Re: [Qemu-devel] [PATCH] tcg/aarch64: Use ADR for shorter jumps

[Richard Henderson]

On 07/12/2017 12:14 PM, Pranith Kumar wrote:
> Use ADR instruction for shorter jumps.
>
> I was going through rth's email and realized that I should have done
> this the first time.
>
> Signed-off-by: Pranith Kumar <address@hidden>
> ---
>   tcg/aarch64/tcg-target.inc.c | 14 +++++++++-----
>   1 file changed, 9 insertions(+), 5 deletions(-)
>
> diff --git a/tcg/aarch64/tcg-target.inc.c b/tcg/aarch64/tcg-target.inc.c
> index 04bc369a92..5121ebc1a1 100644
> --- a/tcg/aarch64/tcg-target.inc.c
> +++ b/tcg/aarch64/tcg-target.inc.c
> @@ -886,12 +886,16 @@ void aarch64_tb_set_jmp_target(uintptr_t jmp_addr, 
> uintptr_t addr)
>           i1 = I3206_B | ((offset >> 2) & 0x3ffffff);
>           i2 = NOP;
>       } else {
> -        offset = (addr >> 12) - (jmp_addr >> 12);
> +        if (offset == sextract64(offset, 0, 21)) {
> +            i1 = I3406_ADR;
> +            i2 = NOP;
> +        } else {

This is dead code because it is covered by the direct jump above.
B has a 26-bit range, whereas ADR has a 21-bit range.


r~