tree 2bddc87d36076445f042b6935f4419a77265acd9
parent 0750107074c39f7b846515de47c2857cbdb7e3d6
author Cherry Mui <cherryyz@google.com> 1655320164 -0400
committer Cherry Mui <cherryyz@google.com> 1656541618 +0000

cmd/internal/obj/arm64: save LR and SP in one instruction for small frames

When we create a thread with signals blocked. But glibc's
pthread_sigmask doesn't really allow us to block SIGSETXID. So we
may get a signal early on before the signal stack is set. If we
get a signal on the current stack, it will clobber anything below
the SP. This CL makes it to save LR and decrement SP in a single
MOVD.W instruction for small frames, so we don't write below the
SP.

We used to use a single MOVD.W instruction before CL 379075.
CL 379075 changed to use an STP instruction to save the LR and FP,
then decrementing the SP. This CL changes it back, just this part
(epilogues and large frame prologues are unchanged). For small
frames, it is the same number of instructions either way.

This decreases the size of a "small" frame from 0x1f0 to 0xf0.
For frame sizes in between, it could benefit from using an
STP instruction instead of using the prologue for the "large"
frame case. We don't bother it for now as this is a stop-gap
solution anyway.

This only addresses the issue with small frames. Luckily, all
functions from thread entry to setting up the signal stack have
samll frames.

Other possible ideas:
- Expand the unwind info metadata, separate SP delta and the
  location of the return address, so we can express "SP is
  decremented but the return address is in the LR register". Then
  we can always create the frame first then write the LR, without
  writing anything below the SP (except the frame pointer at SP-8,
  which is minor because it doesn't really affect program
  execution).
- Set up the signal stack immediately in mstart in assembly.

For Go 1.19 we do this simple fix. We plan to do the metadata fix
in Go 1.20 ( #53609 ).

Other LR architectures are addressed in CL 413428.

Fix #53374.

Change-Id: I9d6582ab14ccb06ac61ad43852943d9555e22ae5
Reviewed-on: https://go-review.googlesource.com/c/go/+/412474
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Eric Fang <eric.fang@arm.com>