Re: [Qemu-devel] [PATCH 2/2] target/hppa: prevent trashing of temporary in do_depw_sar()

[Sven Schnelle]

Hi Philippe,

On Fri, Sep 13, 2019 at 12:58:14PM +0200, Philippe Mathieu-Daudé wrote:
> Hi Sven,
> 
> On 9/13/19 12:17 PM, Sven Schnelle wrote:
> > nullify_over() calls brcond which destroys all temporaries.
> > 
> > Signed-off-by: Sven Schnelle <address@hidden>
> > ---
> >  target/hppa/translate.c | 10 ++++++----
> >  1 file changed, 6 insertions(+), 4 deletions(-)
> > 
> > diff --git a/target/hppa/translate.c b/target/hppa/translate.c
> > index b12525d535..c1b2822f60 100644
> > --- a/target/hppa/translate.c
> > +++ b/target/hppa/translate.c
> > @@ -3404,10 +3404,6 @@ static bool do_depw_sar(DisasContext *ctx, unsigned 
> > rt, unsigned c,
> >      TCGv_reg mask, tmp, shift, dest;
> >      unsigned msb = 1U << (len - 1);
> >  
> > -    if (c) {
> > -        nullify_over(ctx);
> > -    }
> > -
> >      dest = dest_gpr(ctx, rt);
> >      shift = tcg_temp_new();
> >      tmp = tcg_temp_new();
> > @@ -3440,11 +3436,17 @@ static bool do_depw_sar(DisasContext *ctx, unsigned 
> > rt, unsigned c,
> >  
> >  static bool trans_depw_sar(DisasContext *ctx, arg_depw_sar *a)
> >  {
> > +    if (a->c) {
> > +        nullify_over(ctx);
> > +    }
> >      return do_depw_sar(ctx, a->t, a->c, a->nz, a->clen, load_gpr(ctx, 
> > a->r));
> >  }
> >  
> >  static bool trans_depwi_sar(DisasContext *ctx, arg_depwi_sar *a)
> >  {
> > +    if (a->c) {
> > +        nullify_over(ctx);
> > +    }
> 
> I don't see how this patch helps or change anything, isn't it the same?
> You clean in the caller rather than the callee.

The Problem is that load_gpr()/load_const() allocate a temporary, which
gets destroyed in do_depw_sar() when nullify_over() is called. If we
move the nullify_over() before doing the load_gpr()/load_const() this
doesn't happen.

> >      return do_depw_sar(ctx, a->t, a->c, a->nz, a->clen, load_const(ctx, 
> > a->i));
> >  }
> >  
> >

Regards
Sven