[pypy-issue] Issue #1925: VERY slow string concatenation (pypy/pypy)

[Yichao Yu]

New issue 1925: VERY slow string concatenation
https://bitbucket.org/pypy/pypy/issue/1925/very-slow-string-concatenation

Yichao Yu:

The following script (admittedly not the best way to build long string) runs ~30-100 times slower on pypy than cpython2/3.

```python
a = ''
for i in range(10000):
    a += 'a'
```

PyPy version: a772923edb32 (latest as of last week) on my laptop.

```
% python -m timeit -s 'a = ""' -- "for i in range(10000): a += 'a'"
1000 loops, best of 3: 1.07 msec per loop
```

```
% python2 -m timeit -s 'a = ""' -- "for i in range(10000): a += 'a'"
1000 loops, best of 3: 678 usec per loop
```

```
% pypy -m timeit -s 'a = ""' -- "for i in range(10000): a += 'a'"
10 loops, best of 3: 29.6 msec per loop
```

PyPy 2.4.0 on a linode vps is even worse

```
% python -m timeit -s 'a = ""' -- "for i in range(10000): a += 'a'" 
1000 loops, best of 3: 1.55 msec per loop
```

```
% python2 -m timeit -s 'a = ""' -- "for i in range(10000): a += 'a'"
1000 loops, best of 3: 978 usec per loop
```

```
% pypy -m timeit -s 'a = ""' -- "for i in range(10000): a += 'a'"
10 loops, best of 3: 94.6 msec per loop
```

The time might not be accurate since the script does not run for very long (~1s) but the significant slow down remains when repeating this multiple time and with more iteration in the loop.