LinuxKPI: avoid -Werror=unused-value in sort() from BUILD_BUG_ON_ZERO()

[bz]

Summary

The BUILD_BUG_ON_ZERO() macro returns an (int)0 if it does not fail
at build time. LinuxKPI sort() has it as a guard for an unsupported
argument but ignores the return value.
This leads to gcc complaining:
error: statement with no effect [-Werror=unused-value]
Cast the result to (void) to silence the error as we are not interested
in it.

Reported by: CI
Sponsored by: The FreeBSD Foundation
MFC after: 3 days

[emaste]

Seems fine, sure.

[siva]

Ah sorry, I accidentally submitted a review for a similar thing, but I think BUILD_BUG_ON(swap) is cleaner?

[bz]

In D55554#1273043, @siva wrote:

Ah sorry, I accidentally submitted a review for a similar thing, but I think BUILD_BUG_ON(swap) is cleaner?

Yeah. I need to push it. I also tried to fix BUILD_BUG_ON_ZERO() but in the end I traded one GNU-extension warning for another. Still pondering if it would be cleaner ..

-#define        BUILD_BUG_ON_ZERO(x)    ((int)sizeof(struct { int:-((x) != 0); }))
+
+/*
+ * Doing the maths avoids -Wgnu-empty-struct;
+ * still -Wno-gnu-folding-constant for NULL input.
+ */
+#define        BUILD_BUG_ON_ZERO(x)                                    \
+    (int)(4 - sizeof(struct {                                  \
+        _Static_assert(!(x), #x " not false");                 \
+        uint32_t ________z;                                    \
+    }))

[emaste]

I think the (void) here is preferable; I believe Linux's BUILD_BUG_ON_ZERO will behave the same way.

[emaste]

Ah, BUILD_BUG_ON as @siva did in D55634 seems cleanest?

[bz]

In D55554#1273049, @emaste wrote:

Ah, BUILD_BUG_ON as @siva did in D55634 seems cleanest?

Hmm, for our implementation of BUILD_BUG_ON that is probably true though the ; @siva can you commit yours (maybe mention the warning in the commit message?)?

[siva]

Sounds good, I'll commit mine and show the warning in the commit message.

[bz]

For https://reviews.freebsd.org/D55634

[siva]

Committed in D55634 with f26cb4757eb74ceace39144933ae198ebf1b4f28.