linux-kernel - Re: [PATCH v3] lib/zlib_inflate/inffast: Check config in C to avoid unused function warning

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Message-ID: <d00bb831-211e-0c7b-2734-1ed7769af2ef@molgen.mpg.de>
Date:   Tue, 21 Sep 2021 12:11:16 +0200
From:   Paul Menzel <pmenzel@...gen.mpg.de>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Zhen Lei <thunder.leizhen@...wei.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: Re: [PATCH v3] lib/zlib_inflate/inffast: Check config in C to avoid
 unused function warning

Dear Linux folks,


Am 20.09.21 um 17:45 schrieb Nathan Chancellor:
> On Mon, Sep 20, 2021 at 10:43:33AM +0200, Paul Menzel wrote:
>> Building Linux for ppc64le with Ubuntu clang version 12.0.0-3ubuntu1~21.04.1
>> shows the warning below.
>>
>>      arch/powerpc/boot/inffast.c:20:1: warning: unused function 'get_unaligned16' [-Wunused-function]
>>      get_unaligned16(const unsigned short *p)
>>      ^
>>      1 warning generated.
>>
>> Fix it, by moving the check from the preprocessor to C, so the compiler
>> sees the use.
>>
>> Signed-off-by: Paul Menzel <pmenzel@...gen.mpg.de>
> 
> Reviewed-by: Nathan Chancellor <nathan@...nel.org>
> Tested-by: Nathan Chancellor <nathan@...nel.org>
> 
>> ---
>> v2: Use IS_ENABLED
>> v3: Use if statement over ternary operator as requested by Christophe
>>
>>   lib/zlib_inflate/inffast.c | 13 ++++++-------
>>   1 file changed, 6 insertions(+), 7 deletions(-)
>>
>> diff --git a/lib/zlib_inflate/inffast.c b/lib/zlib_inflate/inffast.c
>> index f19c4fbe1be7..2843f9bb42ac 100644
>> --- a/lib/zlib_inflate/inffast.c
>> +++ b/lib/zlib_inflate/inffast.c
>> @@ -253,13 +253,12 @@ void inflate_fast(z_streamp strm, unsigned start)
>>   
>>   			sfrom = (unsigned short *)(from);
>>   			loops = len >> 1;
>> -			do
>> -#ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
>> -			    *sout++ = *sfrom++;
>> -#else
>> -			    *sout++ = get_unaligned16(sfrom++);
>> -#endif
>> -			while (--loops);
>> +			do {
>> +			    if (IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS))
>> +				*sout++ = *sfrom++;
>> +			    else
>> +				*sout++ = get_unaligned16(sfrom++);
>> +			} while (--loops);
>>   			out = (unsigned char *)sout;
>>   			from = (unsigned char *)sfrom;
>>   		    } else { /* dist == 1 or dist == 2 */
>> -- 
>> 2.33.0

Just for the record,

I compared both object files by running `objdump -d`, and the result is 
the same.

The binary differed (`vbindiff`), but I guess this is due to the 
increased revision (`make bindeb-pkg`).

without a change (Linus’ current master):

0000 0B50: 00 00 00 00 00 00 00 00  1F 01 00 00 36 00 00 00  ........ 
....6...
                                      ^
0000 0B60: 00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  ........ 
........
0000 0B70: 00 00 00 00 00 00 00 00  29 01 00 00 32 00 00 00  ........ 
)...2...
                                      ^

v2 (ternary operator):

0000 0B50: 00 00 00 00 00 00 00 00  1C 01 00 00 36 00 00 00  ........ 
....6...
0000 0B60: 00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  ........ 
........
0000 0B70: 00 00 00 00 00 00 00 00  26 01 00 00 32 00 00 00  ........ 
&...2...

v3 (if-else statement):

0000 0B50: 00 00 00 00 00 00 00 00  1E 01 00 00 36 00 00 00  ........ 
....6...
0000 0B60: 00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  ........ 
........
0000 0B70: 00 00 00 00 00 00 00 00  28 01 00 00 32 00 00 00  ........ 
(...2...


Kind regards,

Paul