HSAIL Backend: Register getting overwritten as part of Move To Phi

[Doug Simon]

Yep.

Sent from my iPhone

On Jul 19, 2013, at 10:09 PM, "Deneau, Tom" <tom.deneau at amd.com> wrote:

> Doug --
> 
> So I assume this is the following changeset which has already been pushed?
> 
> -- Tom
> 
> Changeset: a61fa3e171e7
> Author:    Doug Simon <doug.simon at oracle.com>
> Date:      2013-07-19 12:45 +0200
> URL:       http://hg.openjdk.java.net/graal/graal/rev/a61fa3e171e7
> 
> fixed bug in EdgeMoveOptimizer triggered by a backend (such as HSAIL) that has conditional branches with explicit input operands (as opposed to an implicit condition flags register)
> 
> ! graal/com.oracle.graal.lir/src/com/oracle/graal/lir/EdgeMoveOptimizer.java
> 
> 
> 
> -----Original Message-----
> From: Doug Simon [mailto:doug.simon at oracle.com] 
> Sent: Friday, July 19, 2013 5:55 AM
> To: Deneau, Tom
> Cc: graal-dev at openjdk.java.net
> Subject: Re: HSAIL Backend: Register getting overwritten as part of Move To Phi
> 
> Tom,
> 
> I found the bug and am pushing through a fix now.
> 
> The issue was that, until now, we've only had backends whose conditional branch instructions operate on a conditional flag set by a preceding (compare) instruction. As such, we never considered (input) operands when moving instructions from successors to in front of a conditional branch. This assumption does not apply to HSAIL (or to PTX?). I've made a change that disables the optimization for conditional branches that have operands.
> 
> -Doug
> 
> On Jul 17, 2013, at 7:27 PM, "Deneau, Tom" <tom.deneau at amd.com> wrote:
> 
>> The problem described below was first noticed last week but then I thought
>> it went away with the large number of checkins on July 15.  But now I see
>> that it is still there.
>> 
>> I am not sure how to debug this problem so looking for any suggestions.
>> To help you reproduce the problem on your side, I have a small webrev that
>> adds the failing test case.   The test case itself uses String.contains and so
>> needed the capability to properly handle accesses to char arrays and char fields,
>> so in addition to the failing test case there is a very small change to the HSAIL
>> backend to support the narrower object types like char.
>> 
>> The webrev can be found at
>> http://cr.openjdk.java.net/~tdeneau/graal-webrevs/graal-webrev4.0/
>> 
>> One other change in this webrev, I made a change to the okra library loading code
>> originally suggested by Doug.  Doug was suggesting a new environment variable pointing
>> to the native library so we wouldn't need the -Dsun.boot.library.path on the java command
>> line.  Instead I noticed that we were already requiring the directory to be in the PATH
>> environment variable, so the loading code just looks down the PATH.
>> 
>> In the webrev, this change is simply reflected in a change to the version
>> of the okra jar file (since the okra code is not part of graal proper).
>> The change is entirely in the java code, there is no need
>> to rebuild the actual native libraries.
>> 
>> Reproducing the failing test case...
>> 
>> With the patch applied, you should be able to get the test case failure with the following:
>> 
>>  * mx --vm server build product
>>  * mx --vm server unittest @-G:InlineEverything @-Dkerneltester.logLevel=INFO hsail.test.StringContainsAcceptTest
>> 
>> The logLevel=INFO dumps the generated HSAIL to stdout.  When run on my system, the problem
>> code starts at @L10.  We are reading the first char from each of the strings and
>> getting ready to compare them.
>> 
>> @L10:
>>    ld_global_u16 $s9, [$d0 + 24];
>>    ld_global_u16 $s10, [$d3 + 24];
>>    st_spill_s32 $s10, [%spillseg][24];
>>    mov_b32 $s9, 0;                          <<< problem       
>>    cmp_ne_b1_s32 $c0, $s9, $s10;
>> 
>> 
>> -- Tom Deneau
>> 
>> 
>> 
>> -----Original Message-----
>> From: Deneau, Tom 
>> Sent: Monday, July 15, 2013 4:05 PM
>> To: graal-dev at openjdk.java.net
>> Subject: RE: graal/graal: 107 new changesets
>> 
>> Doug or others --
>> 
>> I was about to ask for help on a codegen problem we were seeing but after updating to the latest default,
>> I see that it is gone (so I'm assuming it was not in our backend :) ).  Of course it is possible that the bug
>> is just being hidden by some unrelated change.   Anyway, here is a brief description,
>> maybe you can tell which of the many changesets below would have fixed this, if any.
>> 
>> We were using String.contains as our test case, which was being nicely inlined, and we generated the following 
>> HSAIL code which led to the problem
>> 
>>    ld_global_u16 $s8, [$d3 + 24];         // read a u16 char from the test String
>>    ld_global_u16 $s9, [$d1 + 24];         // read a u16 char from the pattern  
>>    st_spill_s32 $s9, [%spillseg][24];     // register spill
>>    mov_b32 $s8, 0;                        // <<<<--- This code was causing the problem 
>>    cmp_ne_b1_s32 $c0, $s8, $s9;           // compare the two chars but s8 has been clobbered
>> 
>> 
>> When I did a -G:TraceLIRGeneratorLevel=2, I could see that the problematic instruction
>> "mov_b32 $s8, 0"
>> was generated as part of something called
>>      MOVE TO PHI from 276|EndNode to 277|LoopBegin
>> 
>> as part of PhiResolver.dispose, the part that is commented with
>> // generate move for move from non variable to arbitrary destination
>> 
>> Does the above sound like something that was purposely fixed?
>> 
>> -- Tom Deneau
> 
> 
>