golang/go · error

invalid register number: {name}

Error message

invalid register number: {name}

What it means

The Go assembler's ARM64RegisterArrangement returns this error when the numeric register encoding passed in is negative (arm64.go:227-229). After confirming the name prefix is V/Z/P, the function checks the numeric reg value; a negative reg indicates the register name failed to parse to a valid index (the caller represents parse failure as a sentinel negative). The error message echoes the offending name so the developer can locate it.

Source

Thrown at src/cmd/asm/internal/arch/arm64.go:228

	return int64(reg&31)<<16 | int64(op)<<22 | int64(uint16(count)), nil
}

// ARM64RegisterArrangement constructs an ARM64 vector register arrangement.
func ARM64RegisterArrangement(reg int16, name, arng string) (int64, error) {
	var curQ, curSize, prefix uint16
	if name[0] != 'V' && name[0] != 'Z' && name[0] != 'P' {
		return 0, errors.New("expect V0-V31, Z0-Z31, or P0-P15; found: " + name)
	}
	switch name[0] {
	case 'V':
		prefix = 0
	case 'Z':
		prefix = 1
	case 'P':
		prefix = 2
	}
	if reg < 0 {
		return 0, errors.New("invalid register number: " + name)
	}
	switch arng {
	case "B8":
		curSize = 0
		curQ = 0
	case "B16":
		curSize = 0
		curQ = 1
	case "H4":
		curSize = 1
		curQ = 0
	case "H8":
		curSize = 1
		curQ = 1
	case "S2":
		curSize = 2
		curQ = 0
	case "S4":

View on GitHub (pinned to b6b368adc5)

Solutions

1. Use register numbers within range: V0-V31, Z0-Z31, P0-P15.
2. Correct typos in the register token — ensure it is V/Z/P followed by a clean decimal number.
3. If generating assembly programmatically, validate the register index is in [0, 31] (or [0, 15] for P) before emitting.
4. Check the assembler error line in the .s file pointed to by the message and fix the offending operand.

Example fix

; before — out-of-range vector register
LD1 Z32.S, (R0)

; after — use a valid register index
LD1 Z0.S, (R0)

// Validate the register index is in range before emitting assembly.
func validVectorIndex(prefix byte, n int) bool {
    switch prefix {
    case 'V', 'Z': return n >= 0 && n <= 31
    case 'P': return n >= 0 && n <= 15
    }
    return false
}

When it happens

Trigger: Triggered when ARM64RegisterArrangement is called with reg < 0 after the name-prefix check passed (arm64.go:227). This happens when the register-number parsing upstream returned a negative sentinel for a name like 'V99' (out of range) or a malformed token that still began with V/Z/P but had no valid trailing number.

Common situations: Writing V32+ or Z32+ (the valid range is 0-31) in an assembly file. A register token like 'V' with no number, or 'V0x1' with invalid trailing characters. Assembler-fed input where the number parse failed silently and produced the negative sentinel.

Related errors

• invalid register for shift operation
• expect V0-V31, Z0-Z31, or P0-P15; found: {name}
• invalid arrangement in ARM64 register list
• Loong64 extension: invalid LSX/LASX register: {reg}
• globalThis.crypto is not available, polyfill required (crypt

AI-assisted analysis of golang/go@b6b368adc5 (2026-08-12). Data as JSON: /api/errors/06207c74b7cacc37. Report an issue: GitHub.