[PATCH 1/1] tools: ftdgrep: use /* fallthrough */ as needed

[Heinrich Schuchardt]

On 5/11/20 8:40 PM, Tom Rini wrote:
> On Sun, May 10, 2020 at 10:12:07PM +0900, Masahiro Yamada wrote:
>> On Sun, May 10, 2020 at 12:12 AM Heinrich Schuchardt <xypron.glpk at gmx.de> wrote:
>>>
>>> GCC recognizes /* fallthrough */ if -Wimplicit-fallthrough=3 is enabled.
>>
>> FYI.
>>
>> Linux decided to not use /* fallthrough */ any more
>> because Clang does not recognize it.
>>
>> __attribute__((__fallthrough__)) is supported
>> by both Clang and recent GCC.
In fact Linux has a define:

include/linux/compiler_attributes.h:200:# define fallthrough
        __attribute__((__fallthrough__))

And in the code you would use

	case foo:
		fallthrough;
	case bar:

But the Linux kernel still has a lot of lines with

/* fallthrough */

Documentation/process/deprecated.rst:

<cite>
As there have been a long list of flaws `due to missing "break"
statements <https://cwe.mitre.org/data/definitions/484.html>`_, we no
longer allow implicit fall-through. In order to identify intentional
fall-through cases, we have adopted a pseudo-keyword macro "fallthrough"
which expands to gcc's extension `__attribute__((__fallthrough__))
<https://gcc.gnu.org/onlinedocs/gcc/Statement-Attributes.html>`_. (When
the C17/C18  `[[fallthrough]]` syntax is more commonly supported by C
compilers, static analyzers, and IDEs, we can switch to using that
syntax for the macro pseudo-keyword.)
</cite>

Using the attribute is not standard C and not any better than using the
comment. The real target is the C17 syntax.

>>
>>
>> Linux is now doing treewide conversion
>> from /* fallthrough */ to 'fallthrough;'.
>>
>> See include/linux/compiler_attributes.h in Linux.
>>
>> I do not know if U-Boot wants to align with it.
>> (up to Tom ?)
>
> A re-sync on the compiler headers again and making use of this sounds
> like a good idea, yes.
>

We should enable -Wimplicit-fallthrough like the kernel does. This
defaults to -Wimplicit-fallthrough=3 and is happy with both the comment
as well as with the attribute.

@Tom:
Will you update the compiler headers within this release cycle?
Otherwise we should take the patch as is to get us closer to the
-Wimplicit-fallthrough target.

Best regards

Heinrich