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From: Richard Henderson <rth@twiddle.net>
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Date: Fri, 7 Jul 2017 05:26:03 -1000
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Subject: Re: [Qemu-devel] [PATCH v11 23/29] target/arm: [tcg] Port to
 translate_insn
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To: qemu-devel@nongnu.org, Peter Maydell <peter.maydell@linaro.org>, Peter Crosthwaite <crosthwaite.peter@gmail.com>, "Emilio G. Cota" <cota@braap.org>, "open list:ARM" <qemu-arm@nongnu.org>, Paolo Bonzini <pbonzini@redhat.com>, =?UTF-8?Q?Alex_Benn=c3=a9e?= <alex.bennee@linaro.org>

On 07/07/2017 01:13 AM, Lluís Vilanova wrote:
>>> +    if (dc->ss_active && !dc->pstate_ss) {
>>> +        /* Singlestep state is Active-pending.
>>> +         * If we're in this state at the start of a TB then either
>>> +         *  a) we just took an exception to an EL which is being debugged
>>> +         *     and this is the first insn in the exception handler
>>> +         *  b) debug exceptions were masked and we just unmasked them
>>> +         *     without changing EL (eg by clearing PSTATE.D)
>>> +         * In either case we're going to take a swstep exception in the
>>> +         * "did not step an insn" case, and so the syndrome ISV and EX
>>> +         * bits should be zero.
>>> +         */
>>> +        assert(dc->base.num_insns == 1);
>>> +        gen_exception(EXCP_UDEF, syn_swstep(dc->ss_same_el, 0, 0),
>>> +                      default_exception_el(dc));
>>> +        dc->base.is_jmp = DISAS_SKIP;
> 
>> This is surely DISAS_EXC -- see gen_step_complete_exception.
>> Why introduce a new name?
> 
> The original code goes straight to done_generating here, and that's the purpose
> of DISAS_SKIP (skip the code executed between the end of the loop and the
> done_generating label).

That is the purpose of DISAS_EXC too.  We've called a noreturn helper to raise 
an exception and all following code is unreached.  If there *was* any code 
being emitted afterward, that is arguably a bug.


r~