machine: move errors.New calls to globals

Calling errors.New in an error path causes a heap allocation at an
already unfortunate moment. It is more efficient to create these error
values in globals and return these constant globals. If these errors are
not used (because the related code was optimized out), the globals will
also be optimized out.

src/machine/machine_fe310.go

@@ -4,7 +4,6 @@ package machine
 
 import (
 	"device/sifive"
-	"errors"
 	"runtime/interrupt"
 )
 
@@ -195,8 +194,6 @@ type I2CConfig struct {
 	SDA       Pin
 }
 
-var i2cAckExpectedError error = errors.New("I2C write error: expected ACK not NACK")
-
 // Configure is intended to setup the I2C interface.
 func (i2c I2C) Configure(config I2CConfig) error {
 	var i2cClockFrequency uint32 = 32000000
@@ -238,7 +235,7 @@ func (i2c I2C) Tx(addr uint16, w, r []byte) error {
 
 		// ACK received (0: ACK, 1: NACK)
 		if i2c.Bus.CR_SR.HasBits(sifive.I2C_SR_RX_ACK) {
-			return i2cAckExpectedError
+			return errI2CAckExpected
 		}
 
 		// write data
@@ -255,7 +252,7 @@ func (i2c I2C) Tx(addr uint16, w, r []byte) error {
 
 		// ACK received (0: ACK, 1: NACK)
 		if i2c.Bus.CR_SR.HasBits(sifive.I2C_SR_RX_ACK) {
-			return i2cAckExpectedError
+			return errI2CAckExpected
 		}
 
 		// read first byte
@@ -290,7 +287,7 @@ func (i2c I2C) writeByte(data byte) error {
 
 	// ACK received (0: ACK, 1: NACK)
 	if i2c.Bus.CR_SR.HasBits(sifive.I2C_SR_RX_ACK) {
-		return i2cAckExpectedError
+		return errI2CAckExpected
 	}
 
 	return nil