fix(tree): add fallthrough switch markers

[aws-nslick]

Stacked PRs:

• feat(test): parse as c++ source #568
• chore(build): mpi: set mpicxx, too. #567
• chore(build): add AC_PROG_CXX #566
• feat(rdma): constrain C linkage to init #591
• feat(build): add -Wextra to "picky" compiler flags #588
• fix(platform-aws): fill all platform values #595
• fix(tree): use empty brace initializers for zero-initialization #594
• fix(tracing/nvtx): silence -Wmissing-field-initializer warnings #593
• fix(build): check features before mangling CFLAGS #589
• fix(test): fix typing issues #587
• fix(neuron): remove const from ncclNetPlugin_v{4,5} syms #577
• fix(sendrecv): add missing nccl-headers include #576
• fix(rdma): avoid enum/integral comparison #586
• ->fix(tree): add fallthrough switch markers #585
• fix(rdma): use COMM_ID_MASK as invalid id #574
• fix(tree): avoid sign comparison issues #575
• fix(param): move some parameters to unsigned #571
• feat(param): add uint parameter macro #570
• fix(tuner): fix implicit conversions #573
• fix(tuner): avoid gotos #569
• fix(tree): use decltype instead of typeof for cxx #565
• fix(tree): move declarations to top of function #563

---

fix(tree): add fallthrough switch markers

Signed-off-by: Nicholas Sielicki [email protected]

[aws-nslick]

On the top PR, this was amended to use the following macro instead:

#if __has_attribute(__fallthrough__)
# define fallthrough                    __attribute__((__fallthrough__))
#else
# define fallthrough                    do {} while (0)  /* fallthrough */
#endif

Only GCC accepts the comments, clang requires the attribute. GCC also has the attribute. Prefer the attribute if possible, otherwise, use the comment.