Right now `bswap(ulong)` does not work in betterC when compiling with DMD. (It works when compiling with LDC because LDC treats both `bswap(uint)` and `bswap(ulong)` as intrinsic functions.) In the long run all functions in core.bitop probably should either be made into template functions or intrinsics.
@n8sh created dlang/druntime pull request #2621 "Issue 19924 - Make core.bitop.bswap(ulong) work in betterC" mentioning this issue: - Issue 19924 - Make core.bitop.bswap(ulong) work in betterC https://github.com/dlang/druntime/pull/2621
dlang/druntime pull request #2621 "Issue 19924 - Make core.bitop.bswap(ulong) work in betterC" was merged into master: - 23f6cf0fc235c00265c7dcc293de190dbccdbb52 by Nathan Sashihara: Fix Issue 19924 - Make core.bitop.bswap(ulong) work in betterC https://github.com/dlang/druntime/pull/2621