From: Matthias Kaehlcke <mka@chromium.org>
To: Arnd Bergmann <arnd@arndb.de>
Cc: Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@kernel.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
Andrew Morton <akpm@linux-foundation.org>,
James Y Knight <jyknight@google.com>,
Chandler Carruth <chandlerc@google.com>,
Stephen Hines <srhines@google.com>,
Nick Desaulniers <ndesaulniers@google.com>,
Kees Cook <keescook@google.com>,
Guenter Roeck <groeck@chromium.org>,
Greg Hackmann <ghackmann@google.com>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Subject: Re: [GIT PULL] x86/build changes for v4.17
Date: Wed, 4 Apr 2018 14:46:39 -0700 [thread overview]
Message-ID: <20180404214639.GH87376@google.com> (raw)
In-Reply-To: <CAK8P3a2tf3v6seq=OeyBakZ1ZXYzzL=tY=aHvKYv-GtD7sWeTA@mail.gmail.com>
El Wed, Apr 04, 2018 at 11:11:36PM +0200 Arnd Bergmann ha dit:
> On Wed, Apr 4, 2018 at 10:58 PM, Matthias Kaehlcke <mka@chromium.org> wrote:
> > El Wed, Apr 04, 2018 at 10:33:19PM +0200 Arnd Bergmann ha dit:
> >>
> >> In most cases, this is used to implement a fast-path for a helper
> >> function, so not doing it the same way as gcc just results in
> >> slower execution, but I assume we also have code that behaves
> >> differently on clang compared to gcc because of this.
> >
> > I think I didn't come (knowingly) across that one yet. Could you point
> > me to an instance that could be used as an example in a bug report?
>
> This code
>
> #include <linux/math64.h>
> int f(u64 u)
> {
> return div_u64(u, 100000);
> }
>
> results in a call to __do_div64() on 32-bit arm using clang, but
> gets optimized into a set of multiply+shift on gcc.
I understand this is annoying, but it seems I'm missing something:
static inline u64 div_u64(u64 dividend, u32 divisor)
{
u32 remainder;
return div_u64_rem(dividend, divisor, &remainder);
}
static inline u64 div_u64_rem(u64 dividend, u32 divisor, u32 *remainder)
{
*remainder = do_div(dividend, divisor);
return dividend;
}
#define do_div(n, base) __div64_32(&(n), base)
static inline uint32_t __div64_32(uint64_t *n, uint32_t base)
{
register unsigned int __base asm("r4") = base;
register unsigned long long __n asm("r0") = *n;
register unsigned long long __res asm("r2");
register unsigned int __rem asm(__xh);
asm( __asmeq("%0", __xh)
__asmeq("%1", "r2")
__asmeq("%2", "r0")
__asmeq("%3", "r4")
"bl __do_div64"
: "=r" (__rem), "=r" (__res)
: "r" (__n), "r" (__base)
: "ip", "lr", "cc");
*n = __res;
return __rem;
}
There is no reference to __builtin_constant_p(), could you elaborate?
Also you mentioned there are plenty of cases, maybe there is a more
straightforward one?
In any case it seems this derails a bit from the original topic of the
thread. Shall we take this offline?
next prev parent reply other threads:[~2018-04-04 21:46 UTC|newest]
Thread overview: 52+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-04-02 9:50 [GIT PULL] x86/build changes for v4.17 Ingo Molnar
2018-04-02 21:44 ` Linus Torvalds
2018-04-02 22:38 ` Matthias Kaehlcke
2018-04-03 1:26 ` Matthias Kaehlcke
2018-04-03 8:59 ` Peter Zijlstra
2018-04-03 9:51 ` Ingo Molnar
2018-04-03 12:09 ` Peter Zijlstra
2018-04-03 18:06 ` Matthias Kaehlcke
2018-04-03 21:58 ` Nick Desaulniers
2018-04-04 9:19 ` Peter Zijlstra
2018-04-04 9:38 ` Greg KH
2018-04-04 16:49 ` Nick Desaulniers
2018-04-04 17:13 ` Linus Torvalds
2018-04-04 17:46 ` Nick Desaulniers
2018-04-04 23:10 ` Nick Desaulniers
2018-04-04 16:53 ` Nick Desaulniers
2018-04-04 16:59 ` Greg KH
2018-04-04 19:26 ` James Y Knight
2018-04-04 19:42 ` Linus Torvalds
2018-04-04 22:21 ` James Y Knight
2018-04-04 22:29 ` Linus Torvalds
2018-04-05 7:08 ` Peter Zijlstra
2018-04-05 16:21 ` James Y Knight
2018-04-04 19:32 ` Josh Poimboeuf
2018-06-07 19:23 ` Nick Desaulniers
2018-06-07 20:11 ` Greg KH
2018-04-04 9:30 ` Peter Zijlstra
2018-04-04 19:17 ` Matthias Kaehlcke
2018-04-04 20:33 ` Arnd Bergmann
2018-04-04 20:58 ` Matthias Kaehlcke
2018-04-04 21:11 ` Arnd Bergmann
2018-04-04 21:46 ` Matthias Kaehlcke [this message]
2018-04-04 21:59 ` Linus Torvalds
2018-04-04 22:17 ` Matthias Kaehlcke
2018-04-04 22:39 ` Linus Torvalds
2018-04-04 23:31 ` Matthias Kaehlcke
2018-04-05 0:05 ` Linus Torvalds
2018-04-05 0:20 ` Kees Cook
2018-04-05 7:24 ` Peter Zijlstra
2018-04-05 8:04 ` Ingo Molnar
2018-04-05 8:24 ` Peter Zijlstra
2018-04-05 16:43 ` Linus Torvalds
2018-04-05 7:20 ` Peter Zijlstra
2018-04-05 17:46 ` James Y Knight
2018-04-05 18:06 ` Linus Torvalds
2018-04-05 20:51 ` James Y Knight
2018-04-05 21:13 ` Linus Torvalds
2018-04-05 22:51 ` James Y Knight
2018-04-06 2:02 ` Linus Torvalds
2018-04-05 17:47 ` James Y Knight
2018-04-04 23:04 ` Nick Desaulniers
2018-04-03 17:36 ` Linus Torvalds
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