RFR(S): 8243670: Unexpected test result caused by C2 MergeMemNode::Ideal

[Yangfei (Felix)]

Hi Tobias,

> -----Original Message-----
> From: Tobias Hartmann [mailto:tobias.hartmann at oracle.com]
> Sent: Tuesday, May 26, 2020 10:24 PM
> To: Yangfei (Felix) <felix.yang at huawei.com>; hotspot-compiler-
> dev at openjdk.java.net
> Subject: Re: RFR(S): 8243670: Unexpected test result caused by C2
> MergeMemNode::Ideal
> 
> Hi Felix,
> 
> thanks for the details, makes sense to me.
> 
> Isn't the root cause that we are loosing type information and wouldn't that
> be solved by selecting the Phi with the more restrictive _adr_type?

Sorry for the late reply. 
Not sure if I understand correctly.  Do you mean something like this?

diff -r 6ab7805df10d src/hotspot/share/opto/memnode.cpp
--- a/src/hotspot/share/opto/memnode.cpp        Sat Jun 13 01:00:00 2020 +0200
+++ b/src/hotspot/share/opto/memnode.cpp        Mon Jun 15 16:40:57 2020 +0800
@@ -4618,7 +4618,8 @@
         }
         if (phi_mem != NULL) {
           // equivalent phi nodes; revert to the def
-          new_mem = new_base;
+          if (phi_base->adr_type()->higher_equal(phi_mem->adr_type()))
+            new_mem = new_base;
         }
       }
     }



Thanks,
Felix