Re: [PATCH GCC]Improve how we handle overflow for type conversion in scev/ivopts, part I

[Richard Biener <richard dot guenther at gmail dot com>]

On Wed, May 20, 2015 at 11:41 AM, Bin Cheng <bin.cheng@arm.com> wrote:
> Hi,
> As we know, GCC is too conservative when checking overflow behavior in SCEV
> and loop related optimizers.  Result is some variable can't be recognized as
> scalar evolution and thus optimizations are missed.  To be specific,
> optimizers like ivopts and vectorizer are affected.
> This issue is more severe on 64 bit platforms, for example, PR62173 is
> failed on aarch64; scev-3.c and scev-4.c were marked as XFAIL on lp64
> platforms.
>
> As the first part to improve overflow checking in GCC, this patch does below
> improvements:
>   1) Ideally, chrec_convert should be responsible to convert scev like
> "(type){base, step}" to scev like "{(type)base, (type)step} when the result
> scev doesn't overflow; chrec_convert_aggressive should do the conversion if
> the result scev could overflow/wrap.  Unfortunately, current implementation
> may use chrec_convert_aggressive to return a scev that won't overflow.  This
> is because of a) the static parameter "fold_conversions" for
> instantiate_scev_convert can only tracks whether chrec_convert_aggressive
> may be called, rather than if it does some overflow conversion or not;  b)
> the implementation of instantiate_scev_convert sometimes shortcuts the call
> to chrec_convert and misses conversion opportunities.  This patch improves
> this.
>   2) iv->no_overflow computed in simple_iv is too conservative.  With 1)
> fixed, iv->no_overflow should reflects whether chrec_convert_aggressive does
> return an overflow scev.  This patch improves this.
>   3) chrec_convert should be able to prove the resulting scev won't overflow
> with loop niter information.  This patch doesn't finish this, but it
> factored a new interface out of scev_probably_wraps_p for future
> improvement.  And that will be the part II patch.
>
> With the improvements in SCEV, this patch also improves optimizer(IVOPT)
> that uses scev information like below:
>   For array reference in the form of arr[IV], GCC tries to derive new
> address iv {arr+iv.base, iv.step*elem_size} from IV.  If IV overflow wrto a
> type that is narrower than address space, this derivation is not true
> because &arr[IV] isn't a scev.  Root cause why scev-*.c are failed now is
> the overflow information of IV is too conservative.  IVOPT has to be
> conservative to reject &arr[IV] as a scev.  With more accurate overflow
> information, IVOPT can be improved too.  So this patch fixes the mentioned
> long standing issues.
>
> Bootstrap and test on x86_64, x86 and aarch64.
> BTW, test gcc.target/i386/pr49781-1.c failed on x86_64, but I can confirmed
> it's not this patch's fault.
>
> So what's your opinion on this?.

I maybe mixing things up but does

+chrec_convert_aggressive (tree type, tree chrec, bool *fold_conversions)
 {
...
+  if (evolution_function_is_affine_p (chrec))
+    {
+      tree base, step;
+      struct loop *loop;
+
+      loop = get_chrec_loop (chrec);
+      base = CHREC_LEFT (chrec);
+      step = CHREC_RIGHT (chrec);
+      if (convert_affine_scev (loop, type, &base, &step, NULL, true))
+       return build_polynomial_chrec (loop->num, base, step);

^^^ not forget to set *fold_conversions to true?  Or we need to use
convert_affine_scev (..., false)?

+    }

(bah, and the diff somehow messes up -p context :/  which is why I like
context diffs more)

Other from the above the patch looks good to me.

Thanks,
Richard.

> Thanks,
> bin
>
> 2015-05-20  Bin Cheng  <bin.cheng@arm.com>
>
>         PR tree-optimization/62173
>         * tree-ssa-loop-ivopts.c (struct iv): New field.  Reorder fields.
>         (alloc_iv, set_iv): New parameter.
>         (determine_biv_step): Delete.
>         (find_bivs): Inline original determine_biv_step.  Pass new
>         argument to set_iv.
>         (idx_find_step): Use no_overflow information for conversion.
>         * tree-scalar-evolution.c (analyze_scalar_evolution_in_loop): Let
>         resolve_mixers handle folded_casts.
>         (instantiate_scev_name): Change bool parameter to bool pointer.
>         (instantiate_scev_poly, instantiate_scev_binary): Ditto.
>         (instantiate_array_ref, instantiate_scev_not): Ditto.
>         (instantiate_scev_3, instantiate_scev_2): Ditto.
>         (instantiate_scev_1, instantiate_scev_r): Ditto.
>         (instantiate_scev_convert, ): Change parameter.  Pass argument
>         to chrec_convert_aggressive.
>         (instantiate_scev): Change argument.
>         (resolve_mixers): New parameter and set it.
>         (scev_const_prop): New argument.
>         * tree-scalar-evolution.h (resolve_mixers): New parameter.
>         * tree-chrec.c (convert_affine_scev): Call chrec_convert instead
>         of chrec_conert_1.
>         (chrec_convert): New parameter.  Move definition below.
>         (chrec_convert_aggressive): New parameter and set it.  Call
>         convert_affine_scev.
>         * tree-chrec.h (chrec_convert): New parameter.
>         (chrec_convert_aggressive): Ditto.
>         * tree-ssa-loop-niter.c (loop_exits_before_overflow): New function.
>         (scev_probably_wraps_p): Factor loop niter related code into
>         loop_exits_before_overflow.
>
> gcc/testsuite/ChangeLog
> 2015-05-20  Bin Cheng  <bin.cheng@arm.com>
>
>         PR tree-optimization/62173
>         * gcc.dg/tree-ssa/scev-3.c: Remove xfail.
>         * gcc.dg/tree-ssa/scev-4.c: Ditto.
>         * gcc.dg/tree-ssa/scev-8.c: New.