On Mar 10, 2021, at 1:59 PM, Remi Forax <fo...@univ-mlv.fr> wrote: > > Hi all, > slowly reading the Parametric-vm spec. > > With my ASM hat, > "As a structural constraint, it is illegal for a CONSTANT_Parameter constant > to be unused." > > This is different of all other CONSTANTs and doesn't work well with the idea > that you can patch a classfile by copying the existing constant pool and > replace only the method(s) you want, because if i replace a parametric method > by a non parametric one, the existing CONSTANT_Parameter will stay in the > constant pool with no reference to it anymore. > > Being able to patch a classfile like this is very important in term of speed > for some transformers / agents. > > If the VM is able to find that a CONSTANT_Parameter is unused, instead of > throwing an error, why not ignoring it ?
That’s possible. I agree that this makes parametric constants unusual, in a way that doesn’t buy much. Here’s my motivation: The presence of a C_Param forces the JVM to do more work than if it were absent, work that is non-local (a dependency analysis over the CP). Some of that analysis is likely to create detailed CP metadata describing the parametricity of each constant. Having parts of that metadata be unused feels like it could breed bugs. But, it’s just a feeling. I’m not against your request, and maybe I’ll write it into the next draft. (I have a vague memory that there was another reason that occurred to me during prototyping the CP analysis, but it’s been a while now.) — John
