musl/arch/x86_64/bits/reg.h

#define R15    0
#define R14    1
#define R13    2
#define R12    3
#define RBP    4
#define RBX    5
#define R11    6
#define R10    7
#define R9     8
#define R8     9
#define RAX    10
#define RCX    11
#define RDX    12
#define RSI    13
#define RDI    14
#define ORIG_RAX 15
#define RIP    16
#define CS     17
#define EFLAGS 18
#define RSP    19
#define SS     20
#define FS_BASE 21
#define GS_BASE 22
#define DS     23
#define ES     24
#define FS     25
#define GS     26