[Bug middle-end/56309] New: -O3 optimizer generates conditional moves instead of compare and branch resulting in almost 2x slower code

[arturomdn at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56309

             Bug #: 56309
           Summary: -O3 optimizer generates conditional moves instead of
                    compare and branch resulting in almost 2x slower code
    Classification: Unclassified
           Product: gcc
           Version: 4.7.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: arturomdn@gmail.com


Created attachment 29442
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=29442
Self contained source file with parameter x passed by value (slow)

This bug report reflects the analysis of a question asked in stackoverflow

http://stackoverflow.com/questions/14805641/why-does-changing-const-ull-to-const-ull-in-function-parameter-result-in-pe/14819939#14819939

When an unsigned long long parameter to a function is passed by reference
instead of by value the result is a dramatic almost 2x improvement in speed
when compiled with -O3.  Given that the function is inlined this is unexpected.
 Upon closer inspection it was found that the code generated is quite
different, as if passing the parameter by value enables an optimization (use of
x86 conditional moves) that backfires, possibly by suffering an unexpected
stall in the processor.

Two files are attached

by-val-O3.ii
by-ref-O3.ii

They differ only in the way the unsigned long long parameter "x" is passed.

./by-ref-O3
Took 11.85 seconds total.

./by-ref-O3
Took 6.67 seconds total.