[patch, ARM] Fix PR48250, adjust DImode reload address legitimizing

[Chung-Lin Tang]

On 2011/3/29 下午 10:26, Richard Earnshaw wrote:
> On Tue, 2011-03-29 at 18:25 +0800, Chung-Lin Tang wrote:
>> On 2011/3/24 06:51 PM, Richard Earnshaw wrote:
>>>
>>> On Thu, 2011-03-24 at 12:56 +0900, Chung-Lin Tang wrote:
>>>> Hi,
>>>> PR48250 happens under TARGET_NEON, where DImode is included within the
>>>> valid NEON modes. This turns the range of legitimate constant indexes to
>>>> step-4 (coproc load/store), thus arm_legitimize_reload_address() when
>>>> trying to decompose the [reg+index] reload address into
>>>> [(reg+index_high)+index_low], can cause an ICE later when 'index_low'
>>>> part is not aligned to 4.
>>>>
>>>> I'm not sure why the current DImode index is computed as:
>>>> low = ((val & 0xf) ^ 0x8) - 0x8;  the sign-extending into negative
>>>> values, then subtracting back, actually creates further off indexes.
>>>> e.g. in the supplied testcase, [sp+13] was turned into [(sp+16)-3].
>>>>
>>>
>>> Hysterical Raisins... the code there was clearly written for the days
>>> before we had LDRD in the architecture.  At that time the most efficient
>>> way to load a 64-bit object was to use the LDM{ia,ib,da,db}
>>> instructions.  The computation here was (I think), intended to try and
>>> make the most efficient use of an add/sub instruction followed by
>>> LDM/STM offsetting.  At that time the architecture had no unaligned
>>> access either, so dealing with 64-bit that were less than 32-bit aligned
>>> (in those days 32-bit was the maximum alignment) probably wasn't
>>> considered, or couldn't even get through to reload.
>>>
>>
>> I see it now. The code in output_move_double() returning assembly for
>> ldm/stm(db/da/ib) for offsets -8/-4/+4 seems to confirm this.
>>
>> I have changed the patch to let the new code handle the TARGET_LDRD case
>> only.  The pre-LDRD case is still handled by the original code, with an
>> additional & ~0x3 for aligning the offset to 4.
>>
>> I've also added a comment for the pre-TARGET_LDRD case. Please see if
>> the description is accurate enough.
>>
>>>> My patch changes the index decomposing to a more straightforward way; it
>>>> also sort of outlines the way the other reload address indexes are
>>>> broken by using and-masks, is not the most effective.  The address is
>>>> computed by addition, subtracting away the parts to obtain low+high
>>>> should be the optimal way of giving the largest computable index range.
>>>>
>>>> I have included a few Thumb-2 bits in the patch; I know currently
>>>> arm_legitimize_reload_address() is only used under TARGET_ARM, but I
>>>> guess it might eventually be turned into TARGET_32BIT.
>>>>
>>>
>>> I think this needs to be looked at carefully on ARMv4/ARMv4T to check
>>> that it doesn't cause regressions there when we don't have LDRD in the
>>> instruction set.
>>
>> I'll be testing the modified patch under an ARMv4/ARMv4T configuration.
>> Okay for trunk if no regressions?
>>
>> Thanks,
>> Chung-Lin
>>
>> 	PR target/48250
>> 	* config/arm/arm.c (arm_legitimize_reload_address): Adjust
>> 	DImode constant index decomposing under TARGET_LDRD. Clear
>> 	lower two bits for NEON, Thumb-2, and !TARGET_LDRD. Add
>> 	comment for !TARGET_LDRD case.
> 
> This looks technically correct, but I can't help feeling that trying to
> deal with the bottom two bits being non-zero is not really worthwhile.
> This hook is an optimization that's intended to generate better code
> than the default mechanisms that reload provides.  It is allowed to
> fail, but it must say so if it does (by returning false).
> 
> What this hook is trying to do for DImode is to take an address of the
> form (reg + TOO_BIG_CONST) and turn it into two instructions that are
> legitimate:
> 
> 	tmp = (REG + LEGAL_BIG_CONST)
> 	some_use_of (mem (tmp + SMALL_LEGAL_CONST))
> 
> The idea behind the optimization is that LEGAL_BIG_CONST will need fewer
> instructions to generate than TOO_BIG_CONST.  It's unlikely (probably
> impossible in ARM state) that pushing the bottom two bits of the address
> into LEGAL_BIG_CONST part of the offset is going to lead to a better
> code sequence.  
> 
> So I think it would be more sensible to just return false if we have a
> DImode address with the bottom 2 bits non-zero and then let the normal
> reload recovery mechanism take over.

It is supposed to provide better utilization of the full range of
LEGAL_BIG_CONST+SMALL_LEGAL_CONST.  I am not sure, but I guess reload
will resolve it with the reg+LEGAL_BIG_CONST part only, using only (mem
(reg)) for the load/store (correct me if I'm wrong).

Also, the new code slighty improves the reloading, for example currently
[reg+64] is broken into [(reg+72)-8], creating an additional unneeded
reload, which is certainly not good when we have ldrd/strd available.

Thanks,
Chung-Lin