cmd/compile: not use REGTMP in ZeroRange on ARM64

For async preemption, we will be using REGTMP as a temporary
register in injected call on ARM64, which will clobber it. So any
code that uses REGTMP is not safe for async preemption.

For ZeroRange, which is inserted at the function entry where
there is no register live, we could just use a different register
and avoid REGTMP.

Change-Id: I3db763828df6846908c9843a9912597efb9efcdf
Reviewed-on: https://go-review.googlesource.com/c/go/+/203458
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/cmd/compile/internal/arm64/ggen.go

@@ -44,12 +44,16 @@ func zerorange(pp *gc.Progs, p *obj.Prog, off, cnt int64, _ *uint32) *obj.Prog {
 		p.To.Sym = gc.Duffzero
 		p.To.Offset = 4 * (64 - cnt/(2*int64(gc.Widthptr)))
 	} else {
-		p = pp.Appendpp(p, arm64.AMOVD, obj.TYPE_CONST, 0, 8+off-8, obj.TYPE_REG, arm64.REGTMP, 0)
+		// Not using REGTMP, so this is async preemptible (async preemption clobbers REGTMP).
+		// We are at the function entry, where no register is live, so it is okay to clobber
+		// other registers
+		const rtmp = arm64.REG_R20
+		p = pp.Appendpp(p, arm64.AMOVD, obj.TYPE_CONST, 0, 8+off-8, obj.TYPE_REG, rtmp, 0)
 		p = pp.Appendpp(p, arm64.AMOVD, obj.TYPE_REG, arm64.REGSP, 0, obj.TYPE_REG, arm64.REGRT1, 0)
-		p = pp.Appendpp(p, arm64.AADD, obj.TYPE_REG, arm64.REGTMP, 0, obj.TYPE_REG, arm64.REGRT1, 0)
+		p = pp.Appendpp(p, arm64.AADD, obj.TYPE_REG, rtmp, 0, obj.TYPE_REG, arm64.REGRT1, 0)
 		p.Reg = arm64.REGRT1
-		p = pp.Appendpp(p, arm64.AMOVD, obj.TYPE_CONST, 0, cnt, obj.TYPE_REG, arm64.REGTMP, 0)
-		p = pp.Appendpp(p, arm64.AADD, obj.TYPE_REG, arm64.REGTMP, 0, obj.TYPE_REG, arm64.REGRT2, 0)
+		p = pp.Appendpp(p, arm64.AMOVD, obj.TYPE_CONST, 0, cnt, obj.TYPE_REG, rtmp, 0)
+		p = pp.Appendpp(p, arm64.AADD, obj.TYPE_REG, rtmp, 0, obj.TYPE_REG, arm64.REGRT2, 0)
 		p.Reg = arm64.REGRT1
 		p = pp.Appendpp(p, arm64.AMOVD, obj.TYPE_REG, arm64.REGZERO, 0, obj.TYPE_MEM, arm64.REGRT1, int64(gc.Widthptr))
 		p.Scond = arm64.C_XPRE

src/cmd/compile/internal/gc/go.go

@@ -256,8 +256,12 @@ type Arch struct {
 	Use387    bool // should 386 backend use 387 FP instructions instead of sse2.
 	SoftFloat bool
 
-	PadFrame     func(int64) int64
-	ZeroRange    func(*Progs, *obj.Prog, int64, int64, *uint32) *obj.Prog
+	PadFrame func(int64) int64
+
+	// ZeroRange zeroes a range of memory on stack. It is only inserted
+	// at function entry, and it is ok to clobber registers.
+	ZeroRange func(*Progs, *obj.Prog, int64, int64, *uint32) *obj.Prog
+
 	Ginsnop      func(*Progs) *obj.Prog
 	Ginsnopdefer func(*Progs) *obj.Prog // special ginsnop for deferreturn