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

[Richard Earnshaw <rearnsha at arm dot com>]

On Thu, 2011-03-31 at 11:33 +0800, Chung-Lin Tang wrote:
> On 2011/3/30 05:28 PM, Richard Earnshaw wrote:
> > 
> > On Wed, 2011-03-30 at 15:35 +0800, Chung-Lin Tang wrote:
> >> On 2011/3/30 äå 12:23, Richard Earnshaw wrote:
> >>>
> >>> On Tue, 2011-03-29 at 22:53 +0800, Chung-Lin Tang wrote:
> >>>> On 2011/3/29 äå 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 */
> >>
> >> I've looked at the reload code surrounding the call to
> >> LEGITIMIZE_RELOAD_ADDRESS. It looks like for ARM, reload transforms the
> >> address from [reg+#const] to newreg=#const; [reg+newreg]. ARM/Thumb-2
> >> has 16-bits to move that constant, which is much more wider in range
> >> than a 12-bit constant operand + 8-bit index. So I agree that simply
> >> bailing out should be okay.
> >>
> >> OTOH, I'll still add that, for some micro-architectures, register read
> >> ports may be a critical resource; for those cores, handling as many
> >> reloads here as possible by breaking into an address add is still
> >> slightly better than a 'move + [reg+reg]', for the latter load/store
> >> uses one more register read.  So maybe the best should be, to handle
> >> when the 'high' part is a valid add-immediate-operand, and bail out if
> >> not...
> >>
> >> C.L.
> > 
> > If the address is unaligned, then the access is going to be slow anyway;
> > but this is all corner case stuff - the vast majority of accesses will
> > be at natural alignment.  I think it's better to seek clarity in these
> > cases than outright performance in theoretical micro-architectural
> > corner cases.
> > 
> > The largest number of read ports would be needed by store[reg+reg].
> > That's only 3 ports for a normal store (four for a pair of registers),
> > but cores can normally handle this without penalty by reading the
> > address registers in one cycle and the data to be stored in a later
> > cycle -- critical paths tend to be on address generation, not data to be
> > stored.
> 
> Actually, I was thinking of cores with dual-issue, where an additional
> port read may prevent it from happening...
> 
> Anyways, here's a new patch. I've removed the unaligned handling bits as
> you suggested, simply returning false for those cases.
> 
> The points above did inspire another improvement, I think. I have added
> a test to also return false when the high part is not a valid immediate
> operand.  The rationale is, after such a reg=reg+high address compute is
> created, it will still have to be split into multiple adds later, so it
> may be better to let reload turn it into the [reg+reg] form.
> 

Hmm, I think you've missed the point with some of this, which is that
not only is it generally more efficient to try and use offset addressing
but careful selection of the immediate values used in the load and the
ADD insns can often also lead to better reload CSE.  For example:

	ldr	r0, [r2, #4100]  // Offset too large
	ldr	r1, [r2, #4104]  // Offset too large

is best reloaded as
	add	t1, r2, #4096
	ldr	r0, [t1, #4]
	add	t2, r2, #4096
	ldr	r1, [t2, #8]

which of course post-reload CSE can simplify in most cases to eliminate
the second add instruction:

	add	t1, r2, #4096
	ldr	r0, [t1, #4]
	ldr	r1, [t1, #8]

This is true even if the amount of the offset being split out is larger
than a simple legitimate_constant.

The idea here is that we want to split out the bits of the constant as a
mask rather than as subtracting the maximum offset that ldr can handle
(the same principle applies to LDRD too).

A further trick is that we can make use of negative offsets even if the
overall offset is positive and that sometimes this may lead to an
immediate that can be constructed with one fewer add instructions  For
example,

	ldr	r0, [r2, #0x3FFFFC]

This is best reloaded as:

	add	t1, r2, #0x400000
	ldr	r0, [t1, #-4]

The trick for spotting this for a load instruction with N bits of offset
(ie bits N-1:0) is to look at bit N: if it is set, then chose a negative
offset that is going to make bit N and all the bits below it come to
zero in the remainder part.

The final thing to note is that offsets for negative values in Thumb2
are asymmetrical from the positive values that are available.  That
makes selecting the best offset more complicated, and thus using
negative values is less likely to be worth while.

> I'll be testing this patch later, here it is for reviewing.
> 
> Thanks,
> Chung-Lin