Setting up CMake to default to RelWithDebInfo when no build type is specified.

[beanz]

Summary

Turns out if you don't set CMAKE_BUILD_TYPE the default is an empty string. This results in some of the behaviors of debug builds, but not all of them. For example ENABLE_ASSERTIONS is false.

[rnk]

I thought Richard actually used this configuration, but I could be wrong. I'd rather default it to Debug. IIRC ninja doesn't support the "empty string" build type and it just forces the build type to Debug. That seems like good prior art.

[beanz]

I have no objection to making the default Debug rather than RelWithDebInfo. I use RelWithDebInfo primarily because it builds faster, but Debug works too.

The real problem is that the behavior when CMAKE_BUILD_TYPE is the empty string is very inconsistent. The CMAKE_C_FLAGS and CMAKE_CXX_FLAGS variables are empty unless you explicitly set them or the build type, so you actually end up with an unoptimized build with no debug info.

Here's an example compile command from a CMake + Ninja build with no build type set:

/usr/bin/c++   -DGTEST_HAS_RTTI=0 -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -fPIC -fvisibility-inlines-hidden -Wall -W -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -pedantic -Wno-long-long -Wcovered-switch-default -std=c++11 -fcolor-diagnostics -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.10.sdk -Ilib/Support -I/Users/cbieneman/dev/open-source/llvm/lib/Support -Iinclude -I/Users/cbieneman/dev/open-source/llvm/include    -fno-exceptions -fno-rtti -MMD -MT lib/Support/CMakeFiles/LLVMSupport.dir/Allocator.cpp.o -MF lib/Support/CMakeFiles/LLVMSupport.dir/Allocator.cpp.o.d -o lib/Support/CMakeFiles/LLVMSupport.dir/Allocator.cpp.o -c /Users/cbieneman/dev/open-source/llvm/lib/Support/Allocator.cpp

[beanz]

Changed default to Debug based on feedback from rnk.

[rnk]

lgtm

[rnk]

Actually, you should probably conditionalize this on whether the generator is single-target or not. We should have a predicate for that somewhere....

[beanz]

Conditionalized based on CMAKE_CONFIGURATION_TYPES to not set a default for multi-configuration generators.

[chapuni]

I don't prefer "Debug" nor "RelWithDebInfo" by default.
For Clang/LLVM users, I suggest "Release".
To develop with our tree, they may set with debug symbols, I think.

[beanz]

Defaulting to debug makes the behavior almost the same as today sans undefined behavior.

I think that is the right way to go.

[rnk]

lgtm

I agree, the cmake default is closer to debug anyway. We also have a status message.

CMakeLists.txt

 # See docs/CMake.html for instructions about how to build LLVM with CMake.
 
 cmake_minimum_required(VERSION 2.8.8)
 
+if (NOT CMAKE_BUILD_TYPE)
+  message(STATUS "No build type selected, default to Debug")
+  set(CMAKE_BUILD_TYPE "Debug")
+endif()
+
 # FIXME: It may be removed when we use 2.8.12.
 if(CMAKE_VERSION VERSION_LESS 2.8.12)
   # Invalidate a couple of keywords.
   set(cmake_2_8_12_INTERFACE)
   set(cmake_2_8_12_PRIVATE)
 else()
   # Use ${cmake_2_8_12_KEYWORD} intead of KEYWORD in target_link_libraries().
   set(cmake_2_8_12_INTERFACE INTERFACE)