linux-kernel - Re: [PATCH] __div64_32(): straighten up inline asm constraints

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Date:   Mon, 30 Nov 2020 11:33:36 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Nicolas Pitre <nico@...xnic.net>
Cc:     Ard Biesheuvel <ardb@...nel.org>, Antony Yu <swpenim@...il.com>,
        Russell King <linux@...linux.org.uk>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] __div64_32(): straighten up inline asm constraints

On Mon, Nov 30, 2020 at 11:05 AM Nicolas Pitre <nico@...xnic.net> wrote:
>
> The ARM version of __div64_32() encapsulates a call to __do_div64 with
> non-standard argument passing. In particular, __n is a 64-bit input
> argument assigned to r0-r1 and __rem is an output argument sharing half
> of that 40-r1 register pair.

Should `40` be `r0`?

>
> With __n being an input argument, the compiler is in its right to
> presume that r0-r1 would still hold the value of __n past the inline
> assembly statement. Normally, the compiler would have assigned non
> overlapping registers to __n and __rem if the value for __n is needed
> again.
>
> However, here we enforce our own register assignment and gcc fails to
> notice the conflict. In practice this doesn't cause any problem as __n
> is considered dead after the asm statement and *n is overwritten.
> However this is not always guaranteed and clang rightfully complains.
>
> Let's fix it properly by making __n into an input-output variable. This
> makes it clear that those registers representing __n have been modified.
> Then we can extract __rem as the high part of __n with plain C code.
>
> This asm constraint "abuse" was likely relied upon back when gcc didn't
> handle 64-bit values optimally Turns out that gcc is now able to

^ Missing punctuation (period after `optimally`).

> optimize things and produces the same code with this patch applied.
>
> Signed-off-by: Nicolas Pitre <nico@...xnic.net>
> Reviewed-by: Ard Biesheuvel <ardb@...nel.org>
> Tested-by: Ard Biesheuvel <ardb@...nel.org>

Reported-by: Antony Yu <swpenim@...il.com>


> ---
>
> This is related to the thread titled "[RESEND,PATCH] ARM: fix
> __div64_32() error when compiling with clang". My limited compile test
> with clang appears to make it happy. If no more comments I'll push this
> to RMK's patch system.
>
> diff --git a/arch/arm/include/asm/div64.h b/arch/arm/include/asm/div64.h
> index 898e9c78a7..595e538f5b 100644
> --- a/arch/arm/include/asm/div64.h
> +++ b/arch/arm/include/asm/div64.h
> @@ -21,29 +21,20 @@
>   * assembly implementation with completely non standard calling convention
>   * for arguments and results (beware).
>   */
> -
> -#ifdef __ARMEB__
> -#define __xh "r0"
> -#define __xl "r1"
> -#else
> -#define __xl "r0"
> -#define __xh "r1"
> -#endif
> -
>  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)
> +       unsigned int __rem;
> +       asm(    __asmeq("%0", "r0")
>                 __asmeq("%1", "r2")
> -               __asmeq("%2", "r0")
> -               __asmeq("%3", "r4")
> +               __asmeq("%2", "r4")
>                 "bl     __do_div64"
> -               : "=r" (__rem), "=r" (__res)
> -               : "r" (__n), "r" (__base)
> +               : "+r" (__n), "=r" (__res)
> +               : "r" (__base)
>                 : "ip", "lr", "cc");
> +       __rem = __n >> 32;
>         *n = __res;
>         return __rem;

The above 3 statement could be:

```
*n = __res;
return __n >> 32;
```

>  }



-- 
Thanks,
~Nick Desaulniers