[PATCH 0/2] btrfs-progs: Setting implicit-fallthrough by default

[PATCH 0/2] btrfs-progs: Setting implicit-fallthrough by default

[Marcos Paulo de Souza]

From: Marcos Paulo de Souza <mpdesouza@suse.com>

While compiling btrfs-progs using clang I found an issue using
__attribute__(fallthrough), which does not seems to work in clang.

To solve this issue, the code was changed to use /* fallthrough */, which is the
same notation adopted by linux kernel.

Once these places were changed, -Wimplicit-fallthrough was set in Makefile, to
avoid further implicit-fallthrough cases being added in the future.

Marcos Paulo de Souza (2):
  btrfs-progs: utils: Replace __attribute__(fallthrough)
  btrfs-progs: Makefile: Add -Wimplicit-fallthrough

 Makefile       |  1 +
 common/utils.c | 12 ++++++------
 2 files changed, 7 insertions(+), 6 deletions(-)

-- 
2.23.0

[PATCH 1/2] btrfs-progs: utils: Replace __attribute__(fallthrough)

[Marcos Paulo de Souza]

From: Marcos Paulo de Souza <mpdesouza@suse.com>

When compiling with clang, this warning is shown:

common/utils.c:404:3: warning: declaration does not declare anything [-Wmissing-declarations]
                __attribute__ ((fallthrough));

This attribute seems to silence the same warning in GCC. Changing this
attribute with /* fallthrough */ fixes the warning for both gcc and
clang.

Signed-off-by: Marcos Paulo de Souza <mpdesouza@suse.com>
---
 common/utils.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/common/utils.c b/common/utils.c
index 2cf15c33..a88336b3 100644
--- a/common/utils.c
+++ b/common/utils.c
@@ -401,15 +401,15 @@ int pretty_size_snprintf(u64 size, char *str, size_t str_size, unsigned unit_mod
 	case UNITS_TBYTES:
 		base *= mult;
 		num_divs++;
-		__attribute__ ((fallthrough));
+		/* fallthrough */
 	case UNITS_GBYTES:
 		base *= mult;
 		num_divs++;
-		__attribute__ ((fallthrough));
+		/* fallthrough */
 	case UNITS_MBYTES:
 		base *= mult;
 		num_divs++;
-		__attribute__ ((fallthrough));
+		/* fallthrough */
 	case UNITS_KBYTES:
 		num_divs++;
 		break;
@@ -1135,14 +1135,14 @@ int test_num_disk_vs_raid(u64 metadata_profile, u64 data_profile,
 	default:
 	case 4:
 		allowed |= BTRFS_BLOCK_GROUP_RAID10;
-		__attribute__ ((fallthrough));
+		/* fallthrough */
 	case 3:
 		allowed |= BTRFS_BLOCK_GROUP_RAID6;
-		__attribute__ ((fallthrough));
+		/* fallthrough */
 	case 2:
 		allowed |= BTRFS_BLOCK_GROUP_RAID0 | BTRFS_BLOCK_GROUP_RAID1 |
 			BTRFS_BLOCK_GROUP_RAID5;
-		__attribute__ ((fallthrough));
+		/* fallthrough */
 	case 1:
 		allowed |= BTRFS_BLOCK_GROUP_DUP;
 	}
-- 
2.23.0

[PATCH 2/2] btrfs-progs: Makefile: Add -Wimplicit-fallthrough

[Marcos Paulo de Souza]

From: Marcos Paulo de Souza <mpdesouza@suse.com>

Avoid introducing new cases of implicit fallthrough by having this flag
always set.

Signed-off-by: Marcos Paulo de Souza <mpdesouza@suse.com>
---
 Makefile | 1 +
 1 file changed, 1 insertion(+)

diff --git a/Makefile b/Makefile
index 21bf2717..2f04e880 100644
--- a/Makefile
+++ b/Makefile
@@ -86,6 +86,7 @@ CFLAGS = $(SUBST_CFLAGS) \
 	 -D_XOPEN_SOURCE=700  \
 	 -fno-strict-aliasing \
 	 -fPIC \
+	 -Wimplicit-fallthrough \
 	 -I$(TOPDIR) \
 	 -I$(TOPDIR)/libbtrfsutil \
 	 $(DISABLE_WARNING_FLAGS) \
-- 
2.23.0

Re: [PATCH 1/2] btrfs-progs: utils: Replace __attribute__(fallthrough)

[Nikolay Borisov]

On 22.10.19 г. 5:02 ч., Marcos Paulo de Souza wrote:
> From: Marcos Paulo de Souza <mpdesouza@suse.com>
> 
> When compiling with clang, this warning is shown:
> 
> common/utils.c:404:3: warning: declaration does not declare anything [-Wmissing-declarations]
>                 __attribute__ ((fallthrough));
> 
> This attribute seems to silence the same warning in GCC. Changing this
> attribute with /* fallthrough */ fixes the warning for both gcc and
> clang.
> 
> Signed-off-by: Marcos Paulo de Souza <mpdesouza@suse.com>

Which clang version are you using? According to
https://clang.llvm.org/docs/AttributeReference.html#fallthrough this
attribute is supported even with the GNU syntax.

> ---
>  common/utils.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/common/utils.c b/common/utils.c
> index 2cf15c33..a88336b3 100644
> --- a/common/utils.c
> +++ b/common/utils.c
> @@ -401,15 +401,15 @@ int pretty_size_snprintf(u64 size, char *str, size_t str_size, unsigned unit_mod
>  	case UNITS_TBYTES:
>  		base *= mult;
>  		num_divs++;
> -		__attribute__ ((fallthrough));
> +		/* fallthrough */
>  	case UNITS_GBYTES:
>  		base *= mult;
>  		num_divs++;
> -		__attribute__ ((fallthrough));
> +		/* fallthrough */
>  	case UNITS_MBYTES:
>  		base *= mult;
>  		num_divs++;
> -		__attribute__ ((fallthrough));
> +		/* fallthrough */
>  	case UNITS_KBYTES:
>  		num_divs++;
>  		break;
> @@ -1135,14 +1135,14 @@ int test_num_disk_vs_raid(u64 metadata_profile, u64 data_profile,
>  	default:
>  	case 4:
>  		allowed |= BTRFS_BLOCK_GROUP_RAID10;
> -		__attribute__ ((fallthrough));
> +		/* fallthrough */
>  	case 3:
>  		allowed |= BTRFS_BLOCK_GROUP_RAID6;
> -		__attribute__ ((fallthrough));
> +		/* fallthrough */
>  	case 2:
>  		allowed |= BTRFS_BLOCK_GROUP_RAID0 | BTRFS_BLOCK_GROUP_RAID1 |
>  			BTRFS_BLOCK_GROUP_RAID5;
> -		__attribute__ ((fallthrough));
> +		/* fallthrough */
>  	case 1:
>  		allowed |= BTRFS_BLOCK_GROUP_DUP;
>  	}
> 

Re: [PATCH 1/2] btrfs-progs: utils: Replace __attribute__(fallthrough)

[Nikolay Borisov]

On 22.10.19 г. 9:59 ч., Nikolay Borisov wrote:
> 
> 
> On 22.10.19 г. 5:02 ч., Marcos Paulo de Souza wrote:
>> From: Marcos Paulo de Souza <mpdesouza@suse.com>
>>
>> When compiling with clang, this warning is shown:
>>
>> common/utils.c:404:3: warning: declaration does not declare anything [-Wmissing-declarations]
>>                 __attribute__ ((fallthrough));
>>
>> This attribute seems to silence the same warning in GCC. Changing this
>> attribute with /* fallthrough */ fixes the warning for both gcc and
>> clang.
>>
>> Signed-off-by: Marcos Paulo de Souza <mpdesouza@suse.com>
> 
> Which clang version are you using? According to
> https://clang.llvm.org/docs/AttributeReference.html#fallthrough this
> attribute is supported even with the GNU syntax.

Looking at the documentation the gnu syntax is supported in the
'current' / 10, unreleased version. All others, up to version 9 does not
support this syntax.

> 
>> ---
>>  common/utils.c | 12 ++++++------
>>  1 file changed, 6 insertions(+), 6 deletions(-)
>>
>> diff --git a/common/utils.c b/common/utils.c
>> index 2cf15c33..a88336b3 100644
>> --- a/common/utils.c
>> +++ b/common/utils.c
>> @@ -401,15 +401,15 @@ int pretty_size_snprintf(u64 size, char *str, size_t str_size, unsigned unit_mod
>>  	case UNITS_TBYTES:
>>  		base *= mult;
>>  		num_divs++;
>> -		__attribute__ ((fallthrough));
>> +		/* fallthrough */
>>  	case UNITS_GBYTES:
>>  		base *= mult;
>>  		num_divs++;
>> -		__attribute__ ((fallthrough));
>> +		/* fallthrough */
>>  	case UNITS_MBYTES:
>>  		base *= mult;
>>  		num_divs++;
>> -		__attribute__ ((fallthrough));
>> +		/* fallthrough */
>>  	case UNITS_KBYTES:
>>  		num_divs++;
>>  		break;
>> @@ -1135,14 +1135,14 @@ int test_num_disk_vs_raid(u64 metadata_profile, u64 data_profile,
>>  	default:
>>  	case 4:
>>  		allowed |= BTRFS_BLOCK_GROUP_RAID10;
>> -		__attribute__ ((fallthrough));
>> +		/* fallthrough */
>>  	case 3:
>>  		allowed |= BTRFS_BLOCK_GROUP_RAID6;
>> -		__attribute__ ((fallthrough));
>> +		/* fallthrough */
>>  	case 2:
>>  		allowed |= BTRFS_BLOCK_GROUP_RAID0 | BTRFS_BLOCK_GROUP_RAID1 |
>>  			BTRFS_BLOCK_GROUP_RAID5;
>> -		__attribute__ ((fallthrough));
>> +		/* fallthrough */
>>  	case 1:
>>  		allowed |= BTRFS_BLOCK_GROUP_DUP;
>>  	}
>>

Re: [PATCH 1/2] btrfs-progs: utils: Replace __attribute__(fallthrough)

[Marcos Paulo de Souza]

On Tue, 2019-10-22 at 10:01 +0300, Nikolay Borisov wrote:
> 
> On 22.10.19 г. 9:59 ч., Nikolay Borisov wrote:
> > 
> > 
> > On 22.10.19 г. 5:02 ч., Marcos Paulo de Souza wrote:
> >> From: Marcos Paulo de Souza <mpdesouza@suse.com>
> >>
> >> When compiling with clang, this warning is shown:
> >>
> >> common/utils.c:404:3: warning: declaration does not declare
> anything [-Wmissing-declarations]
> >>                 __attribute__ ((fallthrough));
> >>
> >> This attribute seems to silence the same warning in GCC. Changing
> this
> >> attribute with /* fallthrough */ fixes the warning for both gcc
> and
> >> clang.
> >>
> >> Signed-off-by: Marcos Paulo de Souza <mpdesouza@suse.com>
> > 
> > Which clang version are you using? According to
> > https://clang.llvm.org/docs/AttributeReference.html#fallthrough
> this
> > attribute is supported even with the GNU syntax.
> 
> Looking at the documentation the gnu syntax is supported in the
> 'current' / 10, unreleased version. All others, up to version 9 does
> not
> support this syntax.

I'm using the default clang in Tumbleweed (20191009):
clang --version
clang version 8.0.1 (tags/RELEASE_801/final 366581)

> 
> > 
> >> ---
> >>  common/utils.c | 12 ++++++------
> >>  1 file changed, 6 insertions(+), 6 deletions(-)
> >>
> >> diff --git a/common/utils.c b/common/utils.c
> >> index 2cf15c33..a88336b3 100644
> >> --- a/common/utils.c
> >> +++ b/common/utils.c
> >> @@ -401,15 +401,15 @@ int pretty_size_snprintf(u64 size, char
> *str, size_t str_size, unsigned unit_mod
> >>  	case UNITS_TBYTES:
> >>  		base *= mult;
> >>  		num_divs++;
> >> -		__attribute__ ((fallthrough));
> >> +		/* fallthrough */
> >>  	case UNITS_GBYTES:
> >>  		base *= mult;
> >>  		num_divs++;
> >> -		__attribute__ ((fallthrough));
> >> +		/* fallthrough */
> >>  	case UNITS_MBYTES:
> >>  		base *= mult;
> >>  		num_divs++;
> >> -		__attribute__ ((fallthrough));
> >> +		/* fallthrough */
> >>  	case UNITS_KBYTES:
> >>  		num_divs++;
> >>  		break;
> >> @@ -1135,14 +1135,14 @@ int test_num_disk_vs_raid(u64
> metadata_profile, u64 data_profile,
> >>  	default:
> >>  	case 4:
> >>  		allowed |= BTRFS_BLOCK_GROUP_RAID10;
> >> -		__attribute__ ((fallthrough));
> >> +		/* fallthrough */
> >>  	case 3:
> >>  		allowed |= BTRFS_BLOCK_GROUP_RAID6;
> >> -		__attribute__ ((fallthrough));
> >> +		/* fallthrough */
> >>  	case 2:
> >>  		allowed |= BTRFS_BLOCK_GROUP_RAID0 |
> BTRFS_BLOCK_GROUP_RAID1 |
> >>  			BTRFS_BLOCK_GROUP_RAID5;
> >> -		__attribute__ ((fallthrough));
> >> +		/* fallthrough */
> >>  	case 1:
> >>  		allowed |= BTRFS_BLOCK_GROUP_DUP;
> >>  	}
> >>

Re: [PATCH 0/2] btrfs-progs: Setting implicit-fallthrough by default

[David Sterba]

On Mon, Oct 21, 2019 at 11:02:26PM -0300, Marcos Paulo de Souza wrote:
> From: Marcos Paulo de Souza <mpdesouza@suse.com>
> 
> While compiling btrfs-progs using clang I found an issue using
> __attribute__(fallthrough), which does not seems to work in clang.
> 
> To solve this issue, the code was changed to use /* fallthrough */, which is the
> same notation adopted by linux kernel.

I'd suggest to follow what kernel does, IIRC there's some whole-tree
cleanup of all the fall through statements with a unified conversion to
the right(tm) annotation.

> Once these places were changed, -Wimplicit-fallthrough was set in Makefile, to
> avoid further implicit-fallthrough cases being added in the future.

That would be good to add by default, but I'm a bit worried about the
differences between compilers and that we'd have to switch the
annotations again once the attribute support lands. Maybe that's not a
big deal.

Re: [PATCH 0/2] btrfs-progs: Setting implicit-fallthrough by default

[Nikolay Borisov]

On 22.10.19 г. 5:02 ч., Marcos Paulo de Souza wrote:
> From: Marcos Paulo de Souza <mpdesouza@suse.com>
> 
> While compiling btrfs-progs using clang I found an issue using
> __attribute__(fallthrough), which does not seems to work in clang.
> 
> To solve this issue, the code was changed to use /* fallthrough */, which is the
> same notation adopted by linux kernel.
> 
> Once these places were changed, -Wimplicit-fallthrough was set in Makefile, to
> avoid further implicit-fallthrough cases being added in the future.
> 
> Marcos Paulo de Souza (2):
>   btrfs-progs: utils: Replace __attribute__(fallthrough)
>   btrfs-progs: Makefile: Add -Wimplicit-fallthrough
> 
>  Makefile       |  1 +
>  common/utils.c | 12 ++++++------
>  2 files changed, 7 insertions(+), 6 deletions(-)
> 

Overall the patch looks good, it just changes the fallthrough to the
least common denominator which seems to be a simple comment. In clang 10
the currently used attribute method is also going to be supported.

But we'll get most value if we just enable it now, so

Reviewed-by: Nikolay Borisov <nborisov@suse.com>

Re: [PATCH 0/2] btrfs-progs: Setting implicit-fallthrough by default

[David Sterba]

On Tue, Oct 22, 2019 at 03:45:38PM +0300, Nikolay Borisov wrote:
> 
> 
> On 22.10.19 г. 5:02 ч., Marcos Paulo de Souza wrote:
> > From: Marcos Paulo de Souza <mpdesouza@suse.com>
> > 
> > While compiling btrfs-progs using clang I found an issue using
> > __attribute__(fallthrough), which does not seems to work in clang.
> > 
> > To solve this issue, the code was changed to use /* fallthrough */, which is the
> > same notation adopted by linux kernel.
> > 
> > Once these places were changed, -Wimplicit-fallthrough was set in Makefile, to
> > avoid further implicit-fallthrough cases being added in the future.
> > 
> > Marcos Paulo de Souza (2):
> >   btrfs-progs: utils: Replace __attribute__(fallthrough)
> >   btrfs-progs: Makefile: Add -Wimplicit-fallthrough
> > 
> >  Makefile       |  1 +
> >  common/utils.c | 12 ++++++------
> >  2 files changed, 7 insertions(+), 6 deletions(-)
> > 
> 
> Overall the patch looks good, it just changes the fallthrough to the
> least common denominator which seems to be a simple comment. In clang 10
> the currently used attribute method is also going to be supported.
> 
> But we'll get most value if we just enable it now, so
> 
> Reviewed-by: Nikolay Borisov <nborisov@suse.com>

Agreed, I've added the note to the first patch. 1-2 in devel. Thanks.