Set min jump table entries when compiling interpreter

[midronij]

OpenXL on PPC has been known to have some issues with branch tables/indirect branch prediction before, which, when compiling OpenJ9 on AIX with OpenXL, resulted in a ~15% performance regression from before (i.e.: when building with xlC). To eliminate this degredation, the compiler flags -mllvm -ppc-min-jump-table-entries=512 can be used on the souce files containing the interpreter loop.

[midronij]

@zl-wang @pshipton when you have a moment could you please review? The change is very small so it shouldn't take too long

[zl-wang]

Looks like: with this change, variable CMAKE_C_COMPILER_IS_OPENXL is needed for cmake after all (I have impression that Keith requested it to be removed from cmake).

[zl-wang]

LGTM

[midronij]

Looks like: with this change, variable CMAKE_C_COMPILER_IS_OPENXL is needed for cmake after all (I have impression that Keith requested it to be removed from cmake).

I believe the one Keith wanted to remove was actually OMR_ENV_OPENXL. I will be opening a PR for that change (as well as some others that Keith requested) soon

[keithc-ca]

Looks like: with this change, variable CMAKE_C_COMPILER_IS_OPENXL is needed for cmake after all (I have impression that Keith requested it to be removed from cmake).

No, I requested that OMR_ENV_OPENXL not be added to the environment (it serves no purpose in cmake builds).

However, CMAKE_C_COMPILER_IS_OPENXL doesn't seem to fit the existing convention of using OMR_TOOLCONFIG to distingush toolchains. In fact, it appears to identify specific flavours of gnu or xlc, neither of which seem appropriate to me.