro work with no change, but it breaks shunget,
which can no longer look at the value of shend to determine whether to
back up. Szabolcs Nagy suggested a solution which I'm using here:
setting shlim to a negative value is inexpensive to test at shunget
time, and automatically re-trips the cnt>=shlim stop condition in
__shgetc no matter what the original limit was.

commit 2de29bc9 left behind one
reference to pthread_mutex_trylock. fixing this also improves code
generation due to the namespace-safe version being hidde.

HWCAP_SB - speculation barrier instruction available added in linux
commit bd4fb6d270bc423a9a4098108784f7f9254c4e6d
HWCAP_PACA, HWCAP_PACG - pointer authentication instructions available
(address and generic) added in linux commit
7503197562567b57ec14feb3a9d5400ebc56812f

aarch64 pointer authentication code related prctl that allows
reinitializing the key for the thread, added in linux commit
ba830885656414101b2f8ca88786524d4bb5e8c1

NT_MIPS_MSA for ptrace access to mips simd arch reg set, added in linux
commit 3cd640832894b85b5929d5bda74505452c800421
NT_ARM_PAC_MASK for ptrace access to pointer auth code mask, added in
commit ec6e822d1a22d0eef1d1fa260dff751dba9a4258

C-SKY support was added to binutils 2.32 in commit
b8891f8d622a31306062065813fc278d8a94fe21
the elf.h change was added to glibc 2.29 in commit
4975f0c3d0131fdf697be0b1631c265e5fd39088

added in linux commit 4e21565b7fd4d9045765f697887e74a704135fe2

smoothed RTT for SCM_TIMESTAMPING_OPT_STATS control messages.
added in linux commit e8bd8fca6773ef49390269bd467bf940a0841ccf

sockopt to enable gro for udp.
added in linux commit e20cf8d3f1f763ad28a9cb3b41305b8a8a42653e

added in linux commit 5521eb4bca2db733952f068c37bdf3cd656ad23c

for armv8.5 speculative store bypass PSTATE bit support,
added in linux commit d71be2b6c0e19180b5f80a6d42039cc074a693a2

ISO7816 smart cards ioctls.
linux commit ad8c0eaa0a418ae8ef3f9217638bb86439399eac

the actual kernel definitions are

 #define TIOCGISO7816 _IOR('T', 0x42, struct serial_iso7816)
 #define TIOCSISO7816 _IOWR('T', 0x43, struct serial_iso7816)

where struct serial_iso7816 is defined in linux/serial.h as

struct serial_iso7816 {
	__u32   flags;
	__u32   tg;
	__u32   sc_fi;
	__u32   sc_di;
	__u32   clk;
	__u32   reserved[5];
};

prctls to allow per task control of indirect branch speculation on x86.

added in linux commit 9137bb27e60e554dab694eafa4cca241fa3a694f

ipv6 analogue of IP_MULTICAST_ALL sockopt.

added in linux commit 15033f0457dca569b284bef0c8d3ad55fb37eacb

new in linux commit fa788d986a3aac5069378ed04697bd06f83d3488