On Wed, May 20, 2015 at 11:41 AM, Bin Cheng <bin.ch...@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.ch...@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.ch...@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.