[patch,libgfortran] PR37754 [4.4 Regression] READ I/O Performance regression from 4.3 to 4.4

[Jerry DeLisle]

Jack Howarth wrote:
> On Thu, Dec 25, 2008 at 09:32:57AM -0800, Jerry DeLisle wrote:
>> This is a Merry Christmas patch.
>>
>> This patch recovers the performance from this regression by creating a 
>> stream read_char function which is simply a trimmed down version of sread 
>> (fd_read).  I was actually surprised when I saw the test results.  I 
>> suspect that the simplification allows some better optimizations.
>>
>> The patch also refactors next_char in list_read.c to eliminate goto's and 
>> inlining a small portion of the "done:" code.  The refactoring of 
>> next_char alone gains 2.8% over current trunk.  The use of the new 
>> read_char function gains significant additional performance.
>>
>> Using the countlines.f test case in the PR for comparison, average 5 runs.
>>
>> gfortran 4.3: 3.357 seconds
>>
>> gfortran 4.4 current trunk: 3.821 seconds
>>
>> gfortran 4.4 patched: 3.164 seconds
>>
>> This is a 5.7% improvement over 4.3 for this test case and 17% 
>> improvement over current trunk.
>>
>> I also believe this refactoring will make for some easier further 
>> improvements.  I don't know the status of Janne's patch so this patch may 
>> end up being short lived.  However, it is not very intrusive in the sense 
>> that it is mostly reorganizing in simple ways our existing code paths.  
>> Since it involves a regression, I think it would be OK for 4.4
>>
>> Regression tested on x86-64.
>>
>> OK to commit?
>>
>> Jerry
> 
> Jerry,
>     I am seeing about a 10% performance improvement with the patch when
> using...
> 
> gfortran -O countlines.f
> 
> to compile the testcase and using the temp4 file created by the maketemp4.f
> program in the PR. I used average of the last five of ten runs each time to minimize
> effects of any disk caching. What did you use for the test file? I noticed the
> temp4 file has identical lines. It may not be unfair to use the same line length
> but we should probably randomize the contents of the lines.
>               Jack
> ps This was on x86_64-apple-darwin10.
> 
Yes, I used the temp4 file.

So it appears we do get some benefits in performance in a system dependent way. 
    No one is seeing degradation from the patch.  So thats a plus.

Thanks for testing.  Waiting to here from Janne.

Jerry