Hi Nathan,
On Tue, Oct 16, 2018 at 11:15 AM Nathan Chancellor
<natechancellor@xxxxxxxxx> wrote:
>
> There are only a few instances of this warning in an arm64 allyesconfig
> build but none of them appear useful. I believe the intention of the
> warning is to avoid situations like this:
>
> if (condition);
> statement;
>
> where the user really intended
>
> if (condition)
> statement;
>
> However, these instances have already been caught by GCC's warning about
> misleading indentation
Right, the example above is caught by -Wmisleading-indentation.
However, the following is not.
if (condition)
;
So, -Wempty-body is a kind of different thing,
and still useful in my opinion.
> so the remaining warnings are about loops that
> fall into one of three categories:
>
> 1. Execute a function unconditionally (avoiding a useless variable to
> hold the return value):
>
> drivers/isdn/hisax/hfc_pci.c:131:34: warning: if statement has empty body
> [-Wempty-body]
> if (Read_hfc(cs, HFCPCI_INT_S1));
I think this is a real bug,
then -Wempty-body finally caught it.
(but -Wmisleading-indentation cannot catch it.)
It is wrong to enclose a non-effective statement with 'if ();'
just for suppressing another warning.
Read_hfc(cs, HFCPCI_INT_S1);
would emit this warning.
In file included from drivers/isdn/hisax/hfc_pci.c:20:0:
drivers/isdn/hisax/hfc_pci.c: In function ‘reset_hfcpci’:
drivers/isdn/hisax/hfc_pci.h:232:25: warning: statement with no effect
[-Wunused-value]
#define Read_hfc(a, b) (*(((u_char *)a->hw.hfcpci.pci_io) + b))
~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/isdn/hisax/hfc_pci.c:131:2: note: in expansion of macro ‘Read_hfc’
Read_hfc(cs, HFCPCI_INT_S1);
^~~~~~~~
The root cause is missing 'volatile'
while Read_hfc() is supposed to read out a HW register.
#define Read_hfc(a, b) (*(((volatile u_char *)a->hw.hfcpci.pci_io) + b))
will be a correct fix.
(or just use a standard accessor like readb(), ioread8(), etc.)
if (Read_hfc(cs, HFCPCI_INT_S1));
is optimized out by the compiler, so it is not working as expected.
>
> 2. Advancing a value to be used later on in the function like a pointer
> or a count:
>
> drivers/atm/eni.c:244:48: warning: for loop has empty body
> [-Wempty-body]
> for (order = 0; (1 << order) < *size; order++);
> ^
As you noted in the commit log,
Clang's -Wempty-body cares the location of a semi-colon,
while GCC's one does not.
for (order = 0; (1 << order) < *size; order++)
;
is fine, and more readable in my opinion.
> 3. Busy waiting:
>
> drivers/atm/zatm.c:513:7: warning: while loop has empty body
> [-Wempty-body]
> zwait;
> ^
Again, Clang is fine with an empty body in while() loop,
but just picky about the semi-colon location.
For this particular case, how about something like this?
#define zwait do {} while (zin(CMR) & uPD98401_BUSY)
I think an even better fix is
#define zwait() do {} while (zin(CMR) & uPD98401_BUSY)
then, fix-up all
zwait;
to
zwait();
> None of these uses are problematic or need to be addressed.
The first pattern is really problematic, and need to be addressed.
I want to keep -Wempty-body enabled
to find out potential issues.
Please let me know if you see other patterns difficult to fix.
> Clang
> suggests moving the semi-colon to the next line to silence these
> warnings but that defeats the purpose of the compact nature of these
> constructs so just hide the warning behind W=1 so its use can still be
> audited but it won't polute a regular build.
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/42
> Link: https://github.com/ClangBuiltLinux/linux/issues/66
> Signed-off-by: Nathan Chancellor <natechancellor@xxxxxxxxx>
> ---
> scripts/Makefile.extrawarn | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/scripts/Makefile.extrawarn b/scripts/Makefile.extrawarn
> index cf6cd0ef6975..8709d9d6faf1 100644
> --- a/scripts/Makefile.extrawarn
> +++ b/scripts/Makefile.extrawarn
> @@ -11,6 +11,7 @@
> # are not supported by all versions of the compiler
> # ==========================================================================
>
> +KBUILD_CFLAGS += $(call cc-disable-warning, empty-body)
> KBUILD_CFLAGS += $(call cc-disable-warning, packed-not-aligned)
>
> ifeq ("$(origin W)", "command line")
> @@ -32,6 +33,7 @@ warning-1 += $(call cc-option, -Wpacked-not-aligned)
> warning-1 += $(call cc-option, -Wstringop-truncation)
> warning-1 += $(call cc-disable-warning, missing-field-initializers)
> warning-1 += $(call cc-disable-warning, sign-compare)
> +warning-1 += $(call cc-option, -Wempty-body)
>
> warning-2 := -Waggregate-return
> warning-2 += -Wcast-align
> --
> 2.19.1
>
--
Best Regards
Masahiro Yamada
