AArch64: count call stack when working out if scavenging is needed

[t.p.northover]

Summary

We try to guess whether we'll need extra registers or slots to handle large stack offsets (bigger than a single [sp, #N] can handle). This calculation needs to take into account stack that's in the caller's frame, either our own args or involved in calls we make.

Bit of a messy test, but it needs to both have lots of args to actually use significant stack, and be opaque enough that it also uses many registers. So the usual [8 x i64] tricks to quickly use args don't work.

[t.p.northover]

Ping.

[efriedma]

llvm/lib/Target/AArch64/AArch64FrameLowering.cpp

3084

To be clear, this is looking at arguments passed on the stack by the caller of this function definition? If we use a frame index to access them, and we don't have a frame pointer, they need to count towards the stack limit, sure.

Referring to this as the "callee's stack" is a bit confusing.

[t.p.northover]

llvm/lib/Target/AArch64/AArch64FrameLowering.cpp

3084

It is confusing. It's both callee and caller stack space (i.e. used only for args, but might be incoming or outgoing). Regardless, I did word that badly. I'll try to improve it.

[t.p.northover]

Reword comment to avoid confusion.

[efriedma]

LGTM

[t.p.northover]

Thanks. 2002c82278f6 on main.