cmd/internal/obj/arm64: fix frame pointer restore in epilogue

For leaf but nonzero-frame functions.

Currently we're not restoring it properly. We also need to restore
it before popping the stack frame, so that the frame won't get
clobbered by a signal handler in the meantime.

Fixes #63830

Needs a test, but I'm not at all sure how we would actually do that. Leaving for inspiration.

Change-Id: I273a25f2a838f05a959c810145cccc5428eaf164
Reviewed-on: https://go-review.googlesource.com/c/go/+/538635
Reviewed-by: Cherry Mui <[email protected]>
Reviewed-by: Eric Fang <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: David Chase <[email protected]>

Author: randall77

src/cmd/internal/obj/arm64/obj7.go

@@ -850,21 +850,24 @@ func preprocess(ctxt *obj.Link, cursym *obj.LSym, newprog obj.ProgAlloc) {
 			p.To = obj.Addr{}
 			if c.cursym.Func().Text.Mark&LEAF != 0 {
 				if c.autosize != 0 {
+					// Restore frame pointer.
+					// ADD $framesize-8, RSP, R29
 					p.As = AADD
 					p.From.Type = obj.TYPE_CONST
-					p.From.Offset = int64(c.autosize)
+					p.From.Offset = int64(c.autosize) - 8
+					p.Reg = REGSP
 					p.To.Type = obj.TYPE_REG
-					p.To.Reg = REGSP
-					p.Spadj = -c.autosize
+					p.To.Reg = REGFP
 
-					// Frame pointer.
+					// Pop stack frame.
+					// ADD $framesize, RSP, RSP
 					p = obj.Appendp(p, c.newprog)
-					p.As = ASUB
+					p.As = AADD
 					p.From.Type = obj.TYPE_CONST
-					p.From.Offset = 8
-					p.Reg = REGSP
+					p.From.Offset = int64(c.autosize)
 					p.To.Type = obj.TYPE_REG
-					p.To.Reg = REGFP
+					p.To.Reg = REGSP
+					p.Spadj = -c.autosize
 				}
 			} else {
 				aoffset := c.autosize