cmd/internal/obj/arm64: add error checking for system registers.

This CL adds system register error checking test cases. There're two kinds of
error test cases:

1. illegal combination.
MRS should be used in this way: MRS <system register>, <general register>.
MSR should be used in this way: MSR <general register>, <system register>.
Error usage examples:
MRS     R8, VTCR_EL2    // ERROR "illegal combination"
MSR     VTCR_EL2, R8    // ERROR "illegal combination"

2. illegal read or write access.
Error usage examples:
MSR     R7, MIDR_EL1    // ERROR "expected writable system register or pstate"
MRS     OSLAR_EL1, R3   // ERROR "expected readable system register"

This CL reads system registers readable and writeable property to check whether
they're used with legal read or write access. This property is named AccessFlags
in sysRegEnc.go, and it is automatically generated by modifing the system register
generator.

Change-Id: Ic83d5f372de38d1ecd0df1ca56b354ee157f16b4
Reviewed-on: https://go-review.googlesource.com/c/go/+/194917Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/asm/internal/asm/testdata/arm64.s

@@ -1221,7 +1221,7 @@ again:
 	MRS	DBGCLAIMSET_EL1, R7                // c77830d5
 	MSR	R13, DBGCLAIMSET_EL1               // cd7810d5
 	MRS	DBGDTRRX_EL0, R0                   // 000533d5
-	MSR	R29, DBGDTRRX_EL0                  // 1d0513d5
+	MSR	R29, DBGDTRTX_EL0                  // 1d0513d5
 	MRS	DBGDTR_EL0, R27                    // 1b0433d5
 	MSR	R30, DBGDTR_EL0                    // 1e0413d5
 	MRS	DBGPRCR_EL1, R4                    // 841430d5

src/cmd/internal/obj/arm64/a.out.go

@@ -195,11 +195,11 @@ const (
 
 // Special registers, after subtracting obj.RBaseARM64, bit 12 indicates
 // a special register and the low bits select the register.
-// AUTO_SYSREG_END is the last item in the automatically generated system register
+// SYSREG_END is the last item in the automatically generated system register
 // declaration, and it is defined in the sysRegEnc.go file.
 const (
 	REG_SPECIAL = obj.RBaseARM64 + 1<<12
-	REG_DAIFSet = AUTO_SYSREG_END + iota
+	REG_DAIFSet = SYSREG_END + iota
 	REG_DAIFClr
 	REG_PLDL1KEEP
 	REG_PLDL1STRM

src/cmd/internal/obj/arm64/asm7.go

@@ -3543,15 +3543,17 @@ func (c *ctxt7) asmout(p *obj.Prog, o *Optab, out []uint32) {
 	case 35: /* mov SPR,R -> mrs */
 		o1 = c.oprrr(p, AMRS)
 
-		v := uint32(0)
-		// SysRegEnc function returns the system register encoding.
-		_, v = SysRegEnc(p.From.Reg)
+		// SysRegEnc function returns the system register encoding and accessFlags.
+		_, v, accessFlags := SysRegEnc(p.From.Reg)
 		if v == 0 {
 			c.ctxt.Diag("illegal system register:\n%v", p)
 		}
 		if (o1 & (v &^ (3 << 19))) != 0 {
 			c.ctxt.Diag("MRS register value overlap\n%v", p)
 		}
+		if accessFlags&SR_READ == 0 {
+			c.ctxt.Diag("system register is not readable: %v", p)
+		}
 
 		o1 |= v
 		o1 |= uint32(p.To.Reg & 31)
@@ -3559,15 +3561,17 @@ func (c *ctxt7) asmout(p *obj.Prog, o *Optab, out []uint32) {
 	case 36: /* mov R,SPR */
 		o1 = c.oprrr(p, AMSR)
 
-		v := uint32(0)
-		// SysRegEnc function returns the system register encoding.
-		_, v = SysRegEnc(p.To.Reg)
+		// SysRegEnc function returns the system register encoding and accessFlags.
+		_, v, accessFlags := SysRegEnc(p.To.Reg)
 		if v == 0 {
 			c.ctxt.Diag("illegal system register:\n%v", p)
 		}
 		if (o1 & (v &^ (3 << 19))) != 0 {
 			c.ctxt.Diag("MSR register value overlap\n%v", p)
 		}
+		if accessFlags&SR_WRITE == 0 {
+			c.ctxt.Diag("system register is not writable: %v", p)
+		}
 
 		o1 |= v
 		o1 |= uint32(p.From.Reg & 31)

src/cmd/internal/obj/arm64/list7.go

@@ -209,7 +209,7 @@ func rconv(r int) string {
 		return fmt.Sprintf("V%d.%s", r&31, arrange((r>>5)&15))
 	}
 	// Return system register name.
-	name, _ := SysRegEnc(int16(r))
+	name, _, _ := SysRegEnc(int16(r))
 	if name != "" {
 		return name
 	}