[Patch] Fix libstdc++/13341 (speed-up <wchar_t>::do_narrow/widen)

[Paolo Carlini]

Hi,

this is my modest proposal, limited to the wchar_t versions of
do_widen and do_narrow. For the common case it results in a
pretty good speed up:

 for (long i = 0; i < 100000000; ++i)
   ct.widen(i % 128);
   // ct.narrow(i % 128, '*');

On my P4-2400, -O2:

3.4
---
widen:   7.650u 0.000s 0:07.66 99.8%     0+0k 0+0io 167pf+0w
narrow: 11.650u 0.000s 0:11.69 99.6%     0+0k 0+0io 168pf+0w

3.4 + patch
-----------
widen:   1.270u 0.010s 0:01.28 100.0%    0+0k 0+0io 169pf+0w
narrow:  1.270u 0.000s 0:01.27 100.0%    0+0k 0+0io 169pf+0w


Tested x86-linux (gnu/generic).

I will wait some 24 hours for comments...

Paolo.

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