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From: "andi-gcc at firstfloor dot org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/60989] New: automatic increase of global array for vectorization doesn't work due to flag_section_anchors==0
Date: Mon, 28 Apr 2014 15:21:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60989

            Bug ID: 60989
           Summary: automatic increase of global array for vectorization
                    doesn't work due to flag_section_anchors==0
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: andi-gcc at firstfloor dot org

Target x86_64-linux

Following simple test case:

#define N 64
#define ALIGN // __attribute__((aligned(64)))
int a[N] ALIGN;
int b[N] ALIGN;

main()
{
        int i;
        for (i = 0; i < N; i++)
                a[i] = b[i] + 1;
}

If I leave the ALIGN commented out  I get really horrible manual alignment
prologue code like
-O3 -mavx512f

   .cfi_startproc
        movl    $a, %eax
        andl    $63, %eax
        shrq    $2, %rax
        negq    %rax
        andl    $15, %eax
        je      .L7
        movl    b(%rip), %edi
        cmpl    $1, %eax
        leal    1(%rdi), %edx
        movl    %edx, a(%rip)
        je      .L8
...
   lots of repeats upto 63


With ALIGN it's a nice short perfectly vectorized loop.

It seems the automatic alignment of global variables doesn't work anymore.

I checked the increase_alignment/ipa_increase_alignment pass in
tree-vectorize.c and both don't run because flag_section_anchors is 0 in the
gate.

I'm not fully sure what the flag means?