From: David Laight <David.Laight@ACULAB.COM>
To: 'Nicholas Piggin' <npiggin@gmail.com>,
Adhemerval Zanella <adhemerval.zanella@linaro.org>,
Rich Felker <dalias@libc.org>
Cc: "libc-dev@lists.llvm.org" <libc-dev@lists.llvm.org>,
"libc-alpha@sourceware.org" <libc-alpha@sourceware.org>,
"linuxppc-dev@lists.ozlabs.org" <linuxppc-dev@lists.ozlabs.org>,
"musl@lists.openwall.com" <musl@lists.openwall.com>
Subject: RE: [musl] Powerpc Linux 'scv' system call ABI proposal take 2
Date: Tue, 21 Apr 2020 12:28:25 +0000 [thread overview]
Message-ID: <b77fa2dc769d42e1a3e68f5edf90d250@AcuMS.aculab.com> (raw)
In-Reply-To: <1587344003.daumxvs1kh.astroid@bobo.none>
From: Nicholas Piggin
> Sent: 20 April 2020 02:10
...
> >> Yes, but does it really matter to optimize this specific usage case
> >> for size? glibc, for instance, tries to leverage the syscall mechanism
> >> by adding some complex pre-processor asm directives. It optimizes
> >> the syscall code size in most cases. For instance, kill in static case
> >> generates on x86_64:
> >>
> >> 0000000000000000 <__kill>:
> >> 0: b8 3e 00 00 00 mov $0x3e,%eax
> >> 5: 0f 05 syscall
> >> 7: 48 3d 01 f0 ff ff cmp $0xfffffffffffff001,%rax
> >> d: 0f 83 00 00 00 00 jae 13 <__kill+0x13>
Hmmm... that cmp + jae is unnecessary here.
It is also a 32bit offset jump.
I also suspect it gets predicted very badly.
> >> 13: c3 retq
> >>
> >> While on musl:
> >>
> >> 0000000000000000 <kill>:
> >> 0: 48 83 ec 08 sub $0x8,%rsp
> >> 4: 48 63 ff movslq %edi,%rdi
> >> 7: 48 63 f6 movslq %esi,%rsi
> >> a: b8 3e 00 00 00 mov $0x3e,%eax
> >> f: 0f 05 syscall
> >> 11: 48 89 c7 mov %rax,%rdi
> >> 14: e8 00 00 00 00 callq 19 <kill+0x19>
> >> 19: 5a pop %rdx
> >> 1a: c3 retq
> >
> > Wow that's some extraordinarily bad codegen going on by gcc... The
> > sign-extension is semantically needed and I don't see a good way
> > around it (glibc's asm is kinda a hack taking advantage of kernel not
> > looking at high bits, I think), but the gratuitous stack adjustment
> > and refusal to generate a tail call isn't. I'll see if we can track
> > down what's going on and get it fixed.
A suitable cast might get rid of the sign extension.
Possibly just (unsigned int).
David
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Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)
next prev parent reply other threads:[~2020-04-21 12:28 UTC|newest]
Thread overview: 62+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-04-15 21:45 Nicholas Piggin
2020-04-15 22:55 ` [musl] " Rich Felker
2020-04-16 0:16 ` Nicholas Piggin
2020-04-16 0:48 ` Rich Felker
2020-04-16 2:24 ` Nicholas Piggin
2020-04-16 2:35 ` Rich Felker
2020-04-16 2:53 ` Nicholas Piggin
2020-04-16 3:03 ` Rich Felker
2020-04-16 3:41 ` Nicholas Piggin
2020-04-16 20:18 ` Florian Weimer
2020-04-16 9:58 ` Szabolcs Nagy
2020-04-20 0:27 ` Nicholas Piggin
2020-04-20 1:29 ` Rich Felker
2020-04-20 2:08 ` Nicholas Piggin
2020-04-20 21:17 ` Szabolcs Nagy
2020-04-21 9:57 ` Florian Weimer
2020-04-16 15:21 ` Jeffrey Walton
2020-04-16 15:40 ` Rich Felker
2020-04-16 4:48 ` Florian Weimer
2020-04-16 15:35 ` Rich Felker
2020-04-16 16:42 ` Florian Weimer
2020-04-16 16:52 ` Rich Felker
2020-04-16 18:12 ` Florian Weimer
2020-04-16 23:02 ` Segher Boessenkool
2020-04-17 0:34 ` Rich Felker
2020-04-17 1:48 ` Segher Boessenkool
2020-04-17 8:34 ` Florian Weimer
2020-04-16 14:16 ` Adhemerval Zanella
2020-04-16 15:37 ` Rich Felker
2020-04-16 17:50 ` Adhemerval Zanella
2020-04-16 17:59 ` Rich Felker
2020-04-16 18:18 ` Adhemerval Zanella
2020-04-16 18:31 ` Rich Felker
2020-04-16 18:44 ` Rich Felker
2020-04-16 18:52 ` Adhemerval Zanella
2020-04-20 0:46 ` Nicholas Piggin
2020-04-20 1:10 ` Nicholas Piggin
2020-04-20 1:34 ` Rich Felker
2020-04-20 2:32 ` Nicholas Piggin
2020-04-20 4:09 ` Rich Felker
2020-04-20 4:31 ` Nicholas Piggin
2020-04-20 17:27 ` Rich Felker
2020-04-22 6:18 ` Nicholas Piggin
2020-04-22 6:29 ` Nicholas Piggin
2020-04-23 2:36 ` Rich Felker
2020-04-23 12:13 ` Adhemerval Zanella
2020-04-23 16:18 ` Rich Felker
2020-04-23 16:35 ` Adhemerval Zanella
2020-04-23 16:43 ` Rich Felker
2020-04-23 17:15 ` Adhemerval Zanella
2020-04-23 17:42 ` Rich Felker
2020-04-25 3:40 ` Nicholas Piggin
2020-04-25 4:52 ` Rich Felker
2020-04-25 3:30 ` Nicholas Piggin
2020-04-21 12:28 ` David Laight [this message]
2020-04-21 14:39 ` Rich Felker
2020-04-21 15:00 ` Adhemerval Zanella
2020-04-21 15:31 ` David Laight
2020-04-22 6:54 ` Nicholas Piggin
2020-04-22 7:15 ` [musl] " Florian Weimer
2020-04-22 7:31 ` Nicholas Piggin
2020-04-22 8:11 ` Florian Weimer
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