linux-kernel - Re: [PATCH] lib: fix 842 build on 32-bit architectures

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Date:	Thu, 14 May 2015 04:54:07 -0400
From:	Dan Streetman <ddstreet@...e.org>
To:	linux-arm-kernel@...ts.infradead.org
Cc:	Dan Streetman <ddstreet@...e.org>, Arnd Bergmann <arnd@...db.de>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] lib: fix 842 build on 32-bit architectures

On Wed, May 13, 2015 at 7:52 PM, Dan Streetman <ddstreet@...e.org> wrote:
>> Building the 842 code on 32-bit ARM currently results in this link
>> error:
>>
>> ERROR: "__aeabi_uldivmod" [lib/842/842_decompress.ko] undefined!
>
> Oops!  Guess I should build/test on 32 bit more.
>
>>
>> The reason is that the __do_index function performs a 64-bit
>> division by a power-of-two number, but it has no insight into
>> the function arguments.

wait, do you mean the 64 bit mod, total % fsize?  That should already
be fixed in Herbert's tree, I changed it to subtraction instead.

In any case, I looked at the code again and I think the fsize
parameter can be removed, and just simply calculated in the function,
it's just a shift.  I'll send a patch.


>>
>> By marking that function inline, the fsize argument is always
>> known at the time that do_index is called, and the compiler is
>> able to replace the extremely expensive 64-bit division with
>> a cheap constant shift operation.
>
> alternately, we know that fsize will always be less than 64 bits,
> at most it's 4<<9 or 8<<8 (both == 1<<11).  So we could just change
> its type to u16.
>
> diff --git a/lib/842/842_decompress.c b/lib/842/842_decompress.c
> index 6b2b45aecde3..285bf6b6959c 100644
> --- a/lib/842/842_decompress.c
> +++ b/lib/842/842_decompress.c
> @@ -169,7 +169,7 @@ static int do_data(struct sw842_param *p, u8 n)
>         return 0;
>  }
>
> -static int __do_index(struct sw842_param *p, u8 size, u8 bits, u64 fsize)
> +static int __do_index(struct sw842_param *p, u8 size, u8 bits, u16 fsize)
>  {
>         u64 index, offset, total = round_down(p->out - p->ostart, 8);
>         int ret;
>
> Or, we could inline it and change the type to u16.  In any case,
>
> Acked-by: Dan Streetman <ddstreet@...e.org>
>
>>
>> Aside from fixing that link error, this approach should also improve
>> both code size and performance on 32-bit architectures significantly.
>>
>> Signed-off-by: Arnd Bergmann <arnd@...db.de>
>> ---
>> Found while building arm32 allmodconfig with gcc-5.0
>>
>> diff --git a/lib/842/842_decompress.c b/lib/842/842_decompress.c
>> index 6b2b45aecde3..285bf6b6959c 100644
>> --- a/lib/842/842_decompress.c
>> +++ b/lib/842/842_decompress.c
>> @@ -169,7 +169,7 @@ static int do_data(struct sw842_param *p, u8 n)
>>       return 0;
>>  }
>>
>> -static int __do_index(struct sw842_param *p, u8 size, u8 bits, u64 fsize)
>> +static inline int __do_index(struct sw842_param *p, u8 size, u8 bits, u64 fsize)
>>  {
>>       u64 index, offset, total = round_down(p->out - p->ostart, 8);
>>       int ret;
>>
--
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