> On Mon, Aug 14, 2023 at 12:27:48PM +0000, David Laight wrote:
> > From: Willy Tarreau
> > > Sent: 14 August 2023 13:10
> > >
> > > Hi David,
> > >
> > > On Mon, Aug 14, 2023 at 11:15:51AM +0000, David Laight wrote:
> > > > From: Zhangjin Wu
> > > > > Sent: 14 August 2023 11:42
> > > > ...
> > > > > [...]
> > > > > > > > Sure it's not pretty, and I'd rather just go back to SET_ERRNO() to be
> > > > > > > > honest, because we're there just because of the temptation to remove
> > > > > > > > lines that were not causing any difficulties :-/
> > > > > > > >
> > > > > > > > I think we can do something in-between and deal only with signed returns,
> > > > > > > > and explicitly place the test for MAX_ERRNO on the two unsigned ones
> > > > > > > > (brk and mmap). It should look approximately like this:
> > > > > > > >
> > > > > > > > #define __sysret(arg) \
> > > > > > > > ({ \
> > > > > > > > __typeof__(arg) __sysret_arg = (arg); \
> > > > > > > > (__sysret_arg < 0) ? ({ /* error ? */ \
> > > > > > > > SET_ERRNO(-__sysret_arg); /* yes: errno != -ret */ \
> > > > > > > > ((__typeof__(arg)) -1); /* return -1 */ \
> > > >
> > > > I'm pretty sure you don't need the explicit cast.
> > > > (It would be needed for a pointer type.)
> > > > Can you use __arg < ? SET_ERRNO(-__arg), -1 : __arg
> > > >
> > > > Thinking, maybe it should be:
> > > >
> > > > #define __sysret(syscall_fn_args)
> > > > ({
> > > > __typeof__(syscall_fn_args) __rval = syscall_fn_args;
> > > > __rval >= 0 ? __rval : SET_ERRNO(-__rval), -1;
> > > > })
> > >
> > > Yeah almost, since arg is necessarily signed in this version, it's
> > > just that I manually edited the previous macro in the mail and limited
> > > the amount of changes to what was necessary. It's just that SET_ERRNO
> > > only is an instruction, not an expression:
> > >
> > > #define SET_ERRNO(v) do { errno = (v); } while (0)
> > >
> > > Thus the return value doesn't even pass through it. That's why it was
> > > so much simpler before. The rationale behind this was to bring the
> > > ability to completely drop errno for programs where you didn't care
> > > about it. It's particularly interesting when you don't need any other
> > > data either as the program gets strunk from a complete section.
> >
> > Actually something like:
> >
> > #define SET_ERRNO(v) (errno = -(long)(v), __typeof__(v)-1)
> >
> > seems to work and allows the errno assignment be removed.
> > Also works for pointer types (after a different compare).
>
> Yes, that's something we can do (with the parenthesis around
> __typeof__(v) though).
>
Yes, we need the parenthesis, this works:
#define SET_ERRNO(v) ( errno = -(long)(v), ((__typeof__(v))-1))
#define __is_unsigned_type(v) ((__typeof__(v))(-1) > (__typeof__(v))1)
#define __is_syserr(v) (__is_unsigned_type(v) ? (long)v & ~(-4095UL) : (v + 1 < (__typeof__(v))1))
#define __sysret(arg) \
({ \
__typeof__(arg) __sysret_arg = (arg); \
(!__is_syserr(__sysret_arg)) ? __sysret_arg : SET_ERRNO(__sysret_arg); \
})
It looks better now, even for 'void *'.
'(long)x & ~(-4095UL)' saves another 2+ bytes in some architectures:
i386: 160 test(s): 158 passed, 2 skipped, 0 failed => status: warning 19256
x86_64: 160 test(s): 158 passed, 2 skipped, 0 failed => status: warning 21740
arm64: 160 test(s): 158 passed, 2 skipped, 0 failed => status: warning 25812
arm: 160 test(s): 158 passed, 2 skipped, 0 failed => status: warning 22856
mips: 160 test(s): 157 passed, 3 skipped, 0 failed => status: warning 22756
ppc: 160 test(s): 158 passed, 2 skipped, 0 failed => status: warning 26380
ppc64: 160 test(s): 158 passed, 2 skipped, 0 failed => status: warning 26756
ppc64le: 160 test(s): 158 passed, 2 skipped, 0 failed => status: warning 27364
riscv: 160 test(s): 158 passed, 2 skipped, 0 failed => status: warning 21758
s390: 160 test(s): 157 passed, 3 skipped, 0 failed => status: warning 21936
BR,
Zhangjin
> > A quick check with godbolt doesn't show any sign extensions happening.
>
> I agree there's none here.
>
> Willy
