tree c2bbd74b1f9f4d4e5b131b0afb881039b5818722
parent 24f7d89a73b7bd77127a56e8a8552c48278d251b
author Cherry Zhang <cherryyz@google.com> 1561488484 -0400
committer Cherry Zhang <cherryyz@google.com> 1561564907 +0000

cmd/compile, runtime: use R20, R21 in ARM64's Duff's devices

Currently we use R16 and R17 for ARM64's Duff's devices.
According to ARM64 ABI, R16 and R17 can be used by the (external)
linker as scratch registers in trampolines. So don't use these
registers to pass information across functions.

It seems unlikely that calling Duff's devices would need a
trampoline in normal cases. But it could happen if the call
target is out of the 128 MB direct jump limit.

The choice of R20 and R21 is kind of arbitrary. The register
allocator allocates from low-numbered registers. High numbered
registers are chosen so it is unlikely to hold a live value and
forces a spill.

Fixes #32773.

Change-Id: Id22d555b5afeadd4efcf62797d1580d641c39218
Reviewed-on: https://go-review.googlesource.com/c/go/+/183842
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Keith Randall <khr@golang.org>