What does the exclamation mark mean in the end of an A64 instruction?

Question

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 1

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.