We shouldn't need to pass -fno-strict-aliasing when building clang with clang.

[beanz]

Summary

The code comments in the Makefile indicate this was put in place to support issues when building clang with GCC. Today clang's strict aliasing works, so we shouldn't pass -fno-strict-aliasing when building with clang.

[beanz]

Should have been calling $(CC) not cc in the makefile. Oops.

[echristo]

This is fine, but would you mind moving it to the cmake and autoconf checks
respectively? Probably near the warning checks where we already check for
compiler.

[dblaikie]

Have you tested this? I assume there are strict aliasing violations in
Clang/LLVM if we've had this turned on for a while.

(does strict aliasing still have the special case for enum type punning?
Becaues I've certainly seen (& fixed) that in a few places in Clang/LLVM)

[beanz]

dblaikie,

I've run this through check-all with no issues.

[beanz]

echristo,

I can move the makefile check to the autoconf check, but I think setting the flags should stay in the clang CMakeLists/Makefile otherwise it will change behavior. Today we only enable this flag for the clang subdirectory, not all of LLVM.

Thoughts?

[thakis]

Makefile

68

The first of these is marked fixed, the second has a "works on trunk" comment (from 2010). Maybe this isn't needed with gcc anymore either?

[beanz]

• Adapting to r245255 which exposes a variable in autoconf for which compiler you are building with.
• Updated the CMake to be more CMake-y

Note: This has been tested by running a check on a stage2 clang built with asan and ubsan.

[echristo]

Sure I guess. I'm still not sure why we can't do this in the configure side of things, but I don't really care that much for now.

CMakeLists.txt

 
 # Configure the Version.inc file.
 configure_file(
   ${CMAKE_CURRENT_SOURCE_DIR}/include/clang/Basic/Version.inc.in
   ${CMAKE_CURRENT_BINARY_DIR}/include/clang/Basic/Version.inc)
 
 # Add appropriate flags for GCC
 if (LLVM_COMPILER_IS_GCC_COMPATIBLE)
-  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fno-common -Woverloaded-virtual -fno-strict-aliasing")
+  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fno-common -Woverloaded-virtual")
+  if (NOT "${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang")
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fno-strict-aliasing")
+  endif ()
 
   # Enable -pedantic for Clang even if it's not enabled for LLVM.
   if (NOT LLVM_ENABLE_PEDANTIC)
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -pedantic -Wno-long-long")
   endif ()
 
   check_cxx_compiler_flag("-Werror -Wnested-anon-types" CXX_SUPPORTS_NO_NESTED_ANON_TYPES_FLAG)
   if( CXX_SUPPORTS_NO_NESTED_ANON_TYPES_FLAG )

Makefile

 CPP.Flags += -DCLANG_REPOSITORY_STRING='"$(CLANG_REPOSITORY_STRING)"'
 endif
 
 # Disable -fstrict-aliasing. Darwin disables it by default (and LLVM doesn't
 # work with it enabled with GCC), Clang/llvm-gcc don't support it yet, and newer
 # GCC's have false positive warnings with it on Linux (which prove a pain to
 # fix). For example:
 #   http://gcc.gnu.org/PR41874
 #   http://gcc.gnu.org/PR41838
 #
-# We can revisit this when LLVM/Clang support it.
+# We don't need to do this if the host compiler is clang.
+ifeq ($(CXX_COMPILER), "clang")
 CXX.Flags += -fno-strict-aliasing
+endif
+
 
 # Set up Clang's tblgen.
 ifndef CLANG_TBLGEN
   ifeq ($(LLVM_CROSS_COMPILING),1)
     CLANG_TBLGEN := $(BuildLLVMToolDir)/clang-tblgen$(BUILD_EXEEXT)
   else
     CLANG_TBLGEN := $(LLVMToolDir)/clang-tblgen$(EXEEXT)
   endif