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Re: [RFC] improve generic strlen.

On Tue, Dec 23, 2014 at 07:57:14PM +0100, OndÅej BÃlka wrote:
> Hi
> As I promised earlier here is working version of improved strlen.
> For long strings on x64 its twice as fast as current C version. Or 50%
> slower when there is 1/128 probability of character 128.
> For simplicity it assumes unaligned loads. For aligned ones I would need
> to add define to mask first bytes.
> Could you try this on arm or other arch to see what improvement is
> possible?
> Also while header is most important it needs platform specific
> optimizations so if this is improvement next step is take assembly and
> fix gcc mistakes.

FYI, the fastest strlen I've found on 32-bit x86 (and perhaps other
32-bit archs; I don't remember) is this:

size_t unrolled_strlen(const char *s) {
	const char *p = s;
	for (;;) {
		if (!p[0]) return p-s;
		if (!p[1]) return p-s+1;	
		if (!p[2]) return p-s+2;
		if (!p[3]) return p-s+3;
		if (!p[4]) return p-s+4;
		if (!p[5]) return p-s+5;
		if (!p[6]) return p-s+6;
		if (!p[7]) return p-s+7;
		if (!p[8]) return p-s+8;
		if (!p[9]) return p-s+9;
		if (!p[10]) return p-s+10;
		if (!p[11]) return p-s+11;
		if (!p[12]) return p-s+12;
		if (!p[13]) return p-s+13;
		if (!p[14]) return p-s+14;
		if (!p[15]) return p-s+15;
		p += 16;

The nice thing is that it really is pure C -- no UB hacks like reading
past the end of the string or aliasing violations. Sadly it can't keep
up with larger-than-32-bit vectorization though, but it's still pretty
close. Also sadly GCC cannot do the unrolling properly automatically;
it generates some very slow code with loop overhead repeated between
every load-and-branch.

On my atoms it runs at 1.15 cycles per byte. I don't have the figures
for other cpus handy.


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