Re: [PATCH] target/m68k: Remove unused variable in ABCD/SBCD memory opcodes

[Laurent Vivier <laurent@vivier.eu>]

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