Add AsmParser support

[dylanmckay]

Implement the AsmParser class so we can then compile assembly files, and clang can handle inline assembly.

[4ntoine]

That's strange to have asm constraints errors if asm parser is not implemented at all.. Error text makes me think asm code was parsed and it "understands" that 'w' is constraints but it's not supported. Probably i'm wrong

[dylanmckay]

The

asm ( assembler template
    : output operands                   (optional)
    : input operands                    (optional)
    : clobbered registers list          (optional)
    );

syntax is not handled at all by LLVM - this is purely a C construct. Clang would parse the assembler template string, the output and input operands, and the clobbered register list. It would then actually hand off the assembler template to LLVM. The above syntax for inline assembler is the exact same syntax for all GCC targets. Therefore it makes perfect sense for Clang to check the target independent code first. Note that in order to actually assemble the assembler template, the output/inputs/clobbers must be known first, so it makes sense that they cause errors first.

Clang "understands" inline assembly syntax and so it knows that w is a constraint. Because AVRAsmParser is not implemented, it does not know that it is a valid AVR constraint, however.

[4ntoine]

Should we implement AVRAsmParser or we can just add constraints description to lib/target/AVRInstrInfo.td somehow ?

[dylanmckay]

We must implement AVRAsmParser in any case. Once Clang resolves the constraints, it attempts to parse the assembler template regardless, using AVRAsmParser. The problem here is that Clang would try and resolve the constraints without first knowing the possible AVR constraints.

Somehow we need to tell clang of what constraints AVR supports. I'm not sure where to add this code - it shouldn't be hard to figure out, a simple grep of the Mips sources will suffice. It doesn't look like they go in AVRInstrInfo.td, as they are not in the InstrInfo.td files of the other targets.

[dylanmckay]

Implemented AVRAsmParser. There are missing features, however. I know for one that no directives are supported (see issue #24).

Report bugs and missing features as issues on the issue tracker.