Re: atomic.h cleanup

[Rich Felker <dalias@libc.org>]

On Sun, Jan 10, 2016 at 01:21:39PM +0100, Markus Wichmann wrote:
> Hi all,
> 
> The development roadmap on the musl wiki lists the ominous point
> "atomic.h cleanup" for 1.2.0.
> 
> I assume you mean a sort of simplification and unification. I noticed
> that for the RISC arch's there are rather liberal amounts of inline
> assembly for the atomic operations. And I have always been taught, that
> as soon as you start copying code, you are probably doing it wrong.
> 
> So first thing I'd do: add a new file, let's call it atomic_debruijn.h.
> It contains an implementation of a_ctz() and a_ctz_64() based on the
> DeBruijn number. That way, all the architectures currently implementing
> a_ctz() in this manner can just include that file, and a lot of
> duplicate code goes out the window.
> 
> Second thing: We can reduce the inline assembly footprint and the amount
> of duplicate code by adding a new file, let's call it atomic_llsc.h,
> that implements a_cas(), a_cas_p(), a_swap(), a_fetch_add(), a_inc(),
> a_dec(), a_and() and a_or() in terms of new functions that would have to
> be defined, namely:
> 
> static inline void a_presync(void) - execute any barrier needed before
> attempting an atomic operation, like "dmb ish" for arm, or "sync" for
> ppc.
> 
> static inline void a_postsync(void) - execute any barrier needed
> afterwards, like "isync" for PPC, or, again, "dmb ish" for ARM.
> 
> static inline int a_ll(int*) - perform an LL on the given pointer and
> return the value there. This would be "lwarx" for PPC, or "ldrex" for
> ARM.
> 
> static inline int a_sc(int*, int) - perform an SC on the given pointer
> with the given value. Return zero iff that failed.
> 
> static inline void* a_ll_p(void*) - same as a_ll(), but with machine
> words instead of int, if that's a difference.
> 
> static inline int a_sc_p(void*, void*) - same as a_sc(), but with
> machine words.
> 
> 
> With these function we can implement e.g. CAS as:
> 
> static inline int a_cas(volatile int *p, int t, int s)
> {
>     int v;
>     do {
>         v = a_ll(p);
>         if (v != t)
>             break;
>     } while (!a_sc(p, s));
>     return v;
> }
> 
> Add some #ifdefs to only activate the pointer variations if they're
> needed (i.e. if we're on 64 bits) and Bob's your uncle.
> 
> The only hardship would be in implementing a_sc(), but that can be
> solved by using a feature often referenced but rarely seen in the wild:
> ASM goto. How that works is that, if the arch's SC instruction returns
> success or failure in a flag and the CPU can jump on that flag (unlike,
> say, microblaze, which can only jump on comparisons), then you encode
> the jump in the assembly snippet but let the compiler handle the targets
> for you. Since in all cases, we want to jump on failure, that's what the
> assembly should do, so for instance for PowerPC:
> 
> static inline int a_sc(volatile int* p, int x)
> {
>     __asm__ goto ("stwcx. %0, 0, %1\n\tbne- %l2" : : "r"(x), "r"(p) : "cc", "memory" : fail);
>     return 1;
> fail:
>     return 0;
> }
> 
> I already tried the compiler results for such a design, but I never
> tried running it for lack of hardware.
> 
> Anyway, this code makes it possible for the compiler to redirect the
> conditional jump on failure to the top of the loop in a_cas(). Since the
> return value isn't used otherwise, the values 1 and 0 never appear in
> the generated assembly.
> 
> What do you say to this design?

Have you read this thread? :)

http://www.openwall.com/lists/musl/2015/05/20/1

I thought at one point it was linked from the wiki but maybe it got
lost.

Basically I have this done already outside of musl as an experiment,
but there are minor details that were holding it up. One annoyance is
that, on some archs, success/failure of "sc" comes via a condition
flag which the C caller can't easily branch on, so there's an extra
conversion to a boolean result inside the asm and extra conversion
back to a test/branch outside the asm. In practice we probably don't
care.

One other issue is that risc-v seems to guarantee, at least on some
implementations, stronger forward-progress guarantees than a normal
ll/sc as long as the ll/sc are in order, within a few instruction
slots of each other, with no branches between. Such conditions cannot
be met without putting them in the same asm block, so we might need to
do a custom version for risc-v if we want to take advantage of the
stronger properties.

Anyway, at this point the main obstacle to finishing the task is doing
the actual merging and testing, not any new coding, I think.

Rich