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From: Richard Henderson <rth@twiddle.net>
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Date: Fri, 4 Nov 2016 06:27:07 -0600
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Subject: Re: [Qemu-devel] [PATCH v2 2/3] target-m68k: implement 680x0 movem
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To: Laurent Vivier <laurent@vivier.eu>, qemu-devel@nongnu.org
Cc: schwab@linux-m68k.org, agraf@suse.de, gerg@uclinux.org

On 11/04/2016 01:59 AM, Laurent Vivier wrote:
> Le 03/11/2016 à 21:47, Richard Henderson a écrit :
>> On 11/02/2016 03:15 PM, Laurent Vivier wrote:
>>> +            for (i = 15; i >= 0; i--, mask >>= 1) {
>>> +                if (mask & 1) {
>>> +                    if ((insn & 7) + 8 == i &&
>>> +                        m68k_feature(s->env, M68K_FEATURE_EXT_FULL)) {
>>> +                        /* M68020+: if the addressing register is the
>>> +                         * register moved to memory, the value written
>>> +                         * is the initial value decremented by the
>>> size of
>>> +                         * the operation
>>> +                         * M68000/M68010: the value is the initial value
>>> +                         */
>>> +                        TCGv tmp = tcg_temp_new();
>>> +                        tcg_gen_sub_i32(tmp, mreg(i), incr);
>>> +                        gen_store(s, opsize, addr, tmp);
>>> +                        tcg_temp_free(tmp);
>>> +                    } else {
>>> +                        gen_store(s, opsize, addr, mreg(i));
>>> +                    }
>>> +                    if (mask != 1) {
>>> +                        tcg_gen_sub_i32(addr, addr, incr);
>>> +                    }
>>> +                }
>>
>> One more thing: This is pre-decrement.  Why are you decrementing after
>> the store?  Seems to me this should be
>>
>>    if (mask & 1) {
>>        tcg_gen_sub_i32(addr, addr, incr);
>>        if (REG(insn, 0) + 8 == i ...)
>>        ...
>>    }
>>
>
> Because it has already been decremented by gen_lea()... so this a
> problem if we have page fault, except if we use your "areg writeback"
> series, and we will.

Ah, I see.  No, gen_lea doesn't writeback; only gen_ea does.  But I wonder if 
this couldn't be cleaned up a tad.  I'll get back to you.


r~