Support AVR-specific input/output constraints

[4ntoine]

https://github.com/dylanmckay/avr-llvm/issues/21 https://github.com/dylanmckay/avr-llvm/issues/20 https://github.com/dylanmckay/avr-llvm/issues/19

Asm code is validated by Clang and then should be passed to unimplemented AvrAsmParser classes. At the moment asm code is validated by Clang and it's not recognized as valid.

[4ntoine]

https://gcc.gnu.org/onlinedocs/gcc/Machine-Constraints.html for AVR:

AVR family—config/avr/constraints.md
l
Registers from r0 to r15 
a
Registers from r16 to r23 
d
Registers from r16 to r31 
w
Registers from r24 to r31. These registers can be used in ‘adiw’ command 
e
Pointer register (r26–r31) 
b
Base pointer register (r28–r31) 
q
Stack pointer register (SPH:SPL) 
t
Temporary register r0 
x
Register pair X (r27:r26) 
y
Register pair Y (r29:r28) 
z
Register pair Z (r31:r30) 
I
Constant greater than −1, less than 64 
J
Constant greater than −64, less than 1 
K
Constant integer 2 
L
Constant integer 0 
M
Constant that fits in 8 bits 
N
Constant integer −1 
O
Constant integer 8, 16, or 24 
P
Constant integer 1 
G
A floating point constant 0.0 
Q
A memory address based on Y or Z pointer with displacement.

[dylanmckay]

I believe that clang passes the assembly constraints as strings to LLVM. This document describes inline assembler expressions in LLVM.

Clang has a list of possible supported constraints for all targets here. I'd assume you would just add the missing constraints for AVR, and then clang will automatically pass them onto LLVM.

Here is a list of constraints that AVR supports, under the title Input and Output Operands.

Reproduced here:

Constraint	Used for	Range
a	Simple upper registers	r16 to r23
b	Base pointer registers pairs	y, z
d	Upper register	r16 to r31
e	Pointer register pairs	x, y, z
q	Stack pointer register	SPH:SPL
r	Any register	r0 to r31
t	Temporary register	r0
w	Special upper register pairs	r24, r26, r28, r30
x	Pointer register pair X	x (r27:r26)
y	Pointer register pair Y	y (r29:r28)
z	Pointer register pair Z	z (r31:r30)
G	Floating point constant	0.0
I	6-bit positive integer constant	0 to 63
J	6-bit negative integer constant	-63 to 0
K	Integer constant	2
L	Integer constant	0
l	Lower registers	r0 to r15
M	8-bit integer constant	0 to 255
N	Integer constant	-1
O	Integer constant	8, 16, 24
P	Integer constant	1
Q	A memory address based on Y or Z pointer with displacement
R	Integer constant	-6 to 5

[4ntoine]

already in progress

[4ntoine]

Note Clang only validates constraints, still need to implement AVRAsmParser