cmd/6g: let the compiler use R15 when it is not needed for GOT indirection

Thanks to Russ for the hints.

Change-Id: Ie35a71d432b9d68bd30c7a364b4dce1bd3db806e
Reviewed-on: https://go-review.googlesource.com/9102Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Michael Hudson-Doyle <michael.hudson@canonical.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/6g/prog.go

@@ -293,4 +293,16 @@ func proginfo(p *obj.Prog) {
 	if p.To.Index != x86.REG_NONE {
 		info.Regindex |= RtoB(int(p.To.Index))
 	}
+	if gc.Ctxt.Flag_dynlink {
+		// When -dynlink is passed, many operations on external names (and
+		// also calling duffzero/duffcopy) use R15 as a scratch register.
+		if p.As == x86.ALEAQ || info.Flags == gc.Pseudo || p.As == obj.ACALL || p.As == obj.ARET || p.As == obj.AJMP {
+			return
+		}
+		if p.As == obj.ADUFFZERO || p.As == obj.ADUFFCOPY || p.From.Name == obj.NAME_EXTERN || p.To.Name == obj.NAME_EXTERN {
+			info.Reguse |= R15
+			info.Regset |= R15
+			return
+		}
+	}
 }

src/cmd/6g/reg.go

@@ -108,6 +108,7 @@ const (
 	DX  = 1 << (x86.REG_DX - x86.REG_AX)
 	DI  = 1 << (x86.REG_DI - x86.REG_AX)
 	SI  = 1 << (x86.REG_SI - x86.REG_AX)
+	R15 = 1 << (x86.REG_R15 - x86.REG_AX)
 )
 
 func RtoB(r int) uint64 {
@@ -125,9 +126,6 @@ func BtoR(b uint64) int {
 		// BP is part of the calling convention if framepointer_enabled.
 		b &^= (1 << (x86.REG_BP - x86.REG_AX))
 	}
-	if gc.Ctxt.Flag_dynlink {
-		b &^= (1 << (x86.REG_R15 - x86.REG_AX))
-	}
 	if b == 0 {
 		return 0
 	}