Unfortunately it requires at least GCC6. If a nice guy can check the generated code on GCC6.
I don't know clang status.
Here the only example, I have found on the web
https://developers.redhat.com/blog/2016/02/25/new-asm-flags-feature-for-x86-in-gcc-6/
Current generated code in GSTextureCache::SourceMap::Add
38b3: bsf eax,esi
38b6: add esp,0x10
38b9: test esi,esi
38bb: jne 387e <GSTextureCache::SourceMap::Add(GSTextureCache::Source*, GIFRegTEX0 const&, GSOffset*)+0x6e>
BSF already set the Z flag when input (esi) is 0. So it would be better
to not put a silly add before the jump and to skip the test operation.