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 08:10:54 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 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.

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PR Comment: https://git.openjdk.org/jdk/pull/17527#issuecomment-1905504206