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From: "glisse at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/58817] New: optimize alloca with constant size
Date: Mon, 21 Oct 2013 04:12:00 -0000
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X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 4.9.0
X-Bugzilla-Keywords: missed-optimization
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58817

            Bug ID: 58817
           Summary: optimize alloca with constant size
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: glisse at gcc dot gnu.org

Hello,

I thought gcc already had this optimization but apparently not. We don't
produce the same code with alloca and with arrays:

void f(int*);
void g(){
#if 1
  const int n=4;
  int a[n];
#elif 1
  int a[4];
#else
  int*a=__builtin_alloca(16);
#endif
  f(a);
}

possibly because arrays give alloca_with_align, not alloca.

There may be reasons why the transformation would be illegal though, I am not
sure.