Is the cost of this instruction going to depend on how many registers were pushed?

LGTM.

LGTM as well

Is the cost of this instruction going to depend on how many registers were pushed?

No. It's cheap enough to execute anyway, the cost between 1 and 24 pushed registers is likely smaller than per-instruction overhead.

One thing I'm wondering about is how safe it is to assume that the stack will be properly cleared after an internal function call before popping the registers back. Otherwise there's a risk that we'd be popping arbitrary data into the registers if the stack wasn't restored to its original state while pushing the registers. While this is a sway implementation concern, let's say I write a sway function that has an asm block which manually mucks with the stack by performing a cfe, would sway be able to properly restore the stack to its original state before the registers are popped while returning from that fn? @vaivaswatha

One thing I'm wondering about is how safe it is to assume that the stack will be properly cleared after an internal function call before popping the registers back. Otherwise there's a risk that we'd be popping arbitrary data into the registers if the stack wasn't restored to its original state while pushing the registers. While this is a sway implementation concern, let's say I write a sway function that has an asm block which manually mucks with the stack by performing a cfe, would sway be able to properly restore the stack to its original state before the registers are popped while returning from that fn? @vaivaswatha

Isn't this a concern today as well? where we push the registers to the stack before a call and pop them afterwards, but just with separate instructions instead of this one single one.

I'd say it's hard for the compiler to do anything safely if ASM blocks are used unsafely.

One workaround I can think of is if the compiler also pushed $sp to the stack before a function call, and after returning it shrinks the stack back to that value automatically (if $sp != $original_sp) before any popping.