static_assert, sfc, and clang issues

[Tyler Retzlaff]

On Tue, Jan 16, 2024 at 09:03:01AM -0800, Stephen Hemminger wrote:
> Ran into a corner case issue, so sending to mailing list for wider discussion.
> 
> One improvement to DPDK code base planned is getting rid of variable length arrays.
> VLA's can cause bugs and are not supported by the Windows compiler.
> Gcc and Clang have a flag to warn on use of VLA's (-Wvla).
> 
> In DPDK one use of VLA's is in the RTE_BUILD_BUG_ON macro.
> The best path is to replace the undefined array access with a better
> static_assert() which is builtin to compilers.
> 
> Using static_assert() is relatively easy, there are a few places where
> it does detect use of non-constant expressions in existing code but these
> are fixable.
> 
> But there is one case where use static_assert() runs into a Clang bug
> which is not fixed in distros: 
> 
> https://github.com/llvm/llvm-project/issues/55821
> 
> The code in question is in the sfc driver which for better or worse
> has lots of assertions. The problem is that clang (except in unreleased trunk)
> can not handle static_assert in a switch leg.
> 
> 		switch (actions->type) {
> 		case RTE_FLOW_ACTION_TYPE_VOID:
> 			SFC_BUILD_SET_OVERFLOW(RTE_FLOW_ACTION_TYPE_VOID,
> 					       actions_set);
> 			break;
> 
> ../drivers/net/sfc/sfc_flow.c:1635:4: error: expected expression
>                         SFC_BUILD_SET_OVERFLOW(RTE_FLOW_ACTION_TYPE_VOID,
>                         ^
> ../drivers/net/sfc/sfc_flow.h:36:2: note: expanded from macro 'SFC_BUILD_SET_OVERFLOW'
>         RTE_BUILD_BUG_ON((_action) >= sizeof(_set) * CHAR_BIT)
>         ^
> 
> 
> There are some workarounds:
> 	0. Ignore it, works on Gcc, and Clang fix is pending.
> 	1. Remove many of these RTE_BUILD_BUG_ON's. They are really not that helpful.
> 	2. Add additional { } to these switch cases so they become basic blocks
> 	   which works around the bug.
>         3. Move the assertions out of the switch
> 
> My preference is #2 but open to other options.

+1 for #2 just make it a block.