<s>
A	O
software	B-Error_Name
regression	I-Error_Name
is	O
a	O
type	O
of	O
software	B-Error_Name
bug	I-Error_Name
where	O
a	O
feature	O
that	O
has	O
worked	O
before	O
stops	O
working	O
.	O
</s>
<s>
This	O
may	O
happen	O
after	O
changes	O
are	O
applied	O
to	O
the	O
software	O
's	O
source	O
code	O
,	O
including	O
the	O
addition	O
of	O
new	O
features	B-Architecture
and	O
bug	B-Error_Name
fixes	O
.	O
</s>
<s>
Various	O
types	O
of	O
software	B-Error_Name
regressions	I-Error_Name
have	O
been	O
identified	O
in	O
practice	O
,	O
including	O
the	O
following	O
:	O
</s>
<s>
Local	O
–	O
a	O
change	O
introduces	O
a	O
new	O
bug	B-Error_Name
in	O
the	O
changed	O
module	O
or	O
component	O
.	O
</s>
<s>
Unmasked	O
–	O
a	O
change	O
unmasks	O
an	O
already	O
existing	O
bug	B-Error_Name
that	O
had	O
no	O
effect	O
before	O
the	O
change	O
.	O
</s>
<s>
Regressions	O
are	O
often	O
caused	O
by	O
encompassed	O
bug	B-Error_Name
fixes	O
included	O
in	O
software	O
patches	O
.	O
</s>
<s>
Automated	B-Application
testing	I-Application
and	O
well-written	O
test	O
cases	O
can	O
reduce	O
the	O
likelihood	O
of	O
a	O
regression	O
.	O
</s>
<s>
Approaches	O
such	O
as	O
statistical	O
significance	O
testing	O
and	O
change	B-General_Concept
point	I-General_Concept
detection	I-General_Concept
are	O
sometimes	O
used	O
to	O
aid	O
in	O
this	O
decision	O
.	O
</s>
<s>
Since	O
debugging	O
and	O
localizing	O
the	O
root	O
cause	O
of	O
a	O
software	B-Error_Name
regression	I-Error_Name
can	O
be	O
expensive	O
,	O
there	O
also	O
exist	O
some	O
methods	O
that	O
try	O
to	O
prevent	O
regressions	O
from	O
being	O
committed	O
into	O
the	O
code	B-General_Concept
repository	I-General_Concept
in	O
the	O
first	O
place	O
.	O
</s>
<s>
For	O
example	O
,	O
Git	B-Application
Hooks	O
enable	O
developers	O
to	O
run	O
test	O
scripts	O
before	O
code	O
changes	O
are	O
committed	O
or	O
pushed	O
to	O
the	O
code	B-General_Concept
repository	I-General_Concept
.	O
</s>
<s>
Software	B-Device
linters	I-Device
are	O
also	O
often	O
added	O
to	O
commit	O
hooks	O
to	O
ensure	O
consistent	O
coding	O
style	O
,	O
thereby	O
minimizing	O
stylistic	O
issues	O
that	O
can	O
make	O
the	O
software	O
prone	O
to	O
regressions	O
.	O
</s>
<s>
Many	O
of	O
the	O
techniques	O
used	O
to	O
find	O
the	O
root	O
cause	O
of	O
non-regression	O
software	B-Error_Name
bugs	I-Error_Name
can	O
also	O
be	O
used	O
to	O
debug	O
software	B-Error_Name
regressions	I-Error_Name
,	O
including	O
breakpoint	O
debugging	O
,	O
print	O
debugging	O
,	O
and	O
program	O
slicing	O
.	O
</s>
<s>
The	O
techniques	O
described	O
below	O
are	O
often	O
used	O
specifically	O
to	O
debug	O
software	B-Error_Name
regressions	I-Error_Name
.	O
</s>
<s>
A	O
common	O
technique	O
used	O
to	O
localize	O
functional	O
regressions	O
is	O
bisection	B-Protocol
,	O
which	O
takes	O
both	O
a	O
buggy	O
commit	O
and	O
a	O
previously	O
working	O
commit	O
as	O
input	O
,	O
and	O
tries	O
to	O
find	O
the	O
root	O
cause	O
by	O
doing	O
a	O
binary	O
search	O
on	O
the	O
commits	O
in	O
between	O
.	O
</s>
<s>
Version	B-Architecture
control	I-Architecture
systems	I-Architecture
such	O
as	O
Git	B-Application
and	O
Mercurial	B-Protocol
provide	O
built-in	O
ways	O
to	O
perform	O
bisection	B-Protocol
on	O
a	O
given	O
pair	O
of	O
commits	O
.	O
</s>
<s>
In	O
the	O
context	O
of	O
software	O
performance	O
regressions	O
,	O
developers	O
often	O
compare	O
the	O
call	B-General_Concept
trees	I-General_Concept
(	O
also	O
known	O
as	O
"	O
timelines	O
"	O
)	O
generated	O
by	O
profilers	O
for	O
both	O
the	O
buggy	O
version	O
and	O
the	O
previously	O
working	O
version	O
,	O
and	O
mechanisms	O
exist	O
to	O
simplify	O
this	O
comparison	O
.	O
</s>
<s>
Web	B-Application
development	I-Application
tools	I-Application
typically	O
provide	O
developers	O
the	O
ability	O
to	O
record	O
these	O
performance	O
profiles	O
.	O
</s>
<s>
Logging	O
also	O
helps	O
with	O
performance	O
regression	O
localization	O
,	O
and	O
similar	O
to	O
call	B-General_Concept
trees	I-General_Concept
,	O
developers	O
can	O
compare	O
systematically-placed	O
performance	O
logs	O
of	O
multiple	O
versions	O
of	O
the	O
same	O
software	O
.	O
</s>
<s>
Bisection	B-Protocol
can	O
also	O
be	O
repurposed	O
for	O
performance	O
regressions	O
by	O
considering	O
commits	O
that	O
perform	O
below	O
(	O
or	O
above	O
)	O
a	O
certain	O
baseline	O
value	O
as	O
buggy	O
,	O
and	O
taking	O
either	O
the	O
left	O
or	O
the	O
right	O
side	O
of	O
the	O
commits	O
based	O
on	O
the	O
results	O
of	O
this	O
comparison	O
.	O
</s>
