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This adds a new opcode which for certain list comprehensions (ones with no if statements and only a single comprehension), preallocates the list to the appropriate size.
Patch is against 2.7, because it was a bit easier. On:
def f():
 for i in range(10000):
 [j for j in range(10000)]
f()
Here's the speedup:
alex@alex-gaynor-laptop:/tmp$ # Fresh 2.7 branch
alex@alex-gaynor-laptop:/tmp$ time ~/projects/cpython/python t.py 
real	0m6.418s
user	0m6.408s
sys	0m0.004s
alex@alex-gaynor-laptop:/tmp$ time ~/projects/cpython/python t.py 
real	0m5.670s
user	0m5.648s
sys	0m0.008s
alex@alex-gaynor-laptop:/tmp$ time ~/projects/cpython/python t.py 
real	0m5.688s
user	0m5.672s
sys	0m0.008s
alex@alex-gaynor-laptop:/tmp$ time ~/projects/cpython/python t.py 
real	0m5.688s
user	0m5.676s
sys	0m0.004s
alex@alex-gaynor-laptop:/tmp$ time ~/projects/cpython/python t.py 
real	0m5.690s
user	0m5.684s
sys	0m0.000s
alex@alex-gaynor-laptop:/tmp$ 
alex@alex-gaynor-laptop:/tmp$ 
alex@alex-gaynor-laptop:/tmp$ # With patch
alex@alex-gaynor-laptop:/tmp$ time ~/projects/cpython/python t.py 
real	0m6.085s
user	0m6.064s
sys	0m0.008s
alex@alex-gaynor-laptop:/tmp$ time ~/projects/cpython/python t.py 
real	0m5.728s
user	0m5.720s
sys	0m0.004s
alex@alex-gaynor-laptop:/tmp$ time ~/projects/cpython/python t.py 
real	0m5.783s
user	0m5.772s
sys	0m0.004s
alex@alex-gaynor-laptop:/tmp$ time ~/projects/cpython/python t.py 
real	0m4.730s
user	0m4.716s
sys	0m0.008s
alex@alex-gaynor-laptop:/tmp$ time ~/projects/cpython/python t.py 
real	0m4.691s
user	0m4.680s
sys	0m0.004s