Re: [Qemu-devel] [PATCH] Fix extlh instruction on Alpha

[Vince Weaver <vince@csl.cornell.edu>]

On Wed, 16 Sep 2009, Aurelien Jarno wrote:

> On Wed, Sep 09, 2009 at 12:08:25PM -0400, Vince Weaver wrote:

> >          } else {
> > +            int l1;
> >              TCGv tmp1, tmp2;
> > -            tmp1 = tcg_temp_new();
> > +            tmp1 = tcg_temp_local_new();
> > +            l1 = gen_new_label();
> > +
> >              tcg_gen_andi_i64(tmp1, cpu_ir[rb], 7);
> >              tcg_gen_shli_i64(tmp1, tmp1, 3);
> > +
> > +            tcg_gen_mov_i64(cpu_ir[rc], cpu_ir[ra]);
> > +            tcg_gen_brcondi_i64(TCG_COND_EQ, tmp1, 0, l1);
> > +
> >              tmp2 = tcg_const_i64(64);
> >              tcg_gen_sub_i64(tmp1, tmp2, tmp1);
> >              tcg_temp_free(tmp2);
> 
> Given that a test costs a lot (partly due to the fact temp local
> variable must be used), I do wonder if doing a AND here wouldn't
> be better:
> 		
>                tcg_gen_andi_i64(tmp1, tmp1, 0x3f);

I'm not sure I follow.

The code is attempting the following:

  tmp1=rb&0x7;
  tmp1=temp1<<3;

  if (tmp1!=0) {
     tmp1=64-tmp1;
     rc=ra<<tmp1;
  }
  else {
     rc=ra;
  }

The problem with the original code is that in the case of tmp1 being 0,
the shift left by 64 would result in 0, instead of the identity.

I tried to avoid the jump but couldn't.  Am I missing something?


> >              tcg_gen_shl_i64(cpu_ir[rc], cpu_ir[ra], tmp1);
> > +
> > +            gen_set_label(l1);
> > +

Vince