linux-kernel - Re: [RFC PATCH v4 10/11] lib: vdso: Allow arches to override the ns shift operation

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Date:   Thu, 16 Jan 2020 11:47:36 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Christophe Leroy <christophe.leroy@....fr>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>, nathanl@...ux.ibm.com,
        Arnd Bergmann <arnd@...db.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Andrew Lutomirski <luto@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        "open list:MIPS" <linux-mips@...r.kernel.org>,
        X86 ML <x86@...nel.org>
Subject: Re: [RFC PATCH v4 10/11] lib: vdso: Allow arches to override the ns
 shift operation

On Thu, Jan 16, 2020 at 9:58 AM Christophe Leroy
<christophe.leroy@....fr> wrote:
>
> On powerpc/32, GCC (8.1) generates pretty bad code for the
> ns >>= vd->shift operation taking into account that the
> shift is always < 32 and the upper part of the result is
> likely to be nul. GCC makes reversed assumptions considering
> the shift to be likely >= 32 and the upper part to be like not nul.
>
> unsigned long long shift(unsigned long long x, unsigned char s)
> {
>         return x >> s;
> }
>
> results in:
>
> 00000018 <shift>:
>   18:   35 25 ff e0     addic.  r9,r5,-32
>   1c:   41 80 00 10     blt     2c <shift+0x14>
>   20:   7c 64 4c 30     srw     r4,r3,r9
>   24:   38 60 00 00     li      r3,0
>   28:   4e 80 00 20     blr
>   2c:   54 69 08 3c     rlwinm  r9,r3,1,0,30
>   30:   21 45 00 1f     subfic  r10,r5,31
>   34:   7c 84 2c 30     srw     r4,r4,r5
>   38:   7d 29 50 30     slw     r9,r9,r10
>   3c:   7c 63 2c 30     srw     r3,r3,r5
>   40:   7d 24 23 78     or      r4,r9,r4
>   44:   4e 80 00 20     blr
>
> Even when forcing the shift with an &= 31, it still considers
> the shift as likely >= 32.
>
> Define a vdso_shift_ns() macro that can be overriden by
> arches.

Would mul_u64_u64_shr() be a good alternative?  Could we adjust it to
assume the shift is less than 32?  That function exists to benefit
32-bit arches.

--Andy