On Thu, 25 Jan 2024 14:01:59 GMT, Quan Anh Mai <qa...@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. ------------- PR Comment: https://git.openjdk.org/jdk/pull/17527#issuecomment-1914052884
