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

[Erik Joelsson]

On Wed, 19 Mar 2025 14:43:28 GMT, Patrick Zhang <qpzhang at openjdk.org> wrote:

> Building jdk with `--with-extra-cflags='-Wno-incompatible-pointer-types'` triggers 1000+ warning messages like `cc1plus: warning: command-line option ‘-Wno-incompatible-pointer-types’ is valid for C/ObjC but not for C++`. 
> 
> The root cause is that `JVM_CFLAGS ` which contains both `EXTRA_CXXFLAGS` and `EXTRA_CFLAGS` when compiling `src/hotspot` C++ source files and building `BUILD_LIBJVM`.
> 
> This PR does:
> 1. Not to append `EXTRA_CFLAGS` or `EXTRA_CXXFLAGS` into `JVM_CFLAGS` before calling `SetupJdkLibrary`, instead let `SetupCompilerFlags` accept and merge `EXTRA_CFLAGS` and `EXTRA_CXXFLAGS` passed from `SetupJdkLibrary` as parameters, so CPP compilation will only see `EXTRA_CXXFLAGS` as expected.
> 2. Correct `PCH_COMMAND` to use `EXTRA_CXXFLAGS` as precompiled.hpp.gch should not be compiled with `EXTRA_CFLAGS`.
> 3. Fixed `STATIC_LIB_CFLAGS` in `Flags.gmk` to `-DSTATIC_BUILD=1`, which was missed by [cbab40bc](https://github.com/openjdk/jdk/commit/cbab40bce45a2f58906be49c841178fa1dfd457e#diff-ab3ce05e795360030f19402fd0c2fad1dc1f7c5e7acc993cc4a2096cf31ccf40R114-R121) for the refactor of building static libs.
> 
> Tests: Passed jdk building on an AArch64 Linux system and tier1 sanity tests, also passed OpenJDK GHA Sanity Checks.

I think I understand what you are trying to achieve, but this isn't the right solution. 

The `EXTRA_[C|CXX]FLAGS` variables are not meant to be parameters to SetupNativeCompilation. In flags.m4 you can see that the intent is to not even export `EXTRA_[C|CXX]FLAGS` from spec.gmk:


  # FIXME: For compatibility, export this as EXTRA_CFLAGS for now.
  EXTRA_CFLAGS="$MACHINE_FLAG $USER_CFLAGS"
  EXTRA_CXXFLAGS="$MACHINE_FLAG $USER_CXXFLAGS"
  EXTRA_LDFLAGS="$MACHINE_FLAG $USER_LDFLAGS"
  EXTRA_ASFLAGS="$USER_ASFLAGS"


We don't want library/executable specific makefiles to have to worry about the extra flags supplied by the user. Those should be handled in lower layers. Given that, I agree with the change in JvmFlags.gmk. We shouldn't add `$(EXTRA_CFLAGS)` there. If we wanted those added, it should be done in configure, where `EXTRA_CXXFLAGS` are added, but not doing that is your whole goal here. Wouldn't just the change in JvmFlags.gmk be enough to solve your issue?

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PR Review: https://git.openjdk.org/jdk/pull/24115#pullrequestreview-2699645789