state of clang(1)'s -Wshift-count-negative and -Wshift-overflow warnings

[Alexander Best]

On Thu Nov  3 11, Dimitry Andric wrote:
> On 2011-11-03 11:45, Alexander Best wrote:
> ...
> > /usr/git-freebsd-head/sys/dev/ath/ath_hal/ar5210/ar5210_power.c:36:3: warning: signed shift result (0x200000000) requires 35 bits to represent, but 'int' only has 32 bits [-Wshift-overflow]
> >                 OS_REG_RMW_FIELD(ah, AR_SCR, AR_SCR_SLE, AR_SCR_SLE_ALLOW);
> >                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > /usr/git-freebsd-head/sys/dev/ath/ath_hal/ah_internal.h:471:42: note: expanded from macro 'OS_REG_RMW_FIELD'
> >                 (OS_REG_READ(_a, _r) &~ (_f)) | (((_v) << _f##_S) & (_f)))
> >                                                        ^
> > /usr/git-freebsd-head/sys/dev/ath/ah_osdep.h:127:49: note: expanded from macro 'OS_REG_WRITE'
> >             (bus_space_handle_t)(_ah)->ah_sh, (_reg), (_val))
> > 
> > iirc, back then, it was labeled as a clang bug. however testing with clang tot,
> > i still get those warnings. so i just wanted to ask again, whether the warnings
> > are really bogus, or if these warnings actually indicate issues during
> > shifting?
> 
> Those warnings are bogus, and due to this bug:
> 
>   http://llvm.org/bugs/show_bug.cgi?id=10030
> 
> Unfortunately, it is still not fixed for the 3.0 release branch, and I
> don't expect it will be fixed for the actual release.

thanks for the info. so how about something like

diff --git a/sys/conf/kern.mk b/sys/conf/kern.mk
index a0a595f..3cb13de 100644
--- a/sys/conf/kern.mk
+++ b/sys/conf/kern.mk
@@ -1,12 +1,28 @@
 # $FreeBSD$
 
 #
-# Warning flags for compiling the kernel and components of the kernel:
+# XXX Disable bogus llvm warnings, complaining about perfectly valid shifts.
+# See http://llvm.org/bugs/show_bug.cgi?id=10030 for more details.
+# 
+.if ${CC:T:Mclang} == "clang"
+NOSHIFTWARNS=  -Wno-shift-count-negative -Wno-shift-count-overflow \
+               -Wno-shift-overflow
+.endif
+

...and then add ${NOSHIFTWARNS} to the end of CWARNFLAGS?

cheers.
alex