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Instead of testing bytes one by one, we can use the following trick from https://graphics.stanford.edu/~seander/bithacks.html: haszero(v) = (v - 0x01010101) & ~v & 0x80808080 The subexpression v - 0x01010101, evaluates to a high bit set in any byte whenever the corresponding byte in v is zero or greater than 0x80. The sub-expression ~v & 0x80808080 evaluates to high bits set in bytes where the byte of v doesn't have its high bit set (so the byte was less than 0x80). Finally, by ANDing these two sub-expressions the result is the high bits set where the bytes in v were zero, since the high bits set due to a value greater than 0x80 in the first sub-expression are masked off by the second. Reviewed-by: Richard Henderson <rth@twiddle.net> Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Read the documentation in qemu-doc.html or on http://wiki.qemu-project.org - QEMU team