cmd/internal/obj/s390x: mark unsafe points

For async preemption, we will be using REGTMP as a temporary
register in injected call on S390X, which will clobber it. So any
code that uses REGTMP is not safe for async preemption.

In the assembler backend, we expand a Prog to multiple machine
instructions and use REGTMP as a temporary register if necessary.
These need to be marked unsafe. Unlike ARM64 and MIPS,
instructions on S390X are variable length so we don't use the
length as a condition. Instead, we set a bit on the Prog whenever
REGTMP is used.

Change-Id: Ie5d14068a950f4c7cea51dff2c4a8bdc19ec9348
Reviewed-on: https://go-review.googlesource.com/c/go/+/204105
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/internal/obj/s390x/a.out.go

@@ -186,6 +186,7 @@ const (
 	// mark flags
 	LEAF = 1 << iota
 	BRANCH
+	USETMP // generated code of this Prog uses REGTMP
 )
 
 const ( // comments from func aclass in asmz.go

src/cmd/internal/obj/s390x/objz.go

@@ -344,7 +344,11 @@ func preprocess(ctxt *obj.Link, cursym *obj.LSym, newprog obj.ProgAlloc) {
 				// Store link register before decrementing 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(p, c.newprog)
+				// This sequence is not async preemptible, as if we open a frame
+				// at the current SP, it will clobber the saved LR.
+				q = c.ctxt.StartUnsafePoint(p, c.newprog)
+
+				q = obj.Appendp(q, c.newprog)
 				q.As = AMOVD
 				q.From.Type = obj.TYPE_REG
 				q.From.Reg = REG_LR
@@ -360,6 +364,8 @@ func preprocess(ctxt *obj.Link, cursym *obj.LSym, newprog obj.ProgAlloc) {
 				q.To.Type = obj.TYPE_REG
 				q.To.Reg = REGSP
 				q.Spadj = autosize
+
+				q = c.ctxt.EndUnsafePoint(q, c.newprog, -1)
 			} else if c.cursym.Func.Text.Mark&LEAF == 0 {
 				// A very few functions that do not return to their caller
 				// (e.g. gogo) are not identified as leaves but still have