[Bug tree-optimization/53397] New: Scimark performance drops by 10x times when compiled -O3 -march=amdfam10 due to generation more prefecthes

["venkataramanan.kumar at amd dot com" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53397

             Bug #: 53397
           Summary: Scimark performance drops by 10x times when compiled
                    -O3 -march=amdfam10 due to generation more prefecthes
    Classification: Unclassified
           Product: gcc
           Version: tree-ssa
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: venkataramanan.kumar@amd.com


With GCC4.7 the benchmark score drops from ~400 Mflops to ~40 mflops. Almost 10
folds.

Prefecth instructions introduced in the innermost loops of
"FFT_transform_internal" ( FFT.c ) in GCC4.7 but not in GCC4.6 which is causing
the slow down. 

Compiling this function alone as a separate test case with
-fno-prefetch-loop-arrays brings back the original score.

The problem is exposed http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=175474

With GCC r175473
--------------------------
gcc -O3 -march=amdfam10 *.c -o Scimark175473 -lm
vekumar@pcedinar5:/local/home/vekumar/SciMark2_bench/SciMark2> ./Scimark175473
**                                                              **
** SciMark2 Numeric Benchmark, see http://math.nist.gov/scimark **
** for details. (Results can be submitted to pozo@nist.gov)     **
**                                                              **
Using       2.00 seconds min time per kenel.
Composite Score:           99.67
FFT             Mflops:   498.35    (N=1024)

With GCC r175474
-------------------------
gcc -O3 -march=amdfam10 *.c -o Scimark175474 -lm
vekumar@pcedinar5:/local/home/vekumar/SciMark2_bench/SciMark2> ./Scimark175474
**                                                              **
** SciMark2 Numeric Benchmark, see http://math.nist.gov/scimark **
** for details. (Results can be submitted to pozo@nist.gov)     **
**                                                              **
Using       2.00 seconds min time per kenel.
Composite Score:            7.73
FFT             Mflops:    38.66    (N=1024)