On Wed, Sep 2, 2015 at 10:12 PM, Richard Biener
<richard.guent...@gmail.com> wrote:
> On Wed, Sep 2, 2015 at 5:26 AM, Bin Cheng <bin.ch...@arm.com> wrote:
>> Hi,
>> This patch is a new approach to fix PR66388. IVO today computes iv_use with
>> iv_cand which has at least same type precision as the use. On 64bit
>> platforms like AArch64, this results in different iv_cand created for each
>> address type iv_use, and register pressure increased. As a matter of fact,
>> the BIV should be used for all iv_uses in some of these cases. It is a
>> latent bug but recently getting worse because of overflow changes.
>>
>> The original approach at
>> https://gcc.gnu.org/ml/gcc-patches/2015-07/msg01484.html can fix the issue
>> except it conflict with IV elimination. Seems to me it is impossible to
>> mitigate the contradiction.
>>
>> This new approach fixes the issue by adding sizetype iv_cand for BIVs
>> directly. In cases if the original BIV is preferred, the sizetype iv_cand
>> will be chosen. As for code generation, the sizetype iv_cand has the same
>> effect as the original BIV. Actually, it's better because BIV needs to be
>> explicitly extended to sizetype to be used in address expression on most
>> targets.
>>
>> One shortage of this approach is it may introduce more iv candidates. To
>> minimize the impact, this patch does sophisticated code analysis and adds
>> sizetype candidate for BIV only if it is used as index. Moreover, it avoids
>> to add candidate of the original type if the BIV is only used as index.
>> Statistics for compiling spec2k6 shows increase of candidate number is
>> modest and can be ignored.
>>
>> There are two more patches following to fix corner cases revealed by this
>> one. In together they bring obvious perf improvement for spec26k/int on
>> aarch64.
>> Spec2k6/int
>> 400.perlbench 3.44%
>> 445.gobmk -0.86%
>> 456.hmmer 14.83%
>> 458.sjeng 2.49%
>> 462.libquantum -0.79%
>> GEOMEAN 1.68%
>>
>> There is also about 0.36% improvement for spec2k6/fp, mostly because of case
>> 436.cactusADM. I believe it can be further improved, but that should be
>> another patch.
>>
>> I also collected benchmark data for x86_64. Spec2k6/fp is not affected. As
>> for spec2k6/int, though the geomean is improved slightly, 400.perlbench is
>> regressed by ~3%. I can see BIVs are chosen for some loops instead of
>> address candidates. Generally, the loop header will be simplified because
>> iv elimination with BIV is simpler; the number of instructions in loop body
>> isn't changed. I suspect the regression comes from different addressing
>> modes. With BIV, complex addressing mode like [base + index << scale +
>> disp] is used, rather than [base + disp]. I guess the former has more
>> micro-ops, thus more expensive. This guess can be confirmed by manually
>> suppressing the complex addressing mode with higher address cost.
>> Now the problem becomes why overall cost of BIV is computed lower while the
>> actual cost is higher. I noticed for most affected loops, loop header is
>> bloated because of iv elimination using the old address candidate. The
>> bloated loop header results in much higher cost than BIV. As a result, BIV
>> is preferred. I also noticed the bloated loop header generally can be
>> simplified (I have a following patch for this). After applying the local
>> patch, the old address candidate is chosen, and most of regression is
>> recovered.
>> Conclusion is I think loop header bloated issue should be blamed for the
>> regression, and it can be resolved.
>>
>> Bootstrap and test on x64_64 and aarch64. It fixes failure of
>> gcc.target/i386/pr49781-1.c, without new breakage.
>>
>> So what do you think?
>
> The data above looks ok to me.
>
> +static struct iv *
> +find_deriving_biv_for_iv (struct ivopts_data *data, struct iv *iv)
> +{
> + aff_tree aff;
> + struct expand_data exp_data;
> +
> + if (!iv->ssa_name || TREE_CODE (iv->ssa_name) != SSA_NAME)
> + return iv;
> +
> + /* Expand IV's ssa_name till the deriving biv is found. */
> + exp_data.data = data;
> + exp_data.biv = NULL;
> + tree_to_aff_combination_expand (iv->ssa_name, TREE_TYPE (iv->ssa_name),
> + &aff, &data->name_expansion_cache,
> + stop_expand, &exp_data);
> + return exp_data.biv;
>
> that's actually "abusing" tree_to_aff_combination_expand for simply walking
> SSA uses and their defs uses recursively until you hit "stop". ISTR past
> discussion to add a generic walk_ssa_use interface for that. Not sure if it
> materialized with a name I can't remember or whether it didn't.
Thanks for reviewing. I didn't found existing interface to walk up
definition chains of ssa vars. In this updated patch, I implemented a
simple function which meets the minimal requirement of walking up
definition chains of BIV variables. I also counted number of
no_overflow BIVs that are not used in address type use. Since
generally there are only two BIVs in a loop, this can prevent us from
visiting definition chains most of time. Statistics shows that number
of call to find_deriving_biv_for_expr plummet.
>
> - add_candidate (data, iv->base, iv->step, true, NULL);
> + /* Check if this biv is used in address type use. */
> + if (iv->no_overflow && iv->have_address_use
> + && INTEGRAL_TYPE_P (TREE_TYPE (iv->base))
> + && TYPE_PRECISION (TREE_TYPE (iv->base)) < TYPE_PRECISION (sizetype))
> + {
> + tree type = unsigned_type_for (sizetype);
>
> sizetype is unsigned.
Fixed.
Bootstrap and test on x86_64, is this OK?
Thanks,
bin
>
> the rest looks ok to me but I really don't like the abuse of
> tree_to_aff_combination_expand...
>
> Thanks,
> Richard.
>
>> Thanks,
>> bin
>>
>> 2015-08-31 Bin Cheng <bin.ch...@arm.com>
>>
>> * tree-affine.c (aff_combination_expand): New parameters.
>> (tree_to_aff_combination_expand): Ditto.
>> * tree-affine.h (aff_combination_expand): New declaration.
>> (tree_to_aff_combination_expand): Ditto.
>> * tree-ssa-loop-ivopts.c (struct iv, iv_cand): New fields.
>> (dump_iv): Dump no_overflow information.
>> (alloc_iv): Initialize new field for struct iv.
>> (struct expand_data): New struct for affine combination expanding.
>> (stop_expand): New callback func for affine combination expanding.
>> (find_deriving_biv_for_iv, record_biv_for_address_use): New
>> functions.
>> (idx_find_step): Call new functions above.
>> (find_depends, add_candidate): New paramter.
>> (add_iv_candidate_for_biv): Add sizetype cand for BIV.
>> (get_computation_aff): Simplify convertion of cand for BIV.
>> (get_computation_cost_at): Step cand's base if necessary.
>>
