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* [Bug rtl-optimization/20376] New: The missed-optimization of general induction variables in the new rtl-level loop optimizer cause performance degradation.
@ 2005-03-08  2:49 canqun at nudt dot edu dot cn
  2005-03-08  3:13 ` [Bug rtl-optimization/20376] " pinskia at gcc dot gnu dot org
                   ` (15 more replies)
  0 siblings, 16 replies; 21+ messages in thread
From: canqun at nudt dot edu dot cn @ 2005-03-08  2:49 UTC (permalink / raw)
  To: gcc-bugs

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Induction variables are variables whose successive values form an arithmetic 
progression over a loop. Induction variables are often divided into bivs 
(basic induction variables), which are explicitly modified by 
the same constant amount during each iteration of a loop, and gives (general 
induction variables), which may be modified or computed by a linear-function 
of a basic induction variable. 

There are three important transformations that apply to them: strength 
reduction, induction-variable removal, and linear-function test replacement. 
For example, we can do strength reduction of address givs which are usually 
used for address calculation of array elements. On platforms with post-
increment load and store instructions, this will make chance to combine a 
load/store with the following address calculation instruction.

Also the induction variable splitting is an effective optimization during loop 
unrolling.

The new loop optimizer only support a limited gives analysis (ref. loop-iv.c), 
and has not yet implemented giv strength reduction and splitting (ref. ¡®loop-
unroll.c¡¯, ¡®analyze_iv_to_split_insn¡¯, and comments in this function, ¡°For 
now we just split the basic induction 
variables. Later this may be extended for example by selecting also addresses 
of memory references.¡±)

I test 171.swim on IA64 Linux system with 1GHz itanium2 CPU. After implemented 
or improved/adjusted several compile optimizations, such as Fortran alias 
analysis (very simple one), loop unrolling (the old one), loop arrays pre-
fetching, and giv optimizations, it costs just 9.1s (28s for GCC-4.0.0) to 
execute the train mode of 171.swim. But, apply those changes on current GCC-
4.0.0 (the old loop unroller was removed), it costs 
13.4 to execute this benchmark program, and I found that giv optimizations are 
the major factor of such performance degradation.

-- 
           Summary: The missed-optimization of general induction variables
                    in the new rtl-level loop optimizer cause performance
                    degradation.
           Product: gcc
           Version: 4.0.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: rtl-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: canqun at nudt dot edu dot cn
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: ia64-unknown-linux
  GCC host triplet: ia64-unknown-linux
GCC target triplet: ia64-unknown-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20376


^ permalink raw reply	[flat|nested] 21+ messages in thread
end of thread, other threads:[~2005-07-14 15:25 UTC | newest]

Thread overview: 21+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2005-03-08  2:49 [Bug rtl-optimization/20376] New: The missed-optimization of general induction variables in the new rtl-level loop optimizer cause performance degradation canqun at nudt dot edu dot cn
2005-03-08  3:13 ` [Bug rtl-optimization/20376] " pinskia at gcc dot gnu dot org
2005-03-08  3:16   ` Diego Novillo
2005-03-08  3:18     ` Andrew Pinski
2005-03-08  3:20       ` Diego Novillo
2005-03-08  3:16 ` dnovillo at redhat dot com
2005-03-08  3:18 ` pinskia at physics dot uc dot edu
2005-03-08  3:30   ` Daniel Berlin
2005-03-08  3:21 ` dnovillo at redhat dot com
2005-03-08  3:30 ` dberlin at dberlin dot org
2005-03-09 17:53 ` sje at cup dot hp dot com
2005-03-27 10:19 ` steven at gcc dot gnu dot org
2005-03-30 11:38 ` steven at gcc dot gnu dot org
2005-03-30 14:43 ` canqun at nudt dot edu dot cn
2005-03-30 19:54 ` steven at gcc dot gnu dot org
2005-06-24 16:39 ` rakdver at gcc dot gnu dot org
2005-06-24 16:56 ` rakdver at gcc dot gnu dot org
2005-06-26 12:13 ` steven at gcc dot gnu dot org
2005-07-13  9:41 ` cvs-commit at gcc dot gnu dot org
2005-07-13 11:38 ` pinskia at gcc dot gnu dot org
2005-07-14 15:33 ` canqun at nudt dot edu dot cn

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