Re: [PATCH 13/20] target/arm: Convert LDR/STR with 12-bit immediate to decodetree

[Peter Maydell]

On Fri, 2 Jun 2023 at 21:51, Philippe Mathieu-Daudé <philmd@linaro.org> wrote:
>
> Hi Peter,
>
> On 2/6/23 17:52, Peter Maydell wrote:
> > Convert the LDR and STR instructions which use a 12-bit immediate
> > offset to decodetree. We can reuse the existing LDR and STR
> > trans functions for these.
> >
> > Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> > ---
> >   target/arm/tcg/a64.decode      |  25 ++++++++
> >   target/arm/tcg/translate-a64.c | 103 +++++----------------------------
> >   2 files changed, 41 insertions(+), 87 deletions(-)
> >
> > diff --git a/target/arm/tcg/a64.decode b/target/arm/tcg/a64.decode
> > index 4dfb7bbdc2e..c3a6d0b740a 100644
> > --- a/target/arm/tcg/a64.decode
> > +++ b/target/arm/tcg/a64.decode
>
>
> > +# Load/store with an unsigned 12 bit immediate, which is scaled by the
> > +# element size. The function gets the sz:imm and returns the scaled 
> > immediate.
> > +%uimm_scaled   10:12 sz:3 !function=uimm_scaled
> > +
> > +@ldst_uimm      .. ... . .. .. ............ rn:5 rt:5 &ldst_imm unpriv=0 
> > p=0 w=0 imm=%uimm_scaled
> > +
> > +STR_i           sz:2 111 0 01 00 ............ ..... ..... @ldst_uimm 
> > sign=0 ext=0
> > +LDR_i           00 111 0 01 01 ............ ..... ..... @ldst_uimm sign=0 
> > ext=1 sz=0
> > +LDR_i           01 111 0 01 01 ............ ..... ..... @ldst_uimm sign=0 
> > ext=1 sz=1
> > +LDR_i           10 111 0 01 01 ............ ..... ..... @ldst_uimm sign=0 
> > ext=1 sz=2
> > +LDR_i           11 111 0 01 01 ............ ..... ..... @ldst_uimm sign=0 
> > ext=0 sz=3
> > +LDR_i           00 111 0 01 10 ............ ..... ..... @ldst_uimm sign=1 
> > ext=0 sz=0
> > +LDR_i           01 111 0 01 10 ............ ..... ..... @ldst_uimm sign=1 
> > ext=0 sz=1
> > +LDR_i           10 111 0 01 10 ............ ..... ..... @ldst_uimm sign=1 
> > ext=0 sz=2
> > +LDR_i           00 111 0 01 11 ............ ..... ..... @ldst_uimm sign=1 
> > ext=1 sz=0
> > +LDR_i           01 111 0 01 11 ............ ..... ..... @ldst_uimm sign=1 
> > ext=1 sz=1
>
> Why not use "sz:2 111 0 01 sign:1 ext:1", returning false for the
> cases not covered?

That would also catch the PRFM case (which is size=0b11 sign=1 ext=0
in your pattern); and it gives the wrong ext value for 11 111 0 01 01.

You could do more of the decode in the trans_ functions and less
here; but my impression is that we tend to prefer decoding in the
.decode file where we can.

thanks
-- PMM