[libc-dev] [musl] Powerpc Linux 'scv' system call ABI proposal take 2
Segher Boessenkool via libc-dev
libc-dev at lists.llvm.org
Thu Apr 16 18:48:31 PDT 2020
On Thu, Apr 16, 2020 at 08:34:42PM -0400, Rich Felker wrote:
> On Thu, Apr 16, 2020 at 06:02:35PM -0500, Segher Boessenkool wrote:
> > On Thu, Apr 16, 2020 at 08:12:19PM +0200, Florian Weimer wrote:
> > > > I think my choice would be just making the inline syscall be a single
> > > > call insn to an asm source file that out-of-lines the loading of TOC
> > > > pointer and call through it or branch based on hwcap so that it's not
> > > > repeated all over the place.
> > >
> > > I don't know how problematic control flow out of an inline asm is on
> > > POWER. But this is basically the -moutline-atomics approach.
> > Control flow out of inline asm (other than with "asm goto") is not
> > allowed at all, just like on any other target (and will not work in
> > practice, either -- just like on any other target). But the suggestion
> > was to use actual assembler code, not inline asm?
> Calling it control flow out of inline asm is something of a misnomer.
> The enclosing state is not discarded or altered; the asm statement
> exits normally, reaching the next instruction in the enclosing
> block/function as soon as the call from the asm statement returns,
> with all register/clobber constraints satisfied.
Ah. That should always Just Work, then -- our ABIs guarantee you can.
> Control flow out of inline asm would be more like longjmp, and it can
> be valid -- for instance, you can implement coroutines this way
> (assuming you switch stack correctly) or do longjmp this way (jumping
> to the location saved by setjmp). But it's not what'd be happening
Yeah, you cannot do that in C, not without making assumptions about what
machine code the compiler generates. GCC explicitly disallows it, too:
'asm' statements may not perform jumps into other 'asm' statements,
only to the listed GOTOLABELS. GCC's optimizers do not know about
other jumps; therefore they cannot take account of them when
deciding how to optimize.
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