Request for reviews (L): 6814842: Load shortening optimizations

[Christian Thalinger]

On Fri, 2009-05-08 at 18:36 +0200, Ulf Zibis wrote:
> Thanks for detailed information.
> 
> Some more questions:
> - Where can I find information about -XX:LoopUnrollLimit?
> - Where can I find information about other -XX hotspot options (for jdk7)?

-Xprintflags gives you a list of all flags and it's values.  More
information about the flags can be found in the various header files
which define them:

src/share/vm/gc_implementation/g1/g1_globals.hpp
src/share/vm/opto/c2_globals.hpp
src/share/vm/c1/c1_globals.hpp
src/share/vm/runtime/globals.hpp

> - How to see the generated optimized code from hotspot? I don't know 
> where to find the option flag docu.

You can use -XX:+PrintOptoAssembly in any debug build to see an output
that is almost identical with the generated code, but it does not print
the actual code (this is part of HotSpot).

-XX:+PrintAssembly uses libraries from binutils to disassemble the
actually generated code (that one is accurate), but you need to compile
the library wrapper yourself.  See:

http://wikis.sun.com/display/HotSpotInternals/PrintAssembly

>  > it's not the code the makes the second one slower, it's the register 
> allocation.
> Does that mean, that sign-extension has faster register access than 
> zero-extension?

No, it's the register allocation in the unrolled loop around the load.

-- Christian