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Re: [Qemu-devel] [PATCH v11 23/29] target/arm: [tcg] Port to translate_i

From: Richard Henderson
Subject: Re: [Qemu-devel] [PATCH v11 23/29] target/arm: [tcg] Port to translate_insn
Date: Fri, 7 Jul 2017 05:26:03 -1000
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1

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.


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