Re: [PATCH GCC]Resolve compilation time known alias checks in vectorizer

[Bin Cheng <Bin dot Cheng at arm dot com>]

And Below is the ChangeLog entry for test cases

gcc/testsuite/ChangeLog
2016-06-07  Bin Cheng  <bin.cheng@arm.com>

	* gcc.dg/vect/vect-35-big-array.c: Refine comment and test.
	* gcc.dg/vect/vect-35.c: Ditto.

BTW, this patch also makes gcc.dg/vect/vect-mask-store-move-1.c fail, but I think it just exposes existing issue in PR65206.  Vectorizer's dependence analyzer should be fixed for this.

Thanks,
bin


> From: gcc-patches-owner@gcc.gnu.org <gcc-patches-owner@gcc.gnu.org> on behalf of Bin Cheng <Bin.Cheng@arm.com>
> Sent: 13 June 2016 11:01
> To: gcc-patches@gcc.gnu.org
> Cc: nd
> Subject: [PATCH GCC]Resolve compilation time known alias checks in vectorizer
>    
> Hi,
> GCC vectorizer generates many unnecessary runtime alias checks known at compilation time.  For some data-reference pairs, alias relation can be resolved at compilation time, in this case, we can 
> simply skip the alias check.  For some other data-reference pairs,  alias relation can be realized at compilation time, in this case, we should return false and simply skip vectorizing the loop.  For the second 
> case, the corresponding loop is vectorized for nothing because the guard alias condition is bound to false anyway.   Vectorizing it not only wastes compilation time, but also slows down generated code 
> because GCC fails to resolve these "false" alias check after vectorization.  Even in cases it can be resolved (by VRP), GCC fails to cleanup all the mess generated in loop  versioning.
> This looks like a common issue in spec2k6.  For example, in 434.zeusmp/ggen.f, there are three loops vectorized but never executed; in 464.h264ref, there are loops in which all runtime alias checks are 
> resolved at compilation time thus loop versioning is proven  unnecessary.  Statistic data also shows that about >100 loops are falsely vectorized currently in my build of spec2k6.
> 
> This patch is based on  https://gcc.gnu.org/ml/gcc-patches/2016-06/msg00399.html, bootstrap and test on x86_64 and AArch64 (ongoing), is it OK?
> 
> Thanks,
> bin
> 
> 2016-06-07  Bin Cheng  <bin.cheng@arm.com>
> 
>         * tree-vect-data-refs.c (vect_no_alias_p): New function.
>         (vect_prune_runtime_alias_test_list): Call vect_no_alias_p to
>         resolve alias checks which are known at compilation time.
>         Truncate vector LOOP_VINFO_MAY_ALIAS_DDRS(loop_vinfo) if all
>         alias checks are resolved at compilation time.