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From "Travis Vitek" <Travis.Vi...@roguewave.com>
Subject RE: svn commit: r663377 - in /stdcxx/branches/4.2.x: include/valarray src/valarray.cpp tests/numerics/26.class.gslice.cpp tests/regress/26.valarray.sub.stdcxx-955.cpp
Date Fri, 06 Jun 2008 18:40:23 GMT
 

Martin Sebor wrote:
>
>vitek@apache.org wrote:
>> Author: vitek
>> Date: Wed Jun  4 14:48:36 2008
>> New Revision: 663377
>> 
>[...]
>> Modified: stdcxx/branches/4.2.x/src/valarray.cpp
>> URL: 
>http://svn.apache.org/viewvc/stdcxx/branches/4.2.x/src/valarray
>.cpp?rev=663377&r1=663376&r2=663377&view=diff
>> 
>===============================================================
>===============
>> --- stdcxx/branches/4.2.x/src/valarray.cpp (original)
>> +++ stdcxx/branches/4.2.x/src/valarray.cpp Wed Jun  4 14:48:36 2008
>> @@ -41,8 +41,12 @@
>>  {
>>      _RWSTD_SIZE_T __n = _C_length.size ();
>>  
>> -    while (__n && _C_r_length [__n - 1] == _C_length [__n - 1] - 1)
>> -        --__n;
>> +    for (/**/; __n; --__n)
>> +    {
>
>The brace should be on the line above :)
>

Blarg!

>> +        if (   _C_length [__n - 1]
>> +            && _C_r_length [__n - 1] != _C_length [__n - 1] - 1)
>
>Also, I wonder if it might help generate more optimal code to write
>the loop like so:
>
>     while (__n) {
>         --__n;
>
>         if (_C_length [n] && _C_r_length [n] != _C_length [n] - 1)
>             break;
>     }
>

It is possible it would help generate better code, but this code changes
the behavior of the loop. We now decrement __n before checking the
condition; when we break __n will be off-by-one. Obviously we can deal
with that by incrementing __n before the break.

I'll take a quick look at some disassembly and decide if it is worth
making the change or not.

>The duplicate check for (0 == n) below could probably be hoisted
>into the loop for even more optimal code, something like this:
>
>     for ( ; ; ) {
>         if (0 == n) {
>             _C_reset = true;
>             break;
>         }
>
>         --n;
>
>         if (_C_length [n] && _C_r_length [n] != _C_length [n] - 1)
>             break;
>     }
>
>> +            break;
>> +    }
>>  
>>      if (0 == __n) {
>>          _C_reset    = true;
>> 

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