llvm dev - Nov 2012 - [LLVMdev] Improved Covered Default Switch detection

If cmake happens to detect GCC for the C compiler and Clang for the C++
compiler, then a manual override of either the C compiler or
SUPPORTS_COVERED_SWITCH_DEFAULT_FLAG is required.  This has been happening on my
Darwin build environments:

-- The C compiler identification is GNU 4.2.1
-- The CXX compiler identification is Clang 4.1.0

Without the manual overrides, all C sources fail to compile with:

[  4%] Building C object lib/Support/CMakeFiles/LLVMSupport.dir/regcomp.c.o
cd /Users/jabbey/src/llvmCommit/build/lib/Support && /usr/bin/gcc 
-D_DEBUG -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS
-fPIC -I/Users/jabbey/src/llvmCommit/build/lib/Support
-I/Users/jabbey/src/llvmCommit/lib/Support
-I/Users/jabbey/src/llvmCommit/build/include
-I/Users/jabbey/src/llvmCommit/include    -Wall -W -Wno-unused-parameter
-Wwrite-strings -pedantic -Wno-long-long -Wcovered-switch-default
-fno-exceptions -o CMakeFiles/LLVMSupport.dir/regcomp.c.o   -c
/Users/jabbey/src/llvmCommit/lib/Support/regcomp.c
cc1: error: unrecognized command line option
"-Wcovered-switch-default"

The problem stems from HandleLLVMOptions.cmake where a single check of the
warning option is performed using check_cxx_compiler_flag, and then upon success
add_llvm_definitions appends to the flag to a list consumed by both C and CXX
compilers.

Attached is a patch which updates HandleLLVMOptions.cmake to:
  - Test C and CXX compiler separately for -Wcovered-switch-default
  - Split SUPPORTS_COVERED_SWITCH_DEFAULT_FLAG into a C and CXX variant
  - Replace set( ${variable} "${variable} arg") with list( append
$variable "arg" )
  - Check cmake_minimum_required of at least CMake 2.4 (where list append was
introduced).

Ok to commit?

Joe
______________________________
Joe Abbey
Director of Software Development
Arxan Technologies
jabbey at arxan.com<mailto:jabbey at arxan.com>
www.arxan.com<http://www.arxan.com/>
Protecting the App Economy™

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On Sun, Nov 25, 2012 at 3:19 PM, Joe Abbey <jabbey at arxan.com>
wrote:
> If cmake happens to detect GCC for the C compiler and Clang for the C++
> compiler, then a manual override of either the C compiler or
> SUPPORTS_COVERED_SWITCH_DEFAULT_FLAG is required.  This has been happening
> on my Darwin build environments:
>
> -- The C compiler identification is GNU 4.2.1
> -- The CXX compiler identification is Clang 4.1.0
How did you end up in this situation?  Something has gone very wrong
if the build system finds clang++ but not clang.

-Eli

On Nov 25, 2012, at 6:49 PM, Eli Friedman <eli.friedman at gmail.com>
wrote:
> On Sun, Nov 25, 2012 at 3:19 PM, Joe Abbey <jabbey at arxan.com>
wrote:
>> If cmake happens to detect GCC for the C compiler and Clang for the C++
>> compiler, then a manual override of either the C compiler or
>> SUPPORTS_COVERED_SWITCH_DEFAULT_FLAG is required.  This has been
happening
>> on my Darwin build environments:
>> 
>> -- The C compiler identification is GNU 4.2.1
>> -- The CXX compiler identification is Clang 4.1.0
> 
> How did you end up in this situation?  Something has gone very wrong
> if the build system finds clang++ but not clang.
> 
I don't think I've done anything terribly unique.  

1) Install Xcode

2) Installing command-line tools

jabbeymbp:llvmCommit jabbey$ ls -l /usr/bin/gcc /usr/bin/g++ /usr/bin/c++
/usr/bin/cc
lrwxr-xr-x  1 root  wheel   7 Nov 20 01:16 /usr/bin/c++ -> clang++
lrwxr-xr-x  1 root  wheel   5 Nov 20 01:16 /usr/bin/cc -> clang
lrwxr-xr-x  1 root  wheel  12 Nov 20 01:17 /usr/bin/g++ -> llvm-g++-4.2
lrwxr-xr-x  1 root  wheel  12 Nov 20 01:17 /usr/bin/gcc -> llvm-gcc-4.2
jabbeymbp:llvmCommit jabbey$ xcode-select --print-path
/Applications/Xcode.app/Contents/Developer
jabbeymbp:llvmCommit jabbey$ cmake --version
cmake version 2.8.8

Is this not as expected?

Joe



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>> 
>> Is this not as expected?
> 
> I don't think so. Unless you ask for something different, CMake should
> use c++ (->clang++) for C++ and cc (->clang) for C as defaults. Doing
> otherwise looks like a bug. Unless OSX has a tradition of broken or
> inferior cc which makes gcc preferable.
> 
> BTW, what I mentioned on my other message was on Linux.
Interesting.  Can you apply my patch and test to see if it works with your
builds?

Joe

>> It's too late here, sorry, but your patch seems plain correct.
>> 
>> The unrelated set(... -> list(APPEND changes shouldn't be mixed
with the
>> fix proper, IMO. Same for the cmake_minimum_required command. LLVM
>> already requires cmake version 2.8 on the top CMakeLists.txt.
> 
> BTW, you want
> 
> set(FOO "${FOO} something")
> 
> instead of
> 
> list(APPEND FOO something)
> 
> because those variables are strings, not lists. Using the `list'
version
> may cause unwanted semicolons in the command line.
Thanks for the extra info.  I understood that list append was
"faster", and wasn't aware of spurious semicolons.

I'll start a new thread with the reduced patch.

:)

Joe

On Sun, Nov 25, 2012 at 3:19 PM, Joe Abbey <jabbey at arxan.com>
wrote:
> If cmake happens to detect GCC for the C compiler and Clang for the C++
> compiler, then a manual override of either the C compiler or
> SUPPORTS_COVERED_SWITCH_DEFAULT_FLAG is required.  This has been happening
> on my Darwin build environments:
>
> -- The C compiler identification is GNU 4.2.1
> -- The CXX compiler identification is Clang 4.1.0
How/why has this been happening, exactly? Seems kind of surprising for
any platform (or user, for that matter) to mismatch between the C and
C++ compiler.

That being said, I'm not sure we should prohibit this configuration,
I'm just not sure how much I'd really want to tweak the build system
to accommodate it given that there is a workaround.
>
> Without the manual overrides, all C sources fail to compile with:
>
> [  4%] Building C object lib/Support/CMakeFiles/LLVMSupport.dir/regcomp.c.o
> cd /Users/jabbey/src/llvmCommit/build/lib/Support && /usr/bin/gcc 
-D_DEBUG
> -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -fPIC
> -I/Users/jabbey/src/llvmCommit/build/lib/Support
> -I/Users/jabbey/src/llvmCommit/lib/Support
> -I/Users/jabbey/src/llvmCommit/build/include
> -I/Users/jabbey/src/llvmCommit/include    -Wall -W -Wno-unused-parameter
> -Wwrite-strings -pedantic -Wno-long-long -Wcovered-switch-default
> -fno-exceptions -o CMakeFiles/LLVMSupport.dir/regcomp.c.o   -c
> /Users/jabbey/src/llvmCommit/lib/Support/regcomp.c
> cc1: error: unrecognized command line option
"-Wcovered-switch-default"
>
> The problem stems from HandleLLVMOptions.cmake where a single check of the
> warning option is performed using check_cxx_compiler_flag, and then upon
> success add_llvm_definitions appends to the flag to a list consumed by both
> C and CXX compilers.
>
> Attached is a patch which updates HandleLLVMOptions.cmake to:
>   - Test C and CXX compiler separately for -Wcovered-switch-default
>   - Split SUPPORTS_COVERED_SWITCH_DEFAULT_FLAG into a C and CXX variant
>   - Replace set( ${variable} "${variable} arg") with list( append
$variable
> "arg" )
>   - Check cmake_minimum_required of at least CMake 2.4 (where list append
> was introduced).
>
> Ok to commit?
>
> Joe
> ______________________________
> Joe Abbey
> Director of Software Development
> Arxan Technologies
> jabbey at arxan.com www.arxan.com
> Protecting the App Economy™
>
>
> _______________________________________________
> llvm-commits mailing list
> llvm-commits at cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
>

On Nov 25, 2012, at 6:22 PM, David Blaikie wrote:
> On Sun, Nov 25, 2012 at 3:19 PM, Joe Abbey <jabbey at arxan.com>
wrote:
>> If cmake happens to detect GCC for the C compiler and Clang for the C++
>> compiler, then a manual override of either the C compiler or
>> SUPPORTS_COVERED_SWITCH_DEFAULT_FLAG is required.  This has been
happening
>> on my Darwin build environments:
>> 
>> -- The C compiler identification is GNU 4.2.1
>> -- The CXX compiler identification is Clang 4.1.0
> 
> How/why has this been happening, exactly? Seems kind of surprising for
> any platform (or user, for that matter) to mismatch between the C and
> C++ compiler.
I think I can answer that. For some reason, cmake starts with 'c++' when
detecting the C++ compiler, but prefers 'gcc' to plain 'cc' when
detecting the C compiler. On most systems, this isn't a problem, because
'c++' (if present) is usually symlinked to 'g++'. On recent
Xcode setups, however, 'cc' and 'c++' are symlinked to
clang--but 'gcc' is still llvm-gcc! Hence, when cmake goes to detect the
compilers, it picks up llvm-gcc for C and clang for C++. (Seriously, when is
Apple finally going to drop llvm-gcc and just symlink gcc to clang?)

IMO, this looks like a cmake bug. It shouldn't be preferring 'gcc'
to 'cc'. You'll have to take it up with them, though; I have no idea
why they look for gcc first. Maybe a look through their VCS logs will help.

Chip