Re: [PATCH] kbuild, LLVMLinux: Add -Werror to cc-option to support clang

From: Behan Webster
Date: Wed Sep 24 2014 - 14:51:01 EST

On 09/24/14 05:07, Michal Marek wrote:
On 2014-09-23 21:28, behanw@xxxxxxxxxxxxxxxxxx wrote:
From: Mark Charlebois <charlebm@xxxxxxxxx>

Clang will warn about unknown warnings but will not return false
You mean unknown options, right?
2 kinds of options: flags and warnings. clang used to merely warn about unused/unsupported flags/warnings. It now returns errors for unknown flags, but not warnings (unless you specify -Werror).

The issue is that a lot of existing projects which use clang expect the former behaviour (I agree that makes no sense, but there you go).

unless -Werror is set. GCC will return false if an unknown
warning is passed.

Adding -Werror make both compiler behave the same.
Can you please limit it to the clang case? Add an internal variable that
either contains -Werror or nothing, depending on the compiler.
I can do that. Will fix.

What I
fear is that if we use -Werror unconditionally and the user (or some
automated build system) decides to add some silly option to KCFLAGS, we
will get silent failures in the cc-option tests.
A valid concern for sure.

BTW, is there a chance that this would be fixed in some later clang
version? Accepting unknown commandline options is a rather unusual
behavior. How are all the ./configure scripts going to cope with it?
Again, clang does error out on unknown compiler flags (as opposed to warnings).

Getting clang to error on unused flags wasn't trivial (this change broke a lot of builds apparently). Fortunately we weren't the only ones who wanted it to behave like gcc in this case. I think it's going to be *much* harder to do the same for warnings. The argument given by supporters of the current situation is that if a warning isn't supported, why break the build? *sigh*

The LLVMLinux project is pushing hard to fix these sorts of things in clang, but some changes are more likely than others.



Behan Webster

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at