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

[Richard Earnshaw]

On Tue, 2011-03-29 at 22:53 +0800, Chung-Lin Tang wrote:
> 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.
> 

Sorry, didn't mean to suggest that we don't want to do something better
for addresses that are a multiple of 4, just that for addresses that
aren't (at least) word-aligned that we should just bail as the code in
that case won't benefit from the optimization.  So something like

       if (mode == DImode || (mode == DFmode && TARGET_SOFT_FLOAT))
	 {
	    if (val & 3)
		return false;  /* No point in trying to handle this.  */
	    ... /* Cases that are useful to handle */

R.


> Thanks,
> Chung-Lin
>