RE: [PATCH v2 02/10] target/hexagon: import README for idef-parser

[Taylor Simpson]

> -----Original Message-----
> From: Richard Henderson <richard.henderson@linaro.org>
> Sent: Thursday, February 25, 2021 2:21 PM
> To: Alessandro Di Federico <ale.qemu@rev.ng>; qemu-devel@nongnu.org
> Cc: Taylor Simpson <tsimpson@quicinc.com>; Brian Cain
> <bcain@quicinc.com>; babush@rev.ng; nizzo@rev.ng; philmd@redhat.com;
> Alessandro Di Federico <ale@rev.ng>
> Subject: Re: [PATCH v2 02/10] target/hexagon: import README for idef-
> parser
>
> On 2/25/21 7:18 AM, Alessandro Di Federico wrote:
> > +Now let's have a quick look at the generated code, line by line.
> > +
> > +::
> > +
> > +   tcg_gen_movi_i32(RdV, 0);
> > +
> > +This code starts by initializing ``RdV``, since reading from that register
> > +without initialization will cause a segmentation fault by QEMU.  This is
> emitted
> > +because a declaration of the ``RdV`` register was parsed, but no reading of
> the
> > +``RdV`` register was found.
>
> This is odd, as is the description of why.  Yes, if RdV is read without
> initialization, TCG middle-end will abort (at least with --enable-debug-tcg
> enabling the assertions).  But you've just said that "no reading" was found.
> So why did you perform this dummy initialization, which will be eliminated
> later?

Which instructions require this?  There must be an attribute that we could 
check to see if it is needed before emitting the TCG.

Taylor