Question on ARM legitimate address for DImode

[Jie Zhang]

On 12/21/2010 06:12 PM, Richard Earnshaw wrote:
>
> On Tue, 2010-12-21 at 12:12 +0800, Jie Zhang wrote:
>> Hi,
>>
>> While working on a bug, I found some code in ARM port that I don't
>> understand.
>>
>> In ARM_LEGITIMIZE_RELOAD_ADDRESS and arm_legitimize_address, we allow a
>> very small offset for DImode addressing.
>>
>> In ARM_LEGITIMIZE_RELOAD_ADDRESS:
>>
>> if (MODE == DImode || (MODE == DFmode&&  TARGET_SOFT_FLOAT))     \
>>     low = ((val&  0xf) ^ 0x8) - 0x8;                               \
>>
>> In arm_legitimize_address
>>
>>     /* VFP addressing modes actually allow greater offsets, but for
>>        now we just stick with the lowest common denominator.  */
>>     if (mode == DImode
>>         || ((TARGET_SOFT_FLOAT || TARGET_VFP)&&  mode == DFmode))
>>       {
>>         low_n = n&  0x0f;
>>         n&= ~0x0f;
>>         if (low_n>  4)
>>           {
>>             n += 16;
>>             low_n -= 16;
>>           }
>>       }
>>
>> AFAIK, we could use two LDRs, or one LDRD, or one VLDR to access DImode
>> in memory when the address is in the form of (REG + CONST_INT). The
>> offset ranges for these three cases are:
>>
>> LDR  -4095,4091
>> LDRD -255,255
>> VLDR -1020,1020&&  (ADDR&  3) == 0
>
> The original code was designed to exploit LDM(IA,IB,DB,DA) which would
> have the offset ranges described.  On earlier ARM chips (certainly up to
> and including ARM7TDMI) it was a significant win to do it that way (add
> a constant to the address register and then use LDM was faster than two
> LDR instructions).
>
> That's no-longer true on modern chips, LDM is often slower than
> individual LDR insns now.
>
Thanks! Now I see. So I think the original code is still needed but 
should be used only for such earlier ARM chips. I will send the updated 
patch to gcc-patches mailing list.


-- 
Jie Zhang