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

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

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.