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Fix unmarshaller prefetch align #64

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Merged
acrisci merged 2 commits into from
Oct 6, 2020
Merged

Fix unmarshaller prefetch align #64

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b9ccaec
unmarshaller: optimize alignment padding formula
rjarry Oct 5, 2020
cdfbb68
unmarshaller: prefetch header alignment bytes
rjarry Oct 5, 2020
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30 changes: 25 additions & 5 deletions dbus_next/_private/unmarshaller.py
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Original file line number Diff line number Diff line change
Expand Up @@ -71,11 +71,30 @@ def read(self, n, prefetch=False):
self.offset += n
return prev

@staticmethod
def _padding(offset, align):
"""
Get padding bytes to get to the next align bytes mark.

For any align value, the correct padding formula is:

(align - (offset % align)) % align

However, if align is a power of 2 (always the case here), the slow MOD
operator can be replaced by a bitwise AND:

(align - (offset & (align - 1))) & (align - 1)

Which can be simplified to:

(-offset) & (align - 1)
"""
return (-offset) & (align - 1)

def align(self, n):
padding = n - self.offset % n
if padding == 0 or padding == n:
return
self.read(padding)
padding = self._padding(self.offset, n)
if padding > 0:
self.read(padding)

def read_byte(self, _=None):
return self.buf[self.read(1)]
Expand Down Expand Up @@ -209,7 +228,8 @@ def _unmarshall(self):
serial = self.read_uint32()

header_len = self.read_uint32()
self.read(header_len + body_len, prefetch=True)
msg_len = header_len + self._padding(header_len, 8) + body_len
self.read(msg_len, prefetch=True)
# backtrack offset since header array length needs to be read again
self.offset -= 4

Expand Down