RFR: 8352284: EXTRA_CFLAGS incorrectly applied to BUILD_LIBJVM src/hotspot C++ source files [v2]

[Patrick Zhang]

On Mon, 24 Mar 2025 18:43:42 GMT, Magnus Ihse Bursie <ihse at openjdk.org> wrote:

>> Patrick Zhang has updated the pull request incrementally with three additional commits since the last revision:
>> 
>>  - Fixed a typo
>>    
>>    Signed-off-by: Patrick Zhang <patrick at os.amperecomputing.com>
>>  - Added comments to describe why EXTRA_CFLAGS is excluded from JVM_CFLAGS
>>    
>>    Signed-off-by: Patrick Zhang <patrick at os.amperecomputing.com>
>>  - Revert the changes of adding new params to SetupNativeCompilation
>>    
>>    Signed-off-by: Patrick Zhang <patrick at os.amperecomputing.com>
>
> make/hotspot/lib/JvmFlags.gmk line 89:
> 
>> 87: endif
>> 88: 
>> 89: # Hotspot currently supports only C++. To prevent compilation conflicts,
> 
> I don't think this comments serves any purpose. It was probably a mistake to include EXTRA_CFLAGS here to begin with. After this PR, no one is going to miss it, so the comment will make no sense.

I was initially unfamiliar with the building process and puzzled by why `CFLAGS` unexpectedly encompassed both `EXTRA_CXXFLAGS` and `EXTRA_CFLAGS`, until delving into all details of `JVM_CFLAGS` at a couple of m4 and gmk files.  `JVM_CFLAGS` is not set up at a single place, I think necessary comments are required to warn any similar intention to append C and Objective-C only options (https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html).


FYI, my notes of where the `JVM_CFLAGS` got configured and used:
1). Initialization: https://github.com/openjdk/jdk/blob/b891bfa7e67c21478475642e2bfa2cdc65a3bffe/make/autoconf/flags-cflags.m4#L893
2). Updated:
https://github.com/openjdk/jdk/blob/b891bfa7e67c21478475642e2bfa2cdc65a3bffe/make/hotspot/lib/JvmFlags.gmk#L89-L103
3). Passed as a parameter `CFLAGS` to `SetupJdkLibrary`, `SetupJdkNativeCompilation` , `SetupNativeCompilation`
https://github.com/openjdk/jdk/blob/b891bfa7e67c21478475642e2bfa2cdc65a3bffe/make/hotspot/lib/CompileJvm.gmk#L181
4). Will take more steps for other potential changes till its arrival at the final combinations of flags:
https://github.com/openjdk/jdk/blob/b891bfa7e67c21478475642e2bfa2cdc65a3bffe/make/common/native/CompileFile.gmk#L159

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PR Review Comment: https://git.openjdk.org/jdk/pull/24115#discussion_r2011278491