[PATCH PR94026] combine missed opportunity to simplify comparisons with zero

[Yangfei (Felix)]

> -----Original Message-----
> From: Segher Boessenkool [mailto:segher@kernel.crashing.org]
> Sent: Friday, March 13, 2020 7:50 AM
> To: Yangfei (Felix) <felix.yang@huawei.com>
> Cc: gcc-patches@gcc.gnu.org; Zhanghaijian (A) <z.zhanghaijian@huawei.com>
> Subject: Re: [PATCH PR94026] combine missed opportunity to simplify
> comparisons with zero
> 
> Hi!
> 
> Please Cc: me on combine patches; you sent it nine days ago, and I didn't see it
> until now.

OK.  

> On Wed, Mar 04, 2020 at 08:39:36AM +0000, Yangfei (Felix) wrote:
> >   This is a simple fix for PR94026.
> >   With this fix, combine will try make an extraction if we are in a equality
> comparison and this is an AND
> >   with a constant which is power of two minus one.  Shift here should be an
> constant.  For example, combine
> >   will transform (compare (and (lshiftrt x 8) 6) 0) to (compare (zero_extract
> (x 2 9)) 0).
> 
> Why is that a good thing?

The reported test case is reduced from spec2017 541.leela_r.  I have pasted original code snippet on the bugzilla.  
We found other compilers like aocc/llvm can catch this pattern and simplify it.  

> (There should be thorough tests on many archs, showing it helps on average,
> and it doesn't regress anything.  I can do that for you, but not right now).

I only have aarch64 & x86_64 linux available and have tested this patch with spec17 on both platforms.  
No obvious improvement & regression witnessed.  This is expected as only one instruction is reduced here.  
It's appreciated if this can be tested on other archs.  

> The code needs more comments, and the commit message should say what is
> done and why you made those choices.
> In general, we should have *fewer* zero_extract, not more.

OK.  I can add more comments & commit message if finally we are inclined to go with this patch.  

Thanks,
Felix