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Date: Wed, 21 Jul 2010 17:46:00 -0000
Subject: [Bug tree-optimization/45021]  New: Redundant prefetches for the vectorized loop
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From: "changpeng dot fang at amd dot com" <gcc-bugzilla@gcc.gnu.org>
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For the following test case, prefetches will be inserted for both the load and
store of a[i] if the loop is vectorized:

float a[1024], b[1024];
void foo(int beta)
{
  int i;
  for(i=0; i<1024; i++)
     a[i] = a[i] + beta * b[i];
}

with gcc -O3 -fprefetch-loop-arrays -march=amdfam10 -S, a piece of the assembly
is:
        movaps  (%rcx), %xmm0
        addl    $4, %edi
        prefetcht0      (%rdx)
        prefetcht0      240(%rcx)
        prefetchw       (%rdx)
        leaq    64(%rax), %rsi
        mulps   %xmm1, %xmm0


If we don't vectorize the loop, we only generate prefetch for the load a[i]:
        addl    $16, %eax
        salq    $2, %rcx
        mulss   %xmm1, %xmm0
        prefetcht0      a+92(%rcx)
        prefetcht0      b+92(%rcx)
        movl    %esi, %ecx


-- 
           Summary: Redundant prefetches for the vectorized loop
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: changpeng dot fang at amd dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45021