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

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Message-Id: <1431561144-29931-1-git-send-email-ddstreet@ieee.org>
Date:	Wed, 13 May 2015 19:52:24 -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@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] lib: fix 842 build on 32-bit architectures

> 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.
> 
> 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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