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From: "ysrumyan at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/58135] New: [x86] Missed opportunities for partial SLP
Date: Mon, 12 Aug 2013 11:59:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58135

            Bug ID: 58135
           Summary: [x86] Missed opportunities for partial SLP
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ysrumyan at gmail dot com

If we consider the following simple test-case 

int a[100];
void foo()
{
  a[0] = a[1] = a[2] = a[3] = 0;
}
SLP vectorization of basic block takes place:

gcc -S -O3 -m32  t.c -ftree-vectorizer-verbose=1      

t.c:4: note: Vectorized basic-block

but if we add at least one more assignment it won't be vectorized:

a[0] = a[1] = a[2] = a[3] = a[4] = 0;

t11.c:4: note: Build SLP failed: unrolling required in basic block SLP

It is clear that gcc can do partial BB vectorization, i.e. vectorize the first
4 assignments only.