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

[Aleksey Shipilev]

On Tue, 23 Jan 2024 08:16:07 GMT, Aleksey Shipilev <shade 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 inspired by `std::is_constant_evaluated` in C++.
>> 
>> Please kindly give your opinion as well as your reviews, thanks very much.
>
> Nice. I had a similar thing stashed in my todo queue. Note that there is already `isCompileConstant` that does similar thing: https://github.com/openjdk/jdk/blob/5a74c2a67ebcb47e51732f03c4be694bdf920469/src/hotspot/share/opto/library_call.cpp#L8189-L8193 -- maybe we should just expose that more widely. I would suggest we just do the private `java.lang.{Integer,...}.isCompileConstant` methods and bind them to that intrinsic.

> @shipilev Thanks a lot for your suggestions. Yes I can just use `inline_isCompileConstant` instead.
> 
> Regarding the place of the method, I'm not really sure as putting in `java.lang.Long` seems out-of-place for an internal mechanism that is obviously not only used in `java.lang`, which will force a new entry in `JavaLangAccess`. 

Ah yes, if you need to use it across module boundaries, putting the private/protected method would require `JavaLangAccess`, which is burdensome. I am just icky about introducing a whole new internal class for this. Is there anything in current `jdk.internal.vm.*` that fits it? Maybe `misc.Unsafe` or `misc.VM`?

> Finally, I think accepting a `long` would be enough (maybe `double`, too?) since `int`, `boolean` etc can be converted losslessly to `long`.

Right, that would work for primitives, since we could probably rely on conversion for constants to be folded. But I also see the value for asking `isCompileConstant(Object)`, which is not easily convertible. So I would just do the overloads for all primitives and `Object`. The C2 intrinsic would not care about the `arg(0)` type, it would reply `isCon` on those constants just the same.

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PR Comment: https://git.openjdk.org/jdk/pull/17527#issuecomment-1905655407