Re: [PATCH] target/arm: Use deposit_z() in BCF opcode

[Philippe Mathieu-Daudé]

On 29/8/23 15:20, Peter Maydell wrote:
> On Tue, 22 Aug 2023 at 10:51, Philippe Mathieu-Daudé <philmd@linaro.org> wrote:
> > When clearing a bitfield we don't need to lead the
>
> "load" ?
>
> > source register. Use deposit_z_i32() with the BFC
> > opcode to save a load_reg() call.
> >
> > Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
> > ---
> >   target/arm/tcg/translate.c | 9 ++++-----
> >   1 file changed, 4 insertions(+), 5 deletions(-)
> >
> > diff --git a/target/arm/tcg/translate.c b/target/arm/tcg/translate.c
> > index b71ac2d0d5..1a6938d1b3 100644
> > --- a/target/arm/tcg/translate.c
> > +++ b/target/arm/tcg/translate.c
> > @@ -7255,7 +7255,7 @@ static bool trans_UBFX(DisasContext *s, arg_UBFX *a)
> >   static bool trans_BFCI(DisasContext *s, arg_BFCI *a)
> >   {
> >       int msb = a->msb, lsb = a->lsb;
> > -    TCGv_i32 t_in, t_rd;
> > +    TCGv_i32 t_rd;
> >       int width;
> >
> >       if (!ENABLE_ARCH_6T2) {
> > @@ -7268,15 +7268,14 @@ static bool trans_BFCI(DisasContext *s, arg_BFCI *a)
> >       }
> >
> >       width = msb + 1 - lsb;
> > +    t_rd = load_reg(s, a->rd);
> >       if (a->rn == 15) {
> >           /* BFC */
> > -        t_in = tcg_constant_i32(0);
> > +        tcg_gen_deposit_z_i32(t_rd, t_rd, lsb, width);
> >       } else {
> >           /* BFI */
> > -        t_in = load_reg(s, a->rn);
> > +        tcg_gen_deposit_i32(t_rd, t_rd, load_reg(s, a->rn), lsb, width);
> >       }
> > -    t_rd = load_reg(s, a->rd);
> > -    tcg_gen_deposit_i32(t_rd, t_rd, t_in, lsb, width);
> >       store_reg(s, a->rd, t_rd);
> >       return true;
>
> The comment says we are saving a load_reg() call, but the
> code change doesn't seem to do that. Before the change:
>   * for BFC we call load_reg for rd
>   * for BFI we call load_reg for rn and rd
>
> After the change:
>   * for BFC we call load_reg for rd
>   * for BFI we call load_reg for rn and rd
>
> So we're not saving any load_reg() calls as far as I can see ?

Indeed you are right.

The optimizations are in tcg_gen_deposit_z_i32() vs using
tcg_gen_deposit_i32(). I'll reword the description.

Thanks,

Phil.