cmd/asm: implement MCR instruction on ARM

It's an oddball that needs special treatment because it is not really
an opcode, but a variant of MRC.

The String method of Prog still needs updating to print it nicely.

Change-Id: I6005b7f2234ccd3d4ac1f658948e3be97cf1f1c2
Reviewed-on: https://go-review.googlesource.com/7220
Reviewed-by: Dave Cheney <dave@cheney.net>
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/asm/internal/arch/arch.go

@@ -186,6 +186,10 @@ func archArm() *Arch {
 	// Annoying aliases.
 	instructions["B"] = obj.AJMP
 	instructions["BL"] = obj.ACALL
+	// MCR differs from MRC by the way fields of the word are encoded.
+	// (Details in arm.go). Here we add the instruction so parse will find
+	// it, but give it an opcode number known only to us.
+	instructions["MCR"] = aMCR
 
 	return &Arch{
 		LinkArch:       &arm.Linkarm,

src/cmd/asm/internal/arch/arm.go

@@ -105,19 +105,26 @@ func IsARMSTREX(op int) bool {
 	return false
 }
 
+// MCR is not defined by the obj/arm; instead we define it privately here.
+// It is encoded as an MRC with a bit inside the instruction word,
+// passed to arch.ARMMRCOffset.
+const aMCR = arm.ALAST + 1
+
 // IsARMMRC reports whether the op (as defined by an arm.A* constant) is
 // MRC or MCR
 func IsARMMRC(op int) bool {
 	switch op {
-	case arm.AMRC /*, arm.AMCR*/ :
+	case arm.AMRC, aMCR: // Note: aMCR is defined in this package.
 		return true
 	}
 	return false
 }
 
 // ARMMRCOffset implements the peculiar encoding of the MRC and MCR instructions.
-func ARMMRCOffset(op int, cond string, x0, x1, x2, x3, x4, x5 int64) (offset int64, ok bool) {
-	// TODO only MRC is defined.
+// The difference between MRC and MCR is represented by a bit high in the word, not
+// in the usual way by the opcode itself. Asm must use AMRC for both instructions, so
+// we return the opcode for MRC so that asm doesn't need to import obj/arm.
+func ARMMRCOffset(op int, cond string, x0, x1, x2, x3, x4, x5 int64) (offset int64, op0 int16, ok bool) {
 	op1 := int64(0)
 	if op == arm.AMRC {
 		op1 = 1
@@ -136,7 +143,7 @@ func ARMMRCOffset(op int, cond string, x0, x1, x2, x3, x4, x5 int64) (offset int
 		((x4 & 15) << 0) | // Crm
 		((x5 & 7) << 5) | // coprocessor information
 		(1 << 4) /* must be set */
-	return offset, true
+	return offset, arm.AMRC, true
 }
 
 // IsARMMULA reports whether the op (as defined by an arm.A* constant) is

src/cmd/asm/internal/asm/asm.go

@@ -567,12 +567,13 @@ func (p *Parser) asmInstruction(op int, cond string, a []obj.Addr) {
 			x4 := int64(p.getRegister(prog, op, &a[4]))
 			x5 := p.getConstant(prog, op, &a[5])
 			// Cond is handled specially for this instruction.
-			offset, ok := arch.ARMMRCOffset(op, cond, x0, x1, x2, x3, x4, x5)
+			offset, MRC, ok := arch.ARMMRCOffset(op, cond, x0, x1, x2, x3, x4, x5)
 			if !ok {
 				p.errorf("unrecognized condition code .%q", cond)
 			}
 			prog.To.Offset = offset
 			cond = ""
+			prog.As = MRC // Both instructions are coded as MRC.
 			break
 		}
 		fallthrough