Add LoongArch register defs

[zhaixiaojuan]

The LoongArch architecture (LoongArch) is an Instruction Set Architecture (ISA) that has a RISC style.

Documentations:
ISA:
https://loongson.github.io/LoongArch-Documentation/LoongArch-Vol1-EN.html
ABI:
https://loongson.github.io/LoongArch-Documentation/LoongArch-ELF-ABI-EN.html
More docs can be found at:
https://loongson.github.io/LoongArch-Documentation/README-EN.html

[philipc]

I see the list of registers in the documentation, but I didn't see anywhere that documents the DWARF values for these registers.

[zhaixiaojuan]

The mapping relationship between registers and DWARF is consistent with the definition in gcc.
https://github.com/gcc-mirror/gcc/blob/master/gcc/config/loongarch/loongarch.h#374

Loongarch registers	Loongarch dwarf2 regnum
$r0-$r31	0-31
$f0-$f31	32-63
$fcc0-$fcc7	64-71

[philipc]

Including $ in the register names seems unusual to me. Does this mean that when using AT&T assembly syntax you end up referring to $r1 as %$r1?

[zhaixiaojuan]

The assembly format of LoongArch64 is different from that of AT&T and does not use %.
The following is the assembly code example of LoongArch64:

addi.d $sp, $sp, -16(0xff0)
st.d $fp, $sp, 8(0x8)
addi.d $fp, $sp, 16(0x10)
pcaddu12i $a0, 7(0x7)
addi.d $a0, $a0, 1772(0x6ec)
pcaddu12i $t0, 7(0x7)