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

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Message-ID: <20150514030636.GE3853@gondor.apana.org.au>
Date:	Thu, 14 May 2015 11:06:36 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	linux-arm-kernel@...ts.infradead.org,
	Dan Streetman <ddstreet@...e.org>,
	linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] lib: fix 842 build on 32-bit architectures

On Wed, May 13, 2015 at 10:56:39PM +0200, Arnd Bergmann wrote:
> Building the 842 code on 32-bit ARM currently results in this link
> error:
> 
> ERROR: "__aeabi_uldivmod" [lib/842/842_decompress.ko] undefined!
> 
> 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.
> 
> 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.
> 
> 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)

Ugh, relying on inlining to work is fragile.  I'm not against
making this inline but please make it work even when it is out-
of-line.

Thanks,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
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