GCC emits a warning (treated as an error) if an inline method is declared and used without the body being present, even if the calling function is in a header file and is not being used. This occurs in a couple place in hotspot:

Compiling src/share/vm/gc_implementation/shared/adaptiveSizePolicy.cpp
/home/km88527/ws/rt_baseline/src/share/vm/oops/markOop.hpp:208: warning: inline
function `bool markOopDesc::must_be_preserved_with_bias(oopDesc*) const' used bu
t never defined
/home/km88527/ws/rt_baseline/src/share/vm/oops/markOop.hpp:232: warning: inline
function `bool markOopDesc::must_be_preserved_with_bias_for_promotion_failure(oo
pDesc*) const' used but never defined
/home/km88527/ws/rt_baseline/src/share/vm/oops/markOop.hpp:243: warning: inline
function `bool markOopDesc::must_be_preserved_with_bias_for_cms_scavenge(klassOo
pDesc*) const' used but never defined
gmake[2]: *** [adaptiveSizePolicy.o] Error 1

Sun's CC doesn't complain about this, and since the linux builds build with precompiled headers, we don't see this in the current builds.