Re: [Qemu-devel] [PATCH 2/2] target-mips: simplify LWL/LDL mask generation

[Paolo Bonzini]

On 14/07/2015 17:45, Aurelien Jarno wrote:
> The LWL/LDL instructions mask the GPR with a mask depending on the
> address alignement. It is currently computed by doing:
> 
>     mask = 0x7fffffffffffffffull >> (t1 ^ 63)
> 
> It's simpler to generate it by doing:
> 
>     mask = (1 << t1) - 1

Using ~(-1 << t1) may let you use an ANDN instruction, and is also the
same number of instructions on x86.

Paolo

> It uses the same number of TCG instructions, but it avoids a 32/64-bit
> constant loading which can take a few instructions on RISC hosts.
> 
> Cc: Leon Alrae <address@hidden>
> Tested-by: Hervé Poussineau <address@hidden>
> Signed-off-by: Aurelien Jarno <address@hidden>
> ---
>  target-mips/translate.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/target-mips/translate.c b/target-mips/translate.c
> index 0ac3bd8..9891209 100644
> --- a/target-mips/translate.c
> +++ b/target-mips/translate.c
> @@ -2153,9 +2153,9 @@ static void gen_ld(DisasContext *ctx, uint32_t opc,
>          tcg_gen_andi_tl(t0, t0, ~7);
>          tcg_gen_qemu_ld_tl(t0, t0, ctx->mem_idx, MO_TEQ);
>          tcg_gen_shl_tl(t0, t0, t1);
> -        tcg_gen_xori_tl(t1, t1, 63);
> -        t2 = tcg_const_tl(0x7fffffffffffffffull);
> -        tcg_gen_shr_tl(t2, t2, t1);
> +        t2 = tcg_const_tl(1);
> +        tcg_gen_shl_tl(t2, t2, t1);
> +        tcg_gen_subi_tl(t2, t2, 1);
>          gen_load_gpr(t1, rt);
>          tcg_gen_and_tl(t1, t1, t2);
>          tcg_temp_free(t2);
> @@ -2246,9 +2246,9 @@ static void gen_ld(DisasContext *ctx, uint32_t opc,
>          tcg_gen_andi_tl(t0, t0, ~3);
>          tcg_gen_qemu_ld_tl(t0, t0, ctx->mem_idx, MO_TEUL);
>          tcg_gen_shl_tl(t0, t0, t1);
> -        tcg_gen_xori_tl(t1, t1, 31);
> -        t2 = tcg_const_tl(0x7fffffffull);
> -        tcg_gen_shr_tl(t2, t2, t1);
> +        t2 = tcg_const_tl(1);
> +        tcg_gen_shl_tl(t2, t2, t1);
> +        tcg_gen_subi_tl(t2, t2, 1);
>          gen_load_gpr(t1, rt);
>          tcg_gen_and_tl(t1, t1, t2);
>          tcg_temp_free(t2);
>