CODEWITHSUNDEEP
arm64
drymon
drymon

Reputation: 143

Explain arm64 instruction STP

I am reading a source code and find the instructions below but I don't understand what it does. Could someone please help me to understand it?

stp x19, x20, [x8], #16

If only stp x19, x20, [x8] I can understand.

Upvotes: 9

Views: 18602

Answers (1)

Siguza
Siguza

Reputation: 23870

It's called the "post-index" variant, and it modifies the address after storing.
The following two are equivalent:

stp x19, x20, [x8], #16

stp x19, x20, [x8]
add x8, x8, #16

There is also a pre-index variant that modified the address before storing.
Again, these two are equivalent:

stp x19, x20, [x8, #16]!

add x8, x8, #16
stp x19, x20, [x8]

In my experience, this is most commonly used for pushing and popping stack frames like so:

some_func:
    stp x29, x30, [sp, #-0x10]!
    // ...
    ldp x29, x30, [sp], #0x10
    ret

Most forms of single-register ldr and str also have these variants, for more information see section C1.3.3 ("Load/Store addressing modes") in the ARMv8 Reference Manual.

Upvotes: 18

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