[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <b77fa2dc769d42e1a3e68f5edf90d250@AcuMS.aculab.com>
Date: Tue, 21 Apr 2020 12:28:25 +0000
From: David Laight <David.Laight@...LAB.COM>
To: 'Nicholas Piggin' <npiggin@...il.com>, Adhemerval Zanella
<adhemerval.zanella@...aro.org>, Rich Felker <dalias@...c.org>
CC: "libc-dev@...ts.llvm.org" <libc-dev@...ts.llvm.org>,
"libc-alpha@...rceware.org" <libc-alpha@...rceware.org>,
"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
"musl@...ts.openwall.com" <musl@...ts.openwall.com>
Subject: RE: Powerpc Linux 'scv' system call ABI proposal take 2
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
-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)
Powered by blists - more mailing lists
Confused about mailing lists and their use?
Read about mailing lists on Wikipedia
and check out these
guidelines on proper formatting of your messages.
