cmd/internal/obj/mips: mark restartable sequences

Following CL 208126, do the same for MIPS.

Change-Id: I95f8fc99a234524119a4d29c7695676dc0ea1025
Reviewed-on: https://go-review.googlesource.com/c/go/+/208217
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

1 files changed, 19 insertions, 6 deletions

diff --git a/src/cmd/internal/obj/mips/asm0.go b/src/cmd/internal/obj/mips/asm0.go
index 957f2d5c93..faa12bf133 100644
--- a/src/cmd/internal/obj/mips/asm0.go
+++ b/src/cmd/internal/obj/mips/asm0.go
@@ -526,16 +526,29 @@ func span0(ctxt *obj.Link, cursym *obj.LSym, newprog obj.ProgAlloc) {
 	// We use REGTMP as a scratch register during call injection,
 	// so instruction sequences that use REGTMP are unsafe to
 	// preempt asynchronously.
-	obj.MarkUnsafePoints(c.ctxt, c.cursym.Func.Text, c.newprog, c.isUnsafePoint, nil)
+	obj.MarkUnsafePoints(c.ctxt, c.cursym.Func.Text, c.newprog, c.isUnsafePoint, c.isRestartable)
 }
 
-// Return whether p is an unsafe point.
+// isUnsafePoint returns whether p is an unsafe point.
 func (c *ctxt0) isUnsafePoint(p *obj.Prog) bool {
-	if p.From.Reg == REGTMP || p.To.Reg == REGTMP || p.Reg == REGTMP {
-		return true
+	// If p explicitly uses REGTMP, it's unsafe to preempt, because the
+	// preemption sequence clobbers REGTMP.
+	return p.From.Reg == REGTMP || p.To.Reg == REGTMP || p.Reg == REGTMP
+}
+
+// isRestartable returns whether p is a multi-instruction sequence that,
+// if preempted, can be restarted.
+func (c *ctxt0) isRestartable(p *obj.Prog) bool {
+	if c.isUnsafePoint(p) {
+		return false
 	}
-	// Most of the multi-instruction sequence uses REGTMP, except
-	// ones marked safe.
+	// If p is a multi-instruction sequence with uses REGTMP inserted by
+	// the assembler in order to materialize a large constant/offset, we
+	// can restart p (at the start of the instruction sequence), recompute
+	// the content of REGTMP, upon async preemption. Currently, all cases
+	// of assembler-inserted REGTMP fall into this category.
+	// If p doesn't use REGTMP, it can be simply preempted, so we don't
+	// mark it.
 	o := c.oplook(p)
 	return o.size > 4 && o.flag&NOTUSETMP == 0
 }