From: Rasmus Villemoes <linux@rasmusvillemoes.dk>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Patrick McLean <chutzpah@gentoo.org>,
Al Viro <viro@zeniv.linux.org.uk>,
Bruce Fields <bfields@redhat.com>,
"Darrick J. Wong" <darrick.wong@oracle.com>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Linux NFS Mailing List <linux-nfs@vger.kernel.org>,
stable <stable@vger.kernel.org>,
Thorsten Leemhuis <regressions@leemhuis.info>
Subject: bit tweaks [was: Re: [nfsd4] potentially hardware breaking regression in 4.14-rc and 4.13.11]
Date: Mon, 13 Nov 2017 23:59:43 +0100 [thread overview]
Message-ID: <87efp1zsww.fsf_-_@rasmusvillemoes.dk> (raw)
In-Reply-To: <CA+55aFzGDyeJctD5Y3paBnysWXbA0cMF1_7mvvzG3n2OAnNhHw@mail.gmail.com> (Linus Torvalds's message of "Wed, 8 Nov 2017 18:40:22 -0800")
On Thu, Nov 09 2017, Linus Torvalds <torvalds@linux-foundation.org> wrote:
> The code disassembles to
>
> 0: 83 c9 08 or $0x8,%ecx
> 3: 40 f6 c6 04 test $0x4,%sil
> 7: 0f 45 d1 cmovne %ecx,%edx
> a: 89 d1 mov %edx,%ecx
> c: 80 cd 04 or $0x4,%ch
> f: 40 f6 c6 08 test $0x8,%sil
> 13: 0f 45 d1 cmovne %ecx,%edx
> 16: 89 d1 mov %edx,%ecx
> 18: 80 cd 08 or $0x8,%ch
> 1b: 40 f6 c6 10 test $0x10,%sil
> 1f: 0f 45 d1 cmovne %ecx,%edx
> 22: 89 d1 mov %edx,%ecx
> 24: 80 cd 10 or $0x10,%ch
> 27: 83 e6 20 and $0x20,%esi
> 2a:* 48 8b b7 30 02 00 00 mov 0x230(%rdi),%rsi <-- trapping instruction
> 31: 0f 45 d1 cmovne %ecx,%edx
> 34: 83 ca 20 or $0x20,%edx
> 37: 89 f1 mov %esi,%ecx
> 39: 83 e1 10 and $0x10,%ecx
> 3c: 89 cf mov %ecx,%edi
>
> and all those odd cmovne and bit-ops are just the bit selection code
> in flags_by_mnt(), which is inlined through calculate_f_flags (which
> is _also_ inlined) into vfs_statfs().
>
> Sadly, gcc makes a mess of it and actually generates code that looks
> like the original C. I would have hoped that gcc could have turned
>
> if (x & BIT)
> y |= OTHER_BIT;
>
> into
>
> y |= (x & BIT) shifted-by-the-bit-difference-between BIT/OTHER_BIT;
>
> but that doesn't happen.
Actually, new enough gcc (7.1, I think) does contain a pattern that does
this, but unfortunately only if one spells it
y |= (x & BIT) ? OTHER_BIT : 0;
which is half-way to doing it by hand, I suppose. Doing the
- if (mnt_flags & MNT_READONLY)
- flags |= ST_RDONLY;
+ flags |= (mnt_flags & MNT_READONLY) ? ST_RDONLY : 0;
and pasting into godbolt.org, one can apparently get gcc to compile it
to
flags_by_mnt(int):
leal (%rdi,%rdi), %edx
movl %edi, %eax
sarl $6, %eax
movl %edx, %ecx
andl $1, %eax
andl $12, %edx
andl $2, %ecx
orl %ecx, %eax
orl %eax, %edx
movl %edi, %eax
sall $7, %eax
andl $7168, %eax
orl %edx, %eax
ret
Rasmus
next prev parent reply other threads:[~2017-11-13 22:59 UTC|newest]
Thread overview: 82+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-11-09 0:43 [nfsd4] potentially hardware breaking regression in 4.14-rc and 4.13.11 Patrick McLean
2017-11-09 2:40 ` Linus Torvalds
2017-11-09 3:45 ` Al Viro
2017-11-09 19:34 ` Patrick McLean
2017-11-09 19:38 ` Al Viro
2017-11-09 19:42 ` Patrick McLean
2017-11-09 19:37 ` Al Viro
2017-11-09 19:51 ` Patrick McLean
2017-11-09 20:04 ` Linus Torvalds
2017-11-09 21:16 ` Al Viro
2017-11-10 1:58 ` Patrick McLean
2017-11-10 13:53 ` Arnd Bergmann
2017-11-10 18:42 ` Linus Torvalds
2017-11-10 23:26 ` Patrick McLean
2017-11-11 0:27 ` Patrick McLean
2017-11-11 2:36 ` Linus Torvalds
2017-11-11 2:36 ` [kernel-hardening] " Linus Torvalds
2017-11-11 2:36 ` Linus Torvalds
2017-11-11 16:13 ` Kees Cook
2017-11-11 16:13 ` [kernel-hardening] " Kees Cook
2017-11-11 16:13 ` Kees Cook
2017-11-11 17:31 ` Linus Torvalds
2017-11-11 17:31 ` [kernel-hardening] " Linus Torvalds
2017-11-11 17:31 ` Linus Torvalds
2017-11-13 22:48 ` Patrick McLean
2017-11-13 22:48 ` [kernel-hardening] " Patrick McLean
2017-11-13 22:48 ` Patrick McLean
2017-11-17 0:54 ` Kees Cook
2017-11-17 0:54 ` [kernel-hardening] " Kees Cook
2017-11-17 0:54 ` Kees Cook
2017-11-17 19:03 ` Patrick McLean
2017-11-17 19:03 ` [kernel-hardening] " Patrick McLean
2017-11-17 19:03 ` Patrick McLean
2017-11-17 21:26 ` Kees Cook
2017-11-17 21:26 ` [kernel-hardening] " Kees Cook
2017-11-17 21:26 ` Kees Cook
2017-11-18 0:27 ` Patrick McLean
2017-11-18 0:27 ` [kernel-hardening] " Patrick McLean
2017-11-18 0:27 ` Patrick McLean
2017-11-18 0:55 ` Linus Torvalds
2017-11-18 0:55 ` [kernel-hardening] " Linus Torvalds
2017-11-18 0:55 ` Linus Torvalds
2017-11-18 1:54 ` Patrick McLean
2017-11-18 1:54 ` [kernel-hardening] " Patrick McLean
2017-11-18 1:54 ` Patrick McLean
2017-11-18 5:14 ` Kees Cook
2017-11-18 5:14 ` [kernel-hardening] " Kees Cook
2017-11-18 5:14 ` Kees Cook
2017-11-18 5:29 ` Linus Torvalds
2017-11-18 5:29 ` [kernel-hardening] " Linus Torvalds
2017-11-18 5:29 ` Linus Torvalds
2017-11-18 8:20 ` Kees Cook
2017-11-18 8:20 ` [kernel-hardening] " Kees Cook
2017-11-18 8:20 ` Kees Cook
2018-02-21 22:19 ` RANDSTRUCT structs need linux/compiler_types.h (Was: [nfsd4] potentially hardware breaking regression in 4.14-rc and 4.13.11) Maciej S. Szmigiero
2018-02-21 22:47 ` Linus Torvalds
2018-02-21 22:47 ` Linus Torvalds
2018-02-21 23:34 ` Kees Cook
2018-02-21 23:34 ` Kees Cook
2018-03-05 9:27 ` Masahiro Yamada
2018-03-05 9:27 ` Masahiro Yamada
2018-03-05 19:15 ` Kees Cook
2018-03-05 19:18 ` Linus Torvalds
2018-02-21 22:52 ` Kees Cook
2018-02-21 23:24 ` Linus Torvalds
2018-02-22 0:12 ` Kees Cook
2018-02-22 0:22 ` Linus Torvalds
2018-02-22 0:23 ` Kees Cook
2018-02-22 0:27 ` Kees Cook
2017-11-11 1:13 ` [nfsd4] potentially hardware breaking regression in 4.14-rc and 4.13.11 J. Bruce Fields
2017-11-11 2:32 ` Al Viro
2017-11-10 1:47 ` Patrick McLean
2017-11-09 20:47 ` J. Bruce Fields
2017-11-09 23:07 ` Patrick McLean
2017-11-13 22:59 ` Rasmus Villemoes [this message]
2017-11-13 23:30 ` bit tweaks [was: Re: [nfsd4] potentially hardware breaking regression in 4.14-rc and 4.13.11] Linus Torvalds
2017-11-13 23:54 ` Linus Torvalds
2017-11-14 22:24 ` Rasmus Villemoes
2017-11-14 22:43 ` Linus Torvalds
2017-11-14 23:53 ` Rasmus Villemoes
2017-11-15 0:02 ` Linus Torvalds
2017-11-11 2:47 ` [nfsd4] potentially hardware breaking regression in 4.14-rc and 4.13.11 Alan Cox
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