RFR: 8324433: Introduce a way to determine if an expression is evaluated as a constant by the Jit compiler [v7]

[Vladimir Ivanov]

On Thu, 25 Jan 2024 14:01:59 GMT, Quan Anh Mai <qamai at openjdk.org> wrote:

>> Hi,
>> 
>> This patch introduces `JitCompiler::isConstantExpression` which can be used to statically determine whether an expression has been constant-folded by the Jit compiler, leading to more constant-folding opportunities. For example, it can be used in `MemorySessionImpl::checkValidStateRaw` to eliminate the lifetime check on global sessions without imposing additional branches on other non-global sessions. This is similar to `__builtin_constant_p` in GCC and clang.
>> 
>> Please kindly give your opinion as well as your reviews, thanks very much.
>
> Quan Anh Mai has updated the pull request incrementally with one additional commit since the last revision:
> 
>   change expr to val, add examples

I share Roland's concerns w.r.t. profiling. If there's any code guarded by `isCompileConstant(value) == true`, the only way to trigger its profiling is by deoptimizing from C2-generated code.

I added `MHI.isCompileConstant` intrinsic as part of a point fix for a performance problem caused by Java-level code profiling/specialization happening in `java.lang.invoke`. It guards profiling logic which is pruned completely once C2 kicks in. So, absence of profiling is not a problem there. 

Also, there's a constraint on implementation side: the current implementation supports only parse-time folding. If a value turns into a constant later (either during parsing after the call is encountered or during post-parsing phase), it won't have any effect.

So, as it is now (both on API and implementation sides) it's hard to correctly use `isCompileConstant` for more general cases.

It would be helpful to see more examples illustrating possible usage scenarios.

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PR Comment: https://git.openjdk.org/jdk/pull/17527#issuecomment-1914052884