[Buildroot] [PATCHv3 03/15] package/pkg-cmake.mk: determine CMAKE_BUILD_TYPE depending on BR2_ENABLE_RUNTIME_DEBUG

[Thomas De Schampheleire <patrickdepinguin@gmail.com>]

From: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>

The CMAKE_BUILD_TYPE is currently set as 'Debug' in case BR2_ENABLE_DEBUG is
set, and as 'Release' in other cases. However, while the description of
BR2_ENABLE_DEBUG is to enable debug symbols (no runtime impact), the 'Debug'
build type in CMake can actually have runtime impact. For one, because it
does not set -DNDEBUG like is done for 'Release', but also because packages
may do custom things based on it.

The question of which CMAKE_BUILD_TYPE Buildroot should set, be it 'Debug',
'Release', 'RelWithDebInfo' or others, has come up several times in the
past. See some references below:

- July 2016: switch from Debug to RelWithDebInfo:
  https://git.buildroot.org/buildroot/commit/?id=4b0120183404913f7f7788ef4f0f6b51498ef363

- October 2016: switch from RelWithDebInfo back to Debug:
  https://git.buildroot.org/buildroot/commit/?id=104bb29e0490bfb487e2e665448dd3ca07fcc2b5
  and changes to make sure Buildroot's flags are respected:
  https://git.buildroot.org/buildroot/commit/?id=12494ef48f893684d0800e7f6fe39a2ceaed0451

- August 2017: bug #10246 - "BR2_ENABLE_DEBUG does not have the expected
  effect for cmake packages"
  https://bugs.busybox.net/show_bug.cgi?id=10246

- August 2017: mail thread following bug #10246:
  http://lists.busybox.net/pipermail/buildroot/2017-August/200778.html

In the last mail thread, Samuel Martin confirmed that the 'Release' build
type could be used in all cases, because Buildroot is actually making sure
that the optimization flags are those determined by Buildroot, not the
defaults of cmake, thanks to commit 12494ef48f.
But Arnout Vandecappelle objected to using always 'Release', stating that
users may actually want the extra assertions.

With the introduction of BR2_ENABLE_RUNTIME_DEBUG, Buildroot can now cater
for all cases:

- use CMAKE_BUILD_TYPE=Release by default. This makes sure that there is no
  unexpected performance degradation triggered by enabling BR2_ENABLE_DEBUG.

- users can optionally enable BR2_ENABLE_RUNTIME_DEBUG if they want runtime
  debug info like assertions, at the risk of introducing performance
  degradation. In this case, we switch to CMAKE_BUILD_TYPE=Debug.

- orthogonally to the above, BR2_ENABLE_DEBUG still determines passing the
  '-g' flag to enable debug symbols, and BR2_OPTIMIZE_X still determines the
  used optimization flags.

Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
---
 docs/manual/adding-packages-cmake.txt | 2 +-
 package/pkg-cmake.mk                  | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/docs/manual/adding-packages-cmake.txt b/docs/manual/adding-packages-cmake.txt
index 73f0943024..541d7422cf 100644
--- a/docs/manual/adding-packages-cmake.txt
+++ b/docs/manual/adding-packages-cmake.txt
@@ -100,7 +100,7 @@ typical packages will therefore only use a few of them.
   necessary to set them in the package's +*.mk+ file unless you want
   to override them:
 
-** +CMAKE_BUILD_TYPE+ is driven by +BR2_ENABLE_DEBUG+;
+** +CMAKE_BUILD_TYPE+ is driven by +BR2_ENABLE_RUNTIME_DEBUG+;
 ** +CMAKE_INSTALL_PREFIX+;
 ** +BUILD_SHARED_LIBS+ is driven by +BR2_STATIC_LIBS+;
 ** +BUILD_DOC+, +BUILD_DOCS+ are disabled;
diff --git a/package/pkg-cmake.mk b/package/pkg-cmake.mk
index 37078f3c34..4ee100a0c6 100644
--- a/package/pkg-cmake.mk
+++ b/package/pkg-cmake.mk
@@ -266,7 +266,7 @@ define TOOLCHAIN_CMAKE_INSTALL_FILES
 		-e 's#@@CMAKE_SYSTEM_PROCESSOR@@#$(call qstrip,$(CMAKE_SYSTEM_PROCESSOR))#' \
 		-e 's#@@TOOLCHAIN_HAS_CXX@@#$(if $(BR2_INSTALL_LIBSTDCPP),1,0)#' \
 		-e 's#@@TOOLCHAIN_HAS_FORTRAN@@#$(if $(BR2_TOOLCHAIN_HAS_FORTRAN),1,0)#' \
-		-e 's#@@CMAKE_BUILD_TYPE@@#$(if $(BR2_ENABLE_DEBUG),Debug,Release)#' \
+		-e 's#@@CMAKE_BUILD_TYPE@@#$(if $(BR2_ENABLE_RUNTIME_DEBUG),Debug,Release)#' \
 		$(TOPDIR)/support/misc/toolchainfile.cmake.in \
 		> $(HOST_DIR)/share/buildroot/toolchainfile.cmake
 	$(Q)$(INSTALL) -D -m 0644 support/misc/Buildroot.cmake \
-- 
2.26.3