RFR: 8276105: C2: Conv(D|F)2(I|L)Nodes::Ideal should handle rounding correctly [v2]

[Aleksey Shipilev]

On Mon, 1 Nov 2021 15:43:44 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> Happens now in master:
>> 
>> 
>> $ CONF=linux-x86-server-fastdebug make run-test TEST=compiler/loopopts/superword/CoLocatePack.java TEST_VM_OPTS="-XX:UseAVX=0 -XX:UseSSE=0"
>> ...
>> 
>> CompileCommand: compileonly compiler/loopopts/superword/CoLocatePack.test bool compileonly = true
>>  191 ConvF2L === _ 714 [[ 193 ]] !jvms: CoLocatePack::test @ bci:30 (line 70)
>> # To suppress the following error report, specify this argument
>> # after -XX: or in .hotspotrc: SuppressErrorAt=/phaseX.cpp:1128
>> #
>> # A fatal error has been detected by the Java Runtime Environment:
>> #
>> # Internal Error (/home/shade/trunks/jdk/src/hotspot/share/opto/phaseX.cpp:1128), pid=1717516, tid=1717532
>> # fatal error: modified node was not processed by IGVN.transform_old()
>> 
>> 
>> After JDK-8266950 (always `strictfp`), the paths in `Conv(D|F)2(I|L)Nodes::Ideal`-s start to be taken more frequently to round float/double inputs when low SSE is enabled. On those paths, we call `set_req` to rewire current node, but we still return `NULL` from `::Ideal`. I believe that is incorrect, as per `node.cpp` explanation: `NULL` indicates no graph change was done, and `this` should be returned when modification happened. So GVN predictably barfs.
>> 
>> Additional testing:
>>  - [x] Linux x86_32 `tier1` with `-XX:UseAVX=0 -XX:UseSSE=0` (now pass)
>>  - [x] Linux x86_32 `tier1` default (still pass)
>>  - [x] Linux x86_64 `tier1` default
>
> Aleksey Shipilev has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:
> 
>  - Whitespace
>  - Merge branch 'master' into JDK-8276105-c2-igvn
>  - Fix

All right, if anyone has any other comments, we can do those in follow-ups.

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PR: https://git.openjdk.java.net/jdk/pull/6176