RFR: 8323242: Remove vestigial DONT_USE_REGISTER_DEFINES

[Vladimir Kozlov]

On Tue, 5 Mar 2024 08:07:19 GMT, Koichi Sakata <ksakata at openjdk.org> wrote:

> This pull request removes an unnecessary directive.
> 
> There is no definition of DONT_USE_REGISTER_DEFINES in HotSpot or the build system, so this `#ifndef`conditional directive is always true. We can remove it. 
> 
> I built OpenJDK with Zero VM as a test. It was successful.
> 
> 
> $ ./configure --with-jvm-variants=zero --enable-debug
> $ make images 
> $ ./build/macosx-aarch64-zero-fastdebug/jdk/bin/java -version                                                                                                                                         
> openjdk version "23-internal" 2024-09-17                                                                                                                                                                                                                                                 
> OpenJDK Runtime Environment (fastdebug build 23-internal-adhoc.jyukutyo.jyukutyo-jdk)                                                                                                                                                                                                    
> OpenJDK 64-Bit Zero VM (fastdebug build 23-internal-adhoc.jyukutyo.jyukutyo-jdk, interpreted mode)
> 
> 
> It may be possible to remove the `#define noreg` as well because the CONSTANT_REGISTER_DECLARATION macro creates a variable named noreg, but I can't be sure. When I tried removing the noreg definition and building the OpenJDK, the build was successful.

This was from these changes [JDK-8000780](https://github.com/openjdk/jdk/commit/e184d5cc4ec66640366d2d30d8dfaba74a1003a7)

May be @rkennke remember why he added it. May be for some debugging purpose.

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PR Comment: https://git.openjdk.org/jdk/pull/18115#issuecomment-1989328275