Elsewhere in the setobject.c code we do a bitwise-and with the mask
instead of using a conditional to reset to zero on wrap-around.
Using that same technique here use gives cleaner, faster, and more
consistent code.
* Move the test for an exact key match to after a hash match
* Use "used" as a loop counter instead of "fill"
* Minor improvements to variable names and code consistency
The setobject freelist was consuming memory but not providing much value.
Even when a freelisted setobject was available, most of the setobject
fields still needed to be initialized and the small table still required
a memset(). This meant that the custom freelisting scheme for sets was
providing almost no incremental benefit over the default Python freelist
scheme used by _PyObject_Malloc() in Objects/obmalloc.c.
Modern processors tend to make consecutive memory accesses cheaper than
random probes into memory.
Small sets can fit into L1 cache, so they get less benefit. But they do
come out ahead because the consecutive probes don't probe the same key
more than once and because the randomization step occurs less frequently
(or not at all).
For the open addressing step, putting the perturb shift before the index
calculation gets the upper bits into play sooner.
The Gdb prettyprint plugin depended on the dummy object being displayable.
Other solutions besides a unicode object are possible. For now, get it
back up and running.
The identity checks in lookkey() need to be there to prevent the dummy
object from leaking through Py_RichCompareBool() into user code in the
rare circumstance where the dummy's hash value exactly matches the hash
value of the actual key being looked up.
Letting the compiler decide how to optimize the multiply by five
gives it the freedom to make better choices for the best technique
for a given target machine.
For example, GCC on x86_64 produces a little bit better code:
Old-way (3 steps with a data dependency between each step):
shrq $5, %r13
leaq 1(%rbx,%r13), %rax
leaq (%rax,%rbx,4), %rbx
New-way (3 steps with no dependency between the first two steps
which can be run in parallel):
leaq (%rbx,%rbx,4), %rax # i*5
shrq $5, %r13 # perturb >>= PERTURB_SHIFT
leaq 1(%r13,%rax), %rbx # 1 + perturb + i*5
computation as the overflow behavior of signed integers is undefined.
NOTE: This change is smaller compared to 3.2 as much of this cleanup had
already been done. I added the comment that my change in 3.2 added so that the
code would match up. Otherwise this just adds or synchronizes appropriate UL
designations on some constants to be pedantic.
In practice we require compiling everything with -fwrapv which forces overflow
to be defined as twos compliment but this keeps the code cleaner for checkers
or in the case where someone has compiled it without -fwrapv or their
compiler's equivalent.
Found by Clang trunk's Undefined Behavior Sanitizer (UBSan).
Cleanup only - no functionality or hash values change.
computation as the overflow behavior of signed integers is undefined.
In practice we require compiling everything with -fwrapv which forces overflow
to be defined as twos compliment but this keeps the code cleaner for checkers
or in the case where someone has compiled it without -fwrapv or their
compiler's equivalent.
Found by Clang trunk's Undefined Behavior Sanitizer (UBSan).
Cleanup only - no functionality or hash values change.
The fix was already committed to 3.2, but I merged two small changes
recommended by Raymond while I was working on the 2.7 patch to ease
future merges.
svn+ssh://pythondev@svn.python.org/python/branches/py3k
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r78541 | ezio.melotti | 2010-03-01 06:08:34 +0200 (Mon, 01 Mar 2010) | 17 lines
Merged revisions 78515-78516,78522 via svnmerge from
svn+ssh://pythondev@svn.python.org/python/trunk
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r78515 | georg.brandl | 2010-02-28 20:19:17 +0200 (Sun, 28 Feb 2010) | 1 line
#8030: make builtin type docstrings more consistent: use "iterable" instead of "seq(uence)", use "new" to show that set() always returns a new object.
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r78516 | georg.brandl | 2010-02-28 20:26:37 +0200 (Sun, 28 Feb 2010) | 1 line
The set types can also be called without arguments.
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r78522 | ezio.melotti | 2010-03-01 01:59:00 +0200 (Mon, 01 Mar 2010) | 1 line
#8030: more docstring fix for builtin types.
........
................
svn+ssh://pythondev@svn.python.org/python/trunk
........
r78515 | georg.brandl | 2010-02-28 20:19:17 +0200 (Sun, 28 Feb 2010) | 1 line
#8030: make builtin type docstrings more consistent: use "iterable" instead of "seq(uence)", use "new" to show that set() always returns a new object.
........
r78516 | georg.brandl | 2010-02-28 20:26:37 +0200 (Sun, 28 Feb 2010) | 1 line
The set types can also be called without arguments.
........
r78522 | ezio.melotti | 2010-03-01 01:59:00 +0200 (Mon, 01 Mar 2010) | 1 line
#8030: more docstring fix for builtin types.
........
Raymond Hettinger
Victor Stinner
Eric V. Smith
Antoine Pitrou
Gregory P. Smith
Ezio Melotti
David Malcolm
Kristján Valur Jónsson
Benjamin Peterson
Petri Lehtinen
Martin v. Löwis
Mark Dickinson
Brian Curtin
Victor Stinner
Éric Araujo
Georg Brandl
Daniel Stutzbach