[LLVMdev] Asm syntax of Mips m[tf]cX coprocessor instructions

[Jeremy Fitzhardinge]

Hi all,

I'm experimenting with compiling some Mips code which is normally built
with a gcc cross toolchain with clang instead.  This code targets the
Cavium Octeon, and uses some of that CPU's crypto engine features which
are implemented in coprocessor 2.

The inline asm for the crypto code uses instructions of the form "dmtc2
%0, 0xNNNN" - that is the coprocessor register is represented as a
constant, which is the only syntax that gas seems to accept
(specifically, an expression which evaluates to a constant).  Clang's
integrated-as, however, only accepts "dmtc2 %0, $0xNNNN", that is, a
coprocessor register number.

I think clang is correct here, but for cross-tool compatibility it would
be useful if it could also accept the same syntax as gas.  I could hack
up the Octeon SDK headers to switch syntax depending on the compiler,
but I'd really prefer to avoid that (esp since it really depends on the
assembler in use, and I'm not sure if there's preprocessor symbols to
detect whether integrated-as is enabled).

I've had a look at Mips(64)InstrInfo.td which seems to be where the
syntax is defined, but since this is my first encounter with
clang/llvm's internals, I'm not sure how to go about making the change. 
Could someone give me some pointers?  Is this something that InstAlias
can handle?

An alternative workaround would be to just use gas as-is, but it gets
upset by the .cfi_section directives that clang emits.  Is there some
way to suppress those (while keeping the rest of the cfi directives that
this version of gas does understand)?

Thanks,
    J