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From: "anlauf at gmx dot de" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/57037] New: GCC does not generate non-temporal stores on i386 with SSE2+
Date: Mon, 22 Apr 2013 20:29:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57037

             Bug #: 57037
           Summary: GCC does not generate non-temporal stores on i386 with
                    SSE2+
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: anlauf@gmx.de


Hello,

it appears that gcc does not generate non-temporal stores
available on i386 at least with SSE2.  This is an important
optimization for some memory-bandwidth limited codes.

Example: for the stream triad kernel,

subroutine stream_kernel_triad (a, b, c, n, s)
  integer         , intent(in) :: n
  double precision             :: a(*), b(*), c(*)
  double precision, intent(in) :: s

  integer :: j
  do j = 1,n
     a(j) = b(j) + s*c(j)
  end do
end subroutine stream_kernel_triad

the Intel compiler generates vectorized code with a
throughput that is 25% higher on my Core2 than when
disabling the generation of non-temporal stores
(i.e. compiling with "-opt-streaming-stores never").

gfortran (using -Ofast -fprefetch-loop-arrays) exactly
reproduces the performance of the Intel compiler without
temporal stores.  It appears that this is an important
optimization.

Harald