libstdc++/2211: Performance regression for cin.read with respect to g++-2.95.2 ?

[Theodore.Papadopoulo@sophia.inria.fr]

>Number:         2211
>Category:       libstdc++
>Synopsis:       Performance regression for cin.read with respect to g++-2.95.2 ?
>Confidential:   no
>Severity:       non-critical
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Wed Mar 07 11:26:01 PST 2001
>Closed-Date:
>Last-Modified:
>Originator:     Theo Papadopoulo
>Release:        unknown-1.0
>Organization:
>Environment:
g++ mainline from 01/22/2001
Linus redhat 6.2 on x86 (P III)
>Description:
I have a huge performance problem with a call to a libstdc++-v3 
function. istream::read. This is true only for I/O done with cin.

mururoa->ls -l toto
-rw-r--r--   1 papadop  robotvis  1367160 Mar  7 11:38 toto
mururoa->cat toto toto | ./a.out ~/Robotvis++/Libs/Image/tests/images/scully.float.inr
Before ifstream::read: 983970673
After ifstream::read: 983970673
Before cin.read: 983970673
After cin.read: 983970682
Before fread: 983970682
After fread: 983970682

With g++-2.95.2, I get:
mururoa->/usr/local/bin/g++ -g -pg -ftemplate-depth-30 -fPIC -DPIC Test.C
mururoa->cat toto toto | ./a.out ~/Robotvis++/Libs/Image/tests/images/scully.float.inr
Before ifstream::read: 983978303
After ifstream::read: 983978303
Before cin.read: 983978303
After cin.read: 983978303
Before fread: 983978303
After fread: 983978303

[regression 2.95.2 was cin.read(ing) as that as the other ways 
of reading]

The crux of the problem seem to be (in fstream.tcc).

	  // XXX So that istream::getc() will only need to get 1 char,
	  // as opposed to BUF_SIZE.
	  if (__fd == 0)
	    _M_buf_size = 1;

Consequently, characters are read one by one which is extremely 
costly.
>How-To-Repeat:

>Fix:

>Release-Note:
>Audit-Trail:
>Unformatted:
----gnatsweb-attachment----
Content-Type: application/octet-stream; name="Test.C"
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename="Test.C"

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