Re: [PATCH v2 2/4] exfat: optimise and refactor filename up-case conversion

[David Timber <dxdt@dev.snart.me>]

On 5/7/26 21:03, Yuezhang.Mo@sony.com wrote:
>> +static inline bool exfat_illegal_chr(const unsigned short wchar)
>>  {
>> -	while (*str) {
>> -		if (*(str++) == wchar)
>> -			return str;
>> +	switch (wchar) {
>> +	/*
>> +	 * Control characters
>> +	 *
>> +	 * This saves some cmp instructions if the compiler does its job correctly
>> +	 */
>> +	case 0x0000: case 0x0001: case 0x0002: case 0x0003:
>> +	case 0x0004: case 0x0005: case 0x0006: case 0x0007:
>> +	case 0x0008: case 0x0009: case 0x000A: case 0x000B:
>> +	case 0x000C: case 0x000D: case 0x000E: case 0x000F:
>> +	case 0x0010: case 0x0011: case 0x0012: case 0x0013:
>> +	case 0x0014: case 0x0015: case 0x0016: case 0x0017:
>> +	case 0x0018: case 0x0019: case 0x001A: case 0x001B:
>> +	case 0x001C: case 0x001D: case 0x001E: case 0x001F:
>> +
>> +	case 0x0022: /* QUOTATION MARK:    (") */
>> +	case 0x002A: /* ASTERISK:          (*) */
>> +	case 0x002F: /* FORWARD SLASH:     (/) */
>> +	case 0x003A: /* COLON:             (:) */
>> +	case 0x003C: /* LESS-THAN SIGN:    (<) */
>> +	case 0x003E: /* GREATER-THAN SIGN: (>) */
>> +	case 0x003F: /* QUESTION MARK:     (?) */
>> +	case 0x005C: /* BACKSLASH:         (\) */
>> +	case 0x007C: /* PIPE:              (|) */
>> +		return true;
> For the continuous range 0x0000 to 0x001F, I think it's better to
> handle it like below, which avoids the large jump table, and may be
> just as fast(or even faster).
>
> 	default:
> 		return wchar < 0x0020;
I guess you're right.

That costs one extra cmp instructions but still fits under a cache
granularity on modern hardware(64 bytes) so it wouldn't really show up
in profiling. Better to avoid risking compiler mindlessly producing a
huge jump table.

Either way, still a win since the processor no longer has to iterate
over the array.