[Cocci] [RFC] treewide: cleanup unreachable breaks

[Joe Perches]

On Sat, 2020-10-17 at 20:21 +0200, Julia Lawall wrote:
> On Sat, 17 Oct 2020, Joe Perches wrote:
> > On Sat, 2020-10-17 at 09:09 -0700, trix at redhat.com wrote:
> > > From: Tom Rix <trix at redhat.com>
> > > 
> > > This is a upcoming change to clean up a new warning treewide.
> > > I am wondering if the change could be one mega patch (see below) or
> > > normal patch per file about 100 patches or somewhere half way by collecting
> > > early acks.
> > > 
> > > clang has a number of useful, new warnings see
> > > https://clang.llvm.org/docs/DiagnosticsReference.html
> > > 
> > > This change cleans up -Wunreachable-code-break
> > > https://clang.llvm.org/docs/DiagnosticsReference.html#wunreachable-code-break
> > > for 266 of 485 warnings in this week's linux-next, allyesconfig on x86_64.
> > 
> > Early acks/individual patches by subsystem would be good.
> > Better still would be an automated cocci script.
> 
> Coccinelle is not especially good at this, because it is based on control
> flow, and a return or goto diverts the control flow away from the break.
> A hack to solve the problem is to put an if around the return or goto, but
> that gives the break a meaningless file name and line number.  I collected
> the following list, but it only has 439 results, so fewer than clang.  But
> maybe there are some files that are not considered by clang in the x86
> allyesconfig configuration.
> 
> Probably checkpatch is the best solution here, since it is not
> configuration sensitive and doesn't care about control flow.

Likely the clang compiler is the best option here.

It might be useful to add -Wunreachable-code-break to W=1
or just always enable it if it isn't already enabled.

diff --git a/scripts/Makefile.extrawarn b/scripts/Makefile.extrawarn
index 95e4cdb94fe9..3819787579d5 100644
--- a/scripts/Makefile.extrawarn
+++ b/scripts/Makefile.extrawarn
@@ -32,6 +32,7 @@ KBUILD_CFLAGS += $(call cc-option, -Wunused-but-set-variable)
 KBUILD_CFLAGS += $(call cc-option, -Wunused-const-variable)
 KBUILD_CFLAGS += $(call cc-option, -Wpacked-not-aligned)
 KBUILD_CFLAGS += $(call cc-option, -Wstringop-truncation)
+KBUILD_CFLAGS += $(call cc-option, -Wunreachable-code-break)
 # The following turn off the warnings enabled by -Wextra
 KBUILD_CFLAGS += -Wno-missing-field-initializers
 KBUILD_CFLAGS += -Wno-sign-compare

(and thank you Tom for pushing this forward)

checkpatch can't find instances like:

	case FOO:
		if (foo)
			return 1;
		else
			return 2;
		break;

As it doesn't track flow and relies on the number
of tabs to be the same for any goto/return and break;

checkpatch will warn on:

	case FOO:
		...
		goto bar;
		break;