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authorDaniel Santos <daniel.santos@pobox.com>2013-02-21 16:41:54 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2013-02-21 17:22:16 -0800
commitc361d3e54364d19bb5e803d6e766e94674da7b0e (patch)
treea0d5410b10b0733f1f3a73238f7d487fac479ad5 /include/linux/compiler.h
parenta3ccc497cd17147713363a4bf975f1a269fadb6d (diff)
downloadop-kernel-dev-c361d3e54364d19bb5e803d6e766e94674da7b0e.zip
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compiler.h, bug.h: prevent double error messages with BUILD_BUG{,_ON}
Prior to the introduction of __attribute__((error("msg"))) in gcc 4.3, creating compile-time errors required a little trickery. BUILD_BUG{,_ON} uses this attribute when available to generate compile-time errors, but also uses the negative-sized array trick for older compilers, resulting in two error messages in some cases. The reason it's "some" cases is that as of gcc 4.4, the negative-sized array will not create an error in some situations, like inline functions. This patch replaces the negative-sized array code with the new __compiletime_error_fallback() macro which expands to the same thing unless the the error attribute is available, in which case it expands to do{}while(0), resulting in exactly one compile-time error on all versions of gcc. Note that we are not changing the negative-sized array code for the unoptimized version of BUILD_BUG_ON, since it has the potential to catch problems that would be disabled in later versions of gcc were __compiletime_error_fallback used. The reason is that that an unoptimized build can't always remove calls to an error-attributed function call (like we are using) that should effectively become dead code if it were optimized. However, using a negative-sized array with a similar value will not result in an false-positive (error). The only caveat being that it will also fail to catch valid conditions, which we should be expecting in an unoptimized build anyway. Signed-off-by: Daniel Santos <daniel.santos@pobox.com> Cc: Andi Kleen <ak@linux.intel.com> Cc: Borislav Petkov <bp@alien8.de> Cc: David Rientjes <rientjes@google.com> Cc: Joe Perches <joe@perches.com> Cc: Josh Triplett <josh@joshtriplett.org> Cc: Paul Gortmaker <paul.gortmaker@windriver.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'include/linux/compiler.h')
-rw-r--r--include/linux/compiler.h5
1 files changed, 5 insertions, 0 deletions
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 4c638be..423bb6b 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -307,7 +307,12 @@ void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect);
#endif
#ifndef __compiletime_error
# define __compiletime_error(message)
+# define __compiletime_error_fallback(condition) \
+ do { ((void)sizeof(char[1 - 2*!!(condition)])); } while (0)
+#else
+# define __compiletime_error_fallback(condition) do { } while (0)
#endif
+
/*
* Prevent the compiler from merging or refetching accesses. The compiler
* is also forbidden from reordering successive instances of ACCESS_ONCE(),
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