What does the exclamation mark mean in the end of an A64 instruction?
The documentation for LDP and STP gives an example instruction with an exclamation mark in the end:
LDP X8, X2, [X0, #0x10]!
Also the documentation about porting A32 PUSH/POP instructions into A64 gives the following examples:
PUSH {r0-r1} ---> STP X0, X1, [SP, #-16]!
POP {r0-r1} ---> LDP X0, X1, [SP], #16
Neither of the pages explains what the exclamation mark in the end of the instructions means. What does it?
Answer
The ! means "Register write-back": the base register is used to calculate the address of the transfer, and is updated.
In your example:
LDP X8, X2, [X0, #0x10]!
X0 modified so that after the operation:
X0 = X0 + 0x10
If you do not put the !, X0 is not modified by the operation.
On the second example concerning PUSH/POP, the difference is when the increment is done:
STP X0, X1, [SP, #-16]! stores at address SP-16, and SP is decremented in the same way
LDP X0, X1, [SP], #16 loads from address SP, and after the transfer is performed, stores SP+16 to SP.