Re: [Qemu-devel] [PATCH 2/2] target-mips/translate.c: Add judgement for msb and lsb

[Elta]

On 07/29/2014 06:52 AM, Aurelien Jarno wrote:
> On Mon, Jul 28, 2014 at 11:34:30PM +0100, Peter Maydell wrote:
> > On 28 July 2014 23:32, Aurelien Jarno <address@hidden> wrote:
> > > On Mon, Jul 28, 2014 at 11:01:02PM +0100, Peter Maydell wrote:
> > > > This may be true, but the TCG README doesn't define negative
> > > > lengths as being "unspecified behaviour" (ie guaranteed to at
> > > > least not crash even if the result isn't specified), and in fact the
> > > > implementation of tcg_gen_deposit will assert on negative lengths.
> > > > We shouldn't implement guest unpredictable cases as "crash QEMU".
> > > Well I tried this code under QEMU, and it clearly doesn't crash. It
> > > seems the assert are not enabled with the default configuration options.
> > Try --enable-debug...
> That's my point, it's only in debug mode, not in the default
> configuration.

Maybe remove the tcg_debug_assert in tcg_gen_deposit_i64 and 
tcg_gen_deposit_i64
is a better way. But it may cause other mistake in other architecture, 
i'm not
sure.

> > > That said I agree it's something to avoid, but I don't think triggering
> > > a RI exception is the thing to do (even if it is correct according the
> > > MIPS ISA manual) when real silicon output a random result instead.
> > Yes, you could emit code to do that instead if you like.
> When I said random, it didn't say in the sense of random generator, but
> in the sense a result that might depend on the input value and the
> silicon implementation. It would be silly to emit code just for that,
> but it would be smart for example to skip the deposit op in that case
> instead of triggering an exception.
I think, debug mode shouldn't crash the qemu with an unpredictable 
operation,
so i want to fix it. And you say there shouldn't raise RI, i agree with you.

Or when lsb > msb, just leave the code and do nothing. What do you think 
about
this way?