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

[Richard Henderson <rth@twiddle.net>]

On 07/07/2017 07:33 AM, Peter Maydell wrote:
> On 7 July 2017 at 16:26, Richard Henderson <rth@twiddle.net> wrote:
>> 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.
> 
> One exception to that is a conditionally executed
> exception generating exception -- there will in that
> case be a following label for the condfail case to branch
> to and the code for the condfail path.
> 
> The distinction in the case that this code fragment is touching
> is that the cases handled in current master via 'goto
> done_generating' and in Lluis' patch as
> DISAS_SKIP are the "this insn is going to generate an
> exception without even thinking about conditional
> exception" (ie breakpoints, singlestep); DISAS_EXC
> is for "the instruction itself generates an exception,
> so don't bother with emitting too much unreachable
> code to update the PC etc, but we still need to handle
> the usual end-of-insn condfail path".

Ok.

LLuis, this implies that the DISAS_NORETURN that I talked about elsewhere 
should be the thing handled generically, but that target/arm still needs a 
target-specific define for DISAS_EXC so that the conditional execution handler 
can make the distinction.

> We do a few things in the DISAS_EXC codepath
> (like calling gen_set_condexec()) which strictly speaking
> are pointless but which it didn't seem worth trying to
> avoid just to avoid generating a few extra bytes in the
> generated code in a not-terribly-likely case.

Yeah.  We'd probably be better off just adding dead-code removal to TCG. 
Something that used to be difficult but would now be trivial to do.


r~