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.
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