cmd/internal/obj/mips: store LR before update SP in function prologue

This prevents the traceback code from seeing a half-updated
stack frame when a profiling signal comes during the execution
of function prologue. Also fixes mips64x part of #17381.

Change-Id: Iec9683427e546e3648b2e8b1dde956d13f6eb938
Reviewed-on: https://go-review.googlesource.com/31721
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/cmd/internal/obj/mips/obj0.go

@@ -281,7 +281,21 @@ func preprocess(ctxt *obj.Link, cursym *obj.LSym) {
 			q = p
 
 			if autosize != 0 {
-				q = obj.Appendp(ctxt, p)
+				// Make sure to save link register for non-empty frame, even if
+				// it is a leaf function, so that traceback works.
+				// Store link register before decrement SP, so if a signal comes
+				// during the execution of the function prologue, the traceback
+				// code will not see a half-updated stack frame.
+				q = obj.Appendp(ctxt, q)
+				q.As = AMOVV
+				q.Lineno = p.Lineno
+				q.From.Type = obj.TYPE_REG
+				q.From.Reg = REGLINK
+				q.To.Type = obj.TYPE_MEM
+				q.To.Offset = int64(-autosize)
+				q.To.Reg = REGSP
+
+				q = obj.Appendp(ctxt, q)
 				q.As = AADDV
 				q.Lineno = p.Lineno
 				q.From.Type = obj.TYPE_CONST
@@ -304,15 +318,6 @@ func preprocess(ctxt *obj.Link, cursym *obj.LSym) {
 				break
 			}
 
-			q = obj.Appendp(ctxt, q)
-			q.As = AMOVV
-			q.Lineno = p.Lineno
-			q.From.Type = obj.TYPE_REG
-			q.From.Reg = REGLINK
-			q.To.Type = obj.TYPE_MEM
-			q.To.Offset = int64(0)
-			q.To.Reg = REGSP
-
 			if cursym.Text.From3.Offset&obj.WRAPPER != 0 {
 				// if(g->panic != nil && g->panic->argp == FP) g->panic->argp = bottom-of-frame
 				//