Jeff, many appologies. I slightly misread what you wrote, and came across more strongly that was appropriate: I'm sorry!
Let's see. Ralph correctly points out that LLVM isn't producing anything like Intel syntax. ...
This is not true. LLVM produces something very close to what GAS accepts in intel mode: that is, we produce fully intel syntax (e.g. "DWORD PTR", no opcode size suffixes, etc) but we prepend % onto registers. This is what GAS expects (i.e., it's gas intel mode). I believe there is a GAS option to turn off the % prefix, but we never used it because it had other bugs.
No, %reg is not Intel syntax, no matter what gas thinks.
There are several dialects of "intel mode", and GAS's is just one. Greater variation is due to differences in pseudo ops.
Last June, Aaron Gray offered to fix Intel mode so that it actually produced Intel syntax. He offered was a gigantic cut and paste: http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20050627/026872.html. Then he apparently lost interest, though he now claims he had finished it after all but simply neglected to give us a patch.
Yup, that's true. Note, however, that that patch just moved around existing functionality, it didn't change or add anything (hence the use of the term "refactor").
Specifically, when I said "Actually, that's not true.", what I meant is that this:
"Someone offered to do the Intel version, but did little more
than a huge cut and paste of the AT&T version and then lost interest."
... is not true. The current Intel version is based on our original support for GAS Intel mode, it is not based on AT&T syntax support at all (which, again, came after gas intel syntax support).
As was independently pointed out, we're quite close to supporting MASM (or whatever) intel syntax. Removing the %'s, for example, is trivial. Anyone who wants to do so is welcome to. The current -x86-asm-syntax=intel support is not currently compatible with any assembler that I know of. Patches to make it useful are welcome.