cmd/asm: add support for LDREXB/STREXB

These are ARM Load/Store atomics that operate on bytes and are available starting from armv6k.

See https://developer.arm.com/documentation/dui0379/e/arm-and-thumb-instructions/strex

For six-nine-seven-three-five

Change-Id: I2614ea46334df5c14d822238232cd19783145ddd

Author: mauri870

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

@@ -101,7 +101,7 @@ func IsARMCMP(op obj.As) bool {
 // one of the STREX-like instructions that require special handling.
 func IsARMSTREX(op obj.As) bool {
 	switch op {
-	case arm.ASTREX, arm.ASTREXD, arm.ASWPW, arm.ASWPBU:
+	case arm.ASTREX, arm.ASTREXD, arm.ASTREXB, arm.ASWPW, arm.ASWPBU:
 		return true
 	}
 	return false

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

@@ -260,5 +260,9 @@ TEXT errors(SB),$0
 	STREXD	R0, (R2), R1       // ERROR "cannot use same register as both source and destination"
 	STREXD	R0, (R2), R2       // ERROR "cannot use same register as both source and destination"
 	STREXD	R1, (R4), R7       // ERROR "must be even"
+	STREXB	R0, (R2), R0       // ERROR "cannot use same register as both source and destination"
+	STREXB	R0, (R2), R1       // ERROR "cannot use same register as both source and destination"
+	STREXB	R0, (R2), R2       // ERROR "cannot use same register as both source and destination"
+	STREXB	R1, (R4), R7       // ERROR "must be even"
 
 	END

src/cmd/asm/internal/asm/testdata/armv6.s

@@ -52,8 +52,10 @@ TEXT	foo(SB), DUPOK|NOSPLIT, $0
 	MOVDF	F4, F5        // c45bb7ee
 
 	LDREX	(R8), R9      // 9f9f98e1
+	LDREXB	(R11), R12    // 9fcfdbe1
 	LDREXD	(R11), R12    // 9fcfbbe1
 	STREX	R3, (R4), R5  // STREX  (R4), R3, R5 // 935f84e1
+	STREXB	R8, (R9), g   // STREXB (R9), R8, g  // 98afc9e1
 	STREXD	R8, (R9), g   // STREXD (R9), R8, g  // 98afa9e1
 
 	CMPF    F8, F9        // c89ab4ee10faf1ee

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

@@ -333,7 +333,9 @@ const (
 	ALDREX
 	ASTREX
 	ALDREXD
+	ALDREXB
 	ASTREXD
+	ASTREXB
 
 	ADMB
 

src/cmd/internal/obj/arm/anames.go

@@ -318,7 +318,9 @@ var optab = []Optab{
 	{AMOVW, C_REG, C_NONE, C_FREG, 88, 4, 0, 0, 0, 0},
 	{AMOVW, C_FREG, C_NONE, C_REG, 89, 4, 0, 0, 0, 0},
 	{ALDREXD, C_SOREG, C_NONE, C_REG, 91, 4, 0, 0, 0, 0},
+	{ALDREXB, C_SOREG, C_NONE, C_REG, 91, 4, 0, 0, 0, 0},
 	{ASTREXD, C_SOREG, C_REG, C_REG, 92, 4, 0, 0, 0, 0},
+	{ASTREXB, C_SOREG, C_REG, C_REG, 92, 4, 0, 0, 0, 0},
 	{APLD, C_SOREG, C_NONE, C_NONE, 95, 4, 0, 0, 0, 0},
 	{obj.AUNDEF, C_NONE, C_NONE, C_NONE, 96, 4, 0, 0, 0, 0},
 	{ACLZ, C_REG, C_NONE, C_REG, 97, 4, 0, 0, 0, 0},
@@ -1432,7 +1434,9 @@ func buildop(ctxt *obj.Link) {
 		case ALDREX,
 			ASTREX,
 			ALDREXD,
+			ALDREXB,
 			ASTREXD,
+			ASTREXB,
 			ADMB,
 			APLD,
 			AAND,
@@ -2397,18 +2401,25 @@ func (c *ctxt5) asmout(p *obj.Prog, o *Optab, out []uint32) {
 		o1 |= (uint32(p.From.Reg) & 15) << 16
 		o1 |= (uint32(p.To.Reg) & 15) << 12
 
-	case 91: /* ldrexd oreg,reg */
+	case 91: /* ldrexd/ldrexb oreg,reg */
 		c.aclass(&p.From)
 
 		if c.instoffset != 0 {
 			c.ctxt.Diag("offset must be zero in LDREX")
 		}
-		o1 = 0x1b<<20 | 0xf9f
+
+		switch p.As {
+		case ALDREXD:
+			o1 = 0x1b << 20
+		case ALDREXB:
+			o1 = 0x1d << 20
+		}
+		o1 |= 0xf9f
 		o1 |= (uint32(p.From.Reg) & 15) << 16
 		o1 |= (uint32(p.To.Reg) & 15) << 12
 		o1 |= ((uint32(p.Scond) & C_SCOND) ^ C_SCOND_XOR) << 28
 
-	case 92: /* strexd reg,oreg,reg */
+	case 92: /* strexd/strexb reg,oreg,reg */
 		c.aclass(&p.From)
 
 		if c.instoffset != 0 {
@@ -2420,7 +2431,14 @@ func (c *ctxt5) asmout(p *obj.Prog, o *Optab, out []uint32) {
 		if p.To.Reg == p.From.Reg || p.To.Reg == p.Reg || p.To.Reg == p.Reg+1 {
 			c.ctxt.Diag("cannot use same register as both source and destination: %v", p)
 		}
-		o1 = 0x1a<<20 | 0xf90
+
+		switch p.As {
+		case ASTREXD:
+			o1 = 0x1a << 20
+		case ASTREXB:
+			o1 = 0x1c << 20
+		}
+		o1 |= 0xf90
 		o1 |= (uint32(p.From.Reg) & 15) << 16
 		o1 |= (uint32(p.Reg) & 15) << 0
 		o1 |= (uint32(p.To.Reg) & 15) << 12