Re: [Libguestfs] [PATCH] daemon: Move the useful 'is_zero' function into common code.

On Wed, Apr 19, 2017 at 09:35:16AM -0500, Eric Blake wrote: > On 04/19/2017 09:17 AM, Richard W.M. Jones wrote: > > This is largely a simple refactoring, but it combines another > > definition of this function from virt-builder which had a slightly > > different prototype. > > --- > > > +/* Return true iff the buffer is all zero bytes. > > + * > > + * Note that gcc is smart enough to optimize this properly: > > + * http://stackoverflow.com/questions/1493936/faster-means-of-checking-for-a... > > + */ > > +static inline int > > +is_zero (const char *buffer, size_t size) > > +{ > > + size_t i; > > + > > + for (i = 0; i < size; ++i) { > > + if (buffer[i] != 0) > > + return 0; > > + } > > + > > + return 1; > > +} > > This is still byte-at-a-time.

> Often, you can get even faster results by > exploiting libc's optimizations in memcmp (particularly when it is > comparing pointers with nice alignments), by manually checking that the > first 16 bytes are zero, then letting memcmp do the rest: > > size_t i; > size_t limit = MIN(size, 16); > for (i = 0; i < limit; ++i) > if (buffer[i]) > return 0; > if (size != limit) > return !memcmp(buffer, buffer + 16, size - 16); > return 1;

If performance is critical, QEMU has a permissively licensed impl that uses sse/avx instructions to super-optimize it on x86 (util/bufferiszero.c). It is a large amount of code though, so only worth it if this is a notable bottleneck for libguestfs.