RFR: 8370405: C2: mismatched store from MergeStores wrongly scalarized in allocation elimination [v2]

[Emanuel Peter]

> Note: @oliviermattmann found this bug with his whitebox fuzzer. See also https://github.com/openjdk/jdk/pull/27991
> 
> **Analysis**
> We run Escape Analysis, and see that a local array allocation could possibly be removed, we only have matching `StoreI` to the `int[]`. But there is one `StoreI` that is still in a loop, and so we wait with the actual allocation removal until later, hoping it may go away, or drop out of the loop.
> During loop opts, the `StoreI` drops out of the loop, now there should be nothing in the way of allocation removal.
> But now we run `MergeStores`, and merge two of the `StoreI` into a mismatched `StoreL`.
> 
> Then, we eventually remove the allocation, but don't check again if any new mismatched store has appeared.
> Instead of a `ConI`, we receive a `ConL`, for the first of the two merged `StoreI`. The second merged `StoreI` instead captures the state before the `StoreL`, and that is wrong.
> 
> **Solution**
> We should have some assert, that checks that the captured `field_val` corresponds to the expected `field_type`.
> 
> But the real fix was suggested by @merykitty : apparently he just had a similar issue in Valhalla:
> https://github.com/openjdk/valhalla/blame/60af17ff5995cfa5de075332355f7f475c163865/src/hotspot/share/opto/macro.cpp#L709-L713
> (the idea is to bail out of the elimination if any of the found stores are mismatched.)
> 
> **Details**
> 
> How the bad sequence develops, and which components are involved.
> 
> 1) The `SafePoint` contains a `ConL` and 3 `ConI`. (Correct would have been 4 `ConI`)
> 
>   6  ConI  === 23  [[ 4 ]]  #int:16777216
>   7  ConI  === 23  [[ 4 ]]  #int:256
>   8  ConI  === 23  [[ 4 ]]  #int:1048576
>   9  ConL  === 23  [[ 4 ]]  #long:68719476737
>  54  DefinitionSpillCopy  === _ 27  [[ 16 12 4 ]]
>   4  CallStaticJavaDirect  === 47 29 30 26 32 33 0 34 0 54 9 8 7 6  [[ 5 3 52 ]] Static wrapper for: uncommon_trap(reason='unstable_if' action='reinterpret' debug_id='0') # void ( int ) C=0.000100 Test::test @ bci:38 (line 21) reexecute !jvms: Test::test @ bci:38 (line 21)
> 
> 
> 2) This is then encoded into an `ObjectValue`. A `Type::Long` / `ConL` is converted into a `[int=0, long=ConL]` pair, see:
> https://github.com/openjdk/jdk/blob/da7121aff9eccb046b82a75093034f1cdbd9b9e4/src/hotspot/share/opto/output.cpp#L920-L925
> If I understand it right, there zero is just a placeholder.
> 
> And so we get:
> 
> (rr) p sv->print_fields_on(tty)
>         Fields: 0, 68719476737, 1048576, 256, 16777216
> 
> We can see the `zero`, followed by the `ConL`, and then 3 `ConI`.
> 
> This se...

Emanuel Peter 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 16 additional commits since the last revision:

 - Merge branch 'master' into JDK-8370405-alloc-elimination-and-MergeStores
 - only verify primitive types
 - Apply suggestions from code review
 - more assert adjustment
 - ignore debug flag
 - id for tests, and fix up the assert
 - pass int for short slot
 - another test
 - improve test
 - wip new IR test
 - ... and 6 more: https://git.openjdk.org/jdk/compare/ec5c366a...b6e032c2

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/27997/files
  - new: https://git.openjdk.org/jdk/pull/27997/files/9114d379..b6e032c2

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27997&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27997&range=00-01

  Stats: 6694 lines in 293 files changed: 3969 ins; 1760 del; 965 mod
  Patch: https://git.openjdk.org/jdk/pull/27997.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27997/head:pull/27997

PR: https://git.openjdk.org/jdk/pull/27997