On Tue, May 12, 2020 at 11:04:38PM -0400, Tom Rini wrote:
On Mon, May 11, 2020 at 09:08:03PM +0200, Heinrich Schuchardt wrote:
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@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.
I'm not going to update it for this release cycle. I've done the initial import and build and there's some fairly large changes related to inlining that I want to look at harder to see if we can/should do something about (I don't want to derail this thread, I'll start another). But it's very far from zero size change and given the inline changes I think it'll need real testing.
And since the kernel isn't making a huge use yet of fallthrough; we can afford to look a little harder at things.
I think I've figured out the inline issue which is that we need scripts/Kconfig.include from the kernel, CC_HAS_ASM_INLINE Kconfig option, and re-sync with Kconfiglib, but that's still going to be enough stuff that I don't think it's good to pull in at -rc2.