Counting Characters in UTF-8 Strings Is Fast

Written in 2008 by Kragen Javier Sitaker. This paragraph added in 2016.

On IRC, Aristotle Pagaltzis was pondering how much performance variable-width encodings such as UTF-8 actually cost, because it's commonly suggested that fixed-width encodings such as ISO-8859-1 and UCS-4 are much faster.

He suggested:

Huh, it just occurs to me that strlen is not at all expensive on UTF-8-encoded strings. Not exactly as fast, but if you write it in asm, it only takes one extra instruction to count characters in UTF-8 vs those in an 8-bit encoding, per character. So, if you factor in cache misses, it should make no measurable difference. All you lose with a variable-width encoding is direct random access to arbitrary indices in the string, which is basically a non-use case.

It turned out that he was partly wrong, but mostly right. And along the way, we discovered that GCC's standard implementation of strlen was quite pessimal.

I'm using Linux on a 700MHz Pentium III laptop with GCC 4.1.2, using just the -O flag unless otherwise specified.

My First Assembly Version

First I thought about how to write strlen , and came up with this:

.global my_strlen_s my_strlen_s: push %esi cld mov 8(%esp), %esi xor %ecx, %ecx ## repnz lodsb doesn't work because lodsb doesn't update ZF loop: lodsb test %al, %al loopnz loop mov %ecx, %eax not %eax pop %esi ret

For those who aren't well-versed in 80386 assembly language: lodsb reads a byte from memory at %esi , puts it in %al , and increments %esi ;

reads a byte from memory at , puts it in , and increments ; loopnz decrements %ecx each time through the loop, and jumps back to the label loop: if %ecx isn't zero and if the zero flag ZF isn't set (that's the "nz" part);

decrements each time through the loop, and jumps back to the label if isn't zero and if the zero flag isn't set (that's the "nz" part); test %al, %al sets the zero flag ZF if %al is zero (the C string terminator), among other things.

sets the zero flag if is zero (the C string terminator), among other things. after the loop body has run N times, %ecx is -N, so not %eax converts the negative number -N from %ecx into a positive number N-1.

is -N, so converts the negative number -N from into a positive number N-1. you have to push %esi because it's a callee-saves register. (To my surprise.) But there's a scasb instruction which I should have used instead of lodsb; test .

The inner loop there is three instructions.



GCC's Assembly Version

Then I looked at how GCC does strlen . It turns out it inlines it, even without any extra optimization flags. Here's a sample call, albeit with optimization:

80484d5: bf 00 99 04 08 mov $0x8049900,%edi 80484da: fc cld 80484db: b9 ff ff ff ff mov $0xffffffff,%ecx 80484e0: b0 00 mov $0x0,%al 80484e2: f2 ae repnz scas %es:(%edi),%al 80484e4: f7 d1 not %ecx 80484e6: 49 dec %ecx 80484e7: 89 4c 24 04 mov %ecx,0x4(%esp)

There the inner loop is just the repnz scas . I'd forgotten about SCAS .

A C Version

Here's a reasonable strlen in C:

int my_strlen(char *s) { int i = 0; while (*s++) i++; return i; }

This compiles to the following:

080483c4 <my_strlen>: 80483c4: 55 push %ebp 80483c5: 89 e5 mov %esp,%ebp 80483c7: 8b 55 08 mov 0x8(%ebp),%edx 80483ca: b8 00 00 00 00 mov $0x0,%eax 80483cf: 80 3a 00 cmpb $0x0,(%edx) 80483d2: 74 0c je 80483e0 <my_strlen+0x1c> 80483d4: b8 00 00 00 00 mov $0x0,%eax 80483d9: 40 inc %eax 80483da: 80 3c 10 00 cmpb $0x0,(%eax,%edx,1) 80483de: 75 f9 jne 80483d9 <my_strlen+0x15> 80483e0: 5d pop %ebp 80483e1: c3 ret

So here the inner loop is again three instructions: inc %eax ; cmpb 0, (%eax,%edx,1) ; jne . It's been optimized down to while (s[i]) i++; . The loop termination test is duplicated above the top of the loop for the empty-string case.

My UTF-8 Assembly Version

So then I thought about how to do what Aristotle was suggesting. In UTF-8, bytes that start new characters begin either with binary 0 or binary 11; the second and subsequent bytes of multibyte characters have binary 10 as their high bits. So to count the characters, you just have to count the bytes that don't begin with binary 10.

I tried this:

.global my_strlen_utf8_s my_strlen_utf8_s: push %esi cld mov 8(%esp), %esi xor %ecx, %ecx loop2: lodsb test $0x80, %al jz ascii # format 0xxx xxxx test $0x40, %al jz loop2 # format 10xx xxxx: doesn't start new char ascii: test %al, %al loopnz loop2 mov %ecx, %eax not %eax pop %esi ret

So here we jump back to loop2 , rather than decrementing %ecx with loopnz , in the case where the byte starts with 10. And we can skip the test %al, %al test, since 0000 0000 doesn't start with 10.

The inner loop of this version has 5 instructions, including two taken conditional branches, in the usual ASCII case, and 7 instructions for non-ASCII bytes, rather than 3 instructions per byte. That's only two extra instructions in the "usual" case, but if every instruction were one cycle, that would still be a 67% increase in run-time.

Counting instructions and adding up their cycle count isn't a very accurate way to measure performance in these superscalar days, though.

My UTF-8 C Version

A C version looks like this:

int my_strlen_utf8_c(char *s) { int i = 0, j = 0; while (s[i]) { if ((s[i] & 0xc0) != 0x80) j++; i++; } return j; }

GCC compiles it to this:

080483e2 <my_strlen_utf8_c>: 80483e2: 55 push %ebp 80483e3: 89 e5 mov %esp,%ebp 80483e5: 8b 55 08 mov 0x8(%ebp),%edx 80483e8: 0f b6 02 movzbl (%edx),%eax 80483eb: b9 00 00 00 00 mov $0x0,%ecx 80483f0: 84 c0 test %al,%al 80483f2: 74 23 je 8048417 <my_strlen_utf8_c+0x35> 80483f4: b9 00 00 00 00 mov $0x0,%ecx 80483f9: 0f be c0 movsbl %al,%eax 80483fc: 25 c0 00 00 00 and $0xc0,%eax 8048401: 3d 80 00 00 00 cmp $0x80,%eax 8048406: 0f 95 c0 setne %al 8048409: 0f b6 c0 movzbl %al,%eax 804840c: 01 c1 add %eax,%ecx 804840e: 0f b6 42 01 movzbl 0x1(%edx),%eax 8048412: 42 inc %edx 8048413: 84 c0 test %al,%al 8048415: 75 e2 jne 80483f9 <my_strlen_utf8_c+0x17> 8048417: 89 c8 mov %ecx,%eax 8048419: 5d pop %ebp 804841a: c3 ret

An inner loop of 10 instructions --- but containing only a single conditional jump, the jne at the bottom. It uses the and ; cmp ; setne ; movzbl sequence to put either a 0 or a 1 into %eax , depending on whether the byte fetched began with 10, and adds the result into %ecx each time through the loop.

Aristotle's UTF-8 Assembly Version

So after all this, I chatted with Aristotle some more, and it turned out he had a much cleverer trick up his sleeve than I had thought --- or, in fact, than he had thought. He wrote:

But wow, my code is much faster than any of the other variants. Unexpectedly.

Here's his version:

.global ap_strlen_utf8_s ap_strlen_utf8_s: push %esi cld mov 8(%esp), %esi xor %ecx, %ecx loopa: dec %ecx loopb: lodsb shl $1, %al js loopa jc loopb jnz loopa mov %ecx, %eax not %eax pop %esi ret

In this case, the inner loop is 6 instructions, but as few as 3 of them can execute. I hadn't realized that you could get the top two bits of a byte into the carry and sign flags with a single shl instruction like that! Aristotle explains:

Js catches all bytes of the form x1xxxxxx. Jc catches 1xxxxxxx, but because js came first, that can only have been 10xxxxxx; and jnz then catches all 00xxxxxx other than all-0. This runs about 3x as fast as your my_strlen_s --- most of the time, anyway.

Performance Results

So how do these different approaches fare? I wrote a program that creates a 32MB string and timed the different functions on it, in seconds, using wall-clock time. Here are the results from one run, sorted with sort -t: -k1 -k3 -ns . The first few lines are various functions' return values on the given strings.

"": 0 0 0 0 0 0 "hello, world": 12 12 12 12 12 12 "naïve": 6 6 6 5 5 5 "こんにちは": 15 15 15 5 5 5 1: all 'a': 1: my_strlen(string) = 33554431: 0.227555 1: ap_strlen_utf8_s(string) = 33554431: 0.299494 1: strlen(string) = 33554431: 0.314887 1: my_strlen_utf8_c(string) = 33554431: 0.380355 1: my_strlen_s(string) = 33554431: 0.432079 1: my_strlen_utf8_s(string) = 33554431: 0.525443 2: all '\xe3': 2: my_strlen(string) = 33554431: 0.224037 2: ap_strlen_utf8_s(string) = 33554431: 0.299537 2: strlen(string) = 33554431: 0.311552 2: my_strlen_utf8_c(string) = 33554431: 0.378162 2: my_strlen_s(string) = 33554431: 0.436755 2: my_strlen_utf8_s(string) = 33554431: 0.589165 3: all '\x81': 3: my_strlen(string) = 33554431: 0.225011 3: ap_strlen_utf8_s(string) = 0: 0.313525 3: strlen(string) = 33554431: 0.316182 3: my_strlen_utf8_s(string) = 0: 0.322959 3: my_strlen_utf8_c(string) = 0: 0.390958 3: my_strlen_s(string) = 33554431: 0.432342

The 33554431 and 0 numbers are the return values; this ensures that GCC doesn't optimize out the strlen call.

So, on my CPU, the C version of strlen took about 28% less time than the built-in inlined one for this long string; it only uses two registers instead of the three used by the built-in inlined one (the one that uses repnz scasb ); and they both seem to be about 12 bytes. I don't know why GCC inlines the worse one. Most likely it used to be faster than whatever GCC generated at the time and hasn't been revisited.

It's worth noting that while my C version of strlen was always faster than the built-in version, Aristotle's UTF-8 version was always in between.

On Aristotle's Core 2 Duo 1.8GHz (also with GCC 4.1.2 and -O ), the difference was very much greater. Here are his results:

"": 0 0 0 0 0 0 "hello, world": 12 12 12 12 12 12 "naïve": 6 6 6 5 5 5 "こんにちは": 15 15 15 5 5 5 1: all 'a': 1: my_strlen(string) = 33554431: 0.025906 1: ap_strlen_utf8_s(string) = 33554431: 0.039629 1: my_strlen_utf8_c(string) = 33554431: 0.096041 1: strlen(string) = 33554431: 0.114821 1: my_strlen_s(string) = 33554431: 0.116529 1: my_strlen_utf8_s(string) = 33554431: 0.132648 2: all '\xe3': 2: my_strlen(string) = 33554431: 0.025912 2: ap_strlen_utf8_s(string) = 33554431: 0.039583 2: my_strlen_utf8_c(string) = 33554431: 0.095699 2: strlen(string) = 33554431: 0.114452 2: my_strlen_s(string) = 33554431: 0.114622 2: my_strlen_utf8_s(string) = 33554431: 0.136109 3: all '\x81': 3: my_strlen(string) = 33554431: 0.026112 3: my_strlen_utf8_s(string) = 0: 0.039656 3: ap_strlen_utf8_s(string) = 0: 0.039661 3: my_strlen_utf8_c(string) = 0: 0.096416 3: my_strlen_s(string) = 33554431: 0.115327 3: strlen(string) = 33554431: 0.116629

All of this code is online in two files:

Conclusions