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Re: [PATCH] target/m68k: Remove unused variable in ABCD/SBCD memory opco
From: |
Laurent Vivier |
Subject: |
Re: [PATCH] target/m68k: Remove unused variable in ABCD/SBCD memory opcodes |
Date: |
Thu, 6 May 2021 12:29:21 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1 |
Le 05/05/2021 à 18:43, Laurent Vivier a écrit :
> Le 05/05/2021 à 18:03, Philippe Mathieu-Daudé a écrit :
>> The ABCD / SBCD memory opcodes (introduced in commit fb5543d8200)
>> don't use their "addr" variable.
>>
>> Remove the unused variable and pass a NULL argument instead to
>> gen_ea_mode(). This fixes warnings generated when building with
>> CFLAGS=-O3 (using GCC 10.2.1 20201125):
>>
>> target/m68k/translate.c: In function ‘disas_sbcd_mem’:
>> target/m68k/translate.c:897:13: warning: ‘addr’ may be used uninitialized
>> in this function [-Wmaybe-uninitialized]
>> 897 | delay_set_areg(s, reg0, tmp, false);
>> | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>> target/m68k/translate.c:1866:21: note: ‘addr’ was declared here
>> 1866 | TCGv src, dest, addr;
>> | ^~~~
>>
>> target/m68k/translate.c: In function ‘disas_abcd_mem’:
>> target/m68k/translate.c:897:13: warning: ‘addr’ may be used uninitialized
>> in this function [-Wmaybe-uninitialized]
>> 897 | delay_set_areg(s, reg0, tmp, false);
>> | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>> target/m68k/translate.c:1829:21: note: ‘addr’ was declared here
>> 1829 | TCGv src, dest, addr;
>> | ^~~~
>>
>
> It's really strange because ABCD and SBCD support indirect predecrement (mode
> 4, "-(Ay),-(Ax)"), and
> if you look into gen_ea_mode() &addr (addrp) is used with mode 4, it is
> initialized on EA_LOADU to
> be reused on EA_STORE.
>
> The bug is somewhere else...
>
I think I see what is the problem: as the mode is indirect pre-decrement, the
register doesn't need
to be updated and thus the addr is not needed.
But if we replace addrp by NULL, gen_lea_mode() will be called twice and the
register will be
decremented twice (on load and store, rather than only on load).
Thanks,
Laurent