RFR: 8323972: C2 compilation fails with assert(!x->as_Loop()->is_loop_nest_inner_loop()) failed: loop was transformed

[Roland Westrelin]

On Wed, 6 Mar 2024 14:50:30 GMT, Christian Hagedorn <chagedorn at openjdk.org> wrote:

>> Long counted loop are transformed into a loop nest of 2 "regular"
>> loops and in a subsequent loop opts round, the inner loop is
>> transformed into a counted loop. The limit for the inner loop is set,
>> when the loop nest is created, so it's expected there's no need for a
>> loop limit check when the counted loop is created. The assert fires
>> because, when the counted loop is created, it is found that it needs a
>> loop limit check. The reason for that is that the limit is
>> transformed, between nest creation and counted loop creation, in a way
>> that the range of values of the inner loop's limit becomes
>> unknown. The limit when the nest is created is:
>> 
>> 
>>  111  ConL  === 0  [[ 112 ]]  #long:-9223372034707292158
>>  106  Phi  === 105 20 94  [[ 112 ]]  #long:9223372034707292160..9223372034707292164:www !orig=72 !jvms: TestInaccurateInnerLoopLimit::test @ bci:12 (line 40)
>>  112  AddL  === _ 106 111  [[ 122 ]]  !orig=[110]
>>  122  ConvL2I  === _ 112  [[ ]]  #int
>> 
>> 
>> The type of 122 is `2..6` but it is then transformed to:
>> 
>> 
>>  106  Phi  === 105 20 154  [[ 191 130 137 ]]  #long:9223372034707292160..9223372034707292164:www !orig=[72] !jvms: TestInaccurateInnerLoopLimit::test @ bci:12 (line 40)
>>  191  ConvL2I  === _ 106  [[ 196 ]]  #int
>>  195  ConI  === 0  [[ 196 ]]  #int:max-1
>>  196  SubI  === _ 195 191  [[ 201 127 ]]  !orig=[123]
>> 
>> 
>> That is the `(ConvL2I (AddL ...))` is transformed into a `(SubI
>> (ConvL2I ))`. `ConvL2I` for an input that's out of the int range of
>> values returns TypeInt::INT and the bounds of the limit are lost. I
>> propose adding a `CastII` after the `ConvL2I` so the range of values
>> of the limit doesn't get lost.
>
> Marked as reviewed by chagedorn (Reviewer).

@chhagedorn @eme64 thanks for the reviews!

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PR Comment: https://git.openjdk.org/jdk/pull/17965#issuecomment-2006179280