https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93202
--- Comment #7 from Jim Wilson <wilson at gcc dot gnu.org> --- (In reply to Luís Marques from comment #3) > Jim Wilson: I'm not using it, I was only working on the LLVM implementation. > Could you please clarify if following modifiers are also internal only? > > 'C' Print the integer branch condition for comparison OP. > 'A' Print the atomic operation suffix for memory model OP. > 'F' Print a FENCE if the memory model requires a release. 'C' maps an rtx to a string. It is intended to be used for comparisons to emit the appropriate compare instruction, because the instruction names match the gcc internal rtx names. It can't be used for its intended purpose in an asm, as you can't get a comparison operator as an operand to an asm. Since it works with any rtx, it can be used in an asm, but is very unlikely to be useful. 'A' takes a memory order value from stdatomic.h, and emits a .acq if it is one of the memory orders that requires an acquire operation, e.g. __ATOMIC_ACQUIRE. Gcc calls this a memory model internally, and defines the values in memmodel.h. The primary use is for the atomic builtin functions, to map the memory order argument to the right instruction. This takes an integer argument, so in theory it could be used in an asm, but unlikely to be very useful. 'F" is similar, except it is to atomic releases, and emits a fence instruction. This one is a bit of historical accident. The gcc riscv port was written before we had a formal memory model defined, and so to be conservative, it emits fences in a lot of places where we probably don't need them. Now that we do have a formal memory model defined, the gcc port needs to be fixed to implement it, except there is no one to do the work, so it is unclear when it will happen. Meanwhile, the port still emits a lot of fences we don't need via 'F'. This takes an integer argument as above, so likewise in theory could be used in an asm, but unlikely to be useful. And this one has the additional problem that it needs to change in a future gcc release, though we could preserve the current meaning of the 'F' letter and use a new letter if necessary in the rewrite. The useful print operand letters are the ones for registers, constants, and addresses. These random ones used for internal gcc features aren't really useful in asms.
