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

[Adrian Chadd]

Hi,

Please submit a PR and I'll fix the AR5210 code. I'll have to find an
AR5210 though to test it against though...


Adrian

On 3 November 2011 13:08, Dimitry Andric <dim at freebsd.org> wrote:
> On 2011-11-03 20:03, Alexander Best wrote:
>> 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))
> ...
>>> Those warnings are bogus, and due to this bug:
>
> Actually, I was too quick with this one, since it isn't bogus.  The
> macro invocation:
>
>  OS_REG_RMW_FIELD(ah, AR_SCR, AR_SCR_SLE, AR_SCR_SLE_ALLOW);
>
> ultimately expands to:
>
>  bus_space_write_4((bus_space_tag_t)(ah)->ah_st, (bus_space_handle_t)(ah)->ah_sh, (0x4004), ((bus_space_read_4((bus_space_tag_t)(ah)->ah_st, (bus_space_handle_t)(ah)->ah_sh, (0x4004)) &~ (0x00030000)) | (((0x00020000) << 16) & (0x00030000))));
>
> The problem part is ((0x00020000) << 16), which is an overflow.  I'm not
> sure how clang concludes that the result (0x200000000) needs 35 bits to
> represent, as it seems to use 34 bits to me.  But that it doesn't fit
> into a signed integer is crystal-clear.
>
> E.g. this is a real bug!  Probably something in the macro needs to
> explicitly cast to 64 integers, or another workaround must be found.
>
> The other warning:
>
>> In file included from /usr/git-freebsd-head/sys/dev/ath/ath_hal/ah_regdomain.c:99:
>> /usr/git-freebsd-head/sys/dev/ath/ath_hal/ah_regdomain/ah_rd_domains.h:69:15: warning: shift count is negative [-Wshift-count-negative]
>>          .chan11a               = BM4(F1_4950_4980,
>>                                   ^~~~~~~~~~~~~~~~~
>> /usr/git-freebsd-head/sys/dev/ath/ath_hal/ah_regdomain/ah_rd_domains.h:41:4: note: expanded from macro 'BM4'
>>           W1(_fa) | W1(_fb) | W1(_fc) | W1(_fd) }
>>           ^
>> /usr/git-freebsd-head/sys/dev/ath/ath_hal/ah_regdomain/ah_rd_domains.h:34:45: note: expanded from macro 'W1'
>>         (((_a) > 63 && (_a) < 128 ? (((uint64_t) 1)<<((_a)-64)) : (uint64_t) 0))
>>                                                    ^ ~~~~~~~~~
>
> is indeed bogus, since the macro makes sure the shift count never
> becomes negative.  (N.B.: This only happens for global initializations,
> *not* if you would use the same macro in a function.)
>
>
>>>   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?
>
> No, this is a way too big hammer, because it eliminates the useful
> warnings together with the false positives.
>
> It would be better to only apply this band-aid for the specific source
> files that need it, and even then, I would rather wait for the proper
> fix from upstream.
>