From patchwork Tue Jun  8 08:05:33 2010
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Subject: [libstdc++-v3, parallel,
 mode] Correct part lengths calculation for parallel partial_sum
Date: Mon, 07 Jun 2010 22:05:33 -0000
From: Johannes Singler <singler@kit.edu>
X-Patchwork-Id: 54947
Message-Id: <4C0DF9CD.9010807@kit.edu>
To: Jonathan Wakely <jwakely.gcc@gmail.com>
Cc: libstdc++ <libstdc++@gcc.gnu.org>, gcc-patches@gcc.gnu.org

Jonathan Wakely wrote:
> On 8 June 2010 08:21, Johannes Singler wrote:
>> This patch corrects the calculation of the part lengths for parallel
>> partial_sum, leading to the expected behavior for
>> partial_sum_dilation!=1, and thus better performance.
>>
>> Tested x86_64-unknown-linux-gnu: No regressions.
>>
>> Please approve for mainline.
>>
>> 2010-06-08  Johannes Singler  <singler@kit.edu>
>>
>>        * include/parallel/partial_sum.h
>>        (__parallel_partial_sum_linear):
>>        Correctly calculate part lengths for partial_sum_dilation!=1.
> 
> Please declare each variable on its own line, as per
> http://gcc.gnu.org/onlinedocs/libstdc++/manual/source_code_style.html
> 
>       char* c = "abc";  // each variable goes on its own line, always.

Okay, I hope you like the attached version better.

Johannes


Index: include/parallel/partial_sum.h
===================================================================
--- include/parallel/partial_sum.h	(revision 160424)
+++ include/parallel/partial_sum.h	(working copy)
@@ -127,10 +127,13 @@
 	    equally_split(__n, __num_threads + 1, __borders);
 	  else
 	    {
+	      _DifferenceType __first_part_length =
+		  std::max<_DifferenceType>(1, (float)__n /
+		    (1.0 + __s.partial_sum_dilation * (float)__num_threads));
 	      _DifferenceType __chunk_length =
-		((double)__n
-		 / ((double)__num_threads + __s.partial_sum_dilation)),
-		__borderstart = __n - __num_threads * __chunk_length;
+		  (__n - __first_part_length) / __num_threads;
+	      _DifferenceType __borderstart =
+		  __n - __num_threads * __chunk_length;
 	      __borders[0] = 0;
 	      for (_ThreadIndex __i = 1; __i < (__num_threads + 1); ++__i)
 		{