[LLVMbugs] [Bug 928] NEW: Investigate cases where running llvm optimizer twice slows down code

[bugzilla-daemon at cs.uiuc.edu]

http://llvm.org/bugs/show_bug.cgi?id=928

           Summary: Investigate cases where running llvm optimizer twice
                    slows down code
           Product: tools
           Version: 1.7
          Platform: Macintosh
        OS/Version: MacOS X
            Status: NEW
          Severity: normal
          Priority: P2
         Component: llvm-gcc
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: sabre at nondot.org


These two nightly tester tests:

http://llvm.org/nightlytest/test.php?machine=36&night=1063
http://llvm.org/nightlytest/test.php?machine=23&night=1064

Capture performance changes that were due to running llvm-gcc at -O0 instead of -O2.  The basic 
impact is that we're now runing 'gccas' once instead of twice on the input programs (far more realistic).

Some of these programs got slower, e.g. fldry.  This turns out to be a phase-ordering issue.  If these 
can be tracked down and improved, that would be good.

More important, however, are the programs that sped up by disabling the run of the optimizer.  This 
implies that running the optimizer twice on the programs actually *slowed them down*, which is clearly 
bad.

-Chris



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