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


From: Pranith Kumar
Subject: Re: [Qemu-devel] [PATCH] tcg/aarch64: Use ADR for shorter jumps
Date: Wed, 12 Jul 2017 20:14:07 -0400

On Wed, Jul 12, 2017 at 7:08 PM, Richard Henderson <address@hidden> wrote:
> 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.

Hmm, yep. And I guess it doesn't make sense to use ADR for short jumps
because this will be a 2 instruction jump vs 1 instruction for direct
branch.

Thanks,
-- 
Pranith



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