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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/63185] Improve DSE with branches
Date: Fri, 24 Oct 2014 16:50:00 -0000	[thread overview]
Message-ID: <bug-63185-4-OyadhtVFK7@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-63185-4@http.gcc.gnu.org/bugzilla/>

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63185

--- Comment #5 from Marc Glisse <glisse at gcc dot gnu.org> ---
Another example:

#include <time.h>
void f(){
  const int n=1<<14;
  double a[n];
  double b[n];
  double c[n];
  __builtin_memset(a,0,n);
  __builtin_memset(b,0,n);
  __builtin_memset(c,0,n);
  for(int i=0;i<n;++i)
    c[i]=a[i]+b[i];
  time(0);
}

If I make n a constant (using #define), DCE knows that c is not
ref_may_be_aliased (that test could probably be weakened) and removes almost
everything. We don't replace alloca_with_align with an array because the size
is larger than 256 (with a more limited scope the limit would even be 25...).
DSE is likely confused by the loop. And PRE and others don't know that a[i] is
always 0. (llvm removes everything but the final call)


  parent reply	other threads:[~2014-10-24 16:38 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-09-05 10:51 [Bug tree-optimization/63185] New: " glisse at gcc dot gnu.org
2014-09-05 13:03 ` [Bug tree-optimization/63185] " rguenth at gcc dot gnu.org
2014-09-05 13:27 ` glisse at gcc dot gnu.org
2014-10-16 12:16 ` rguenth at gcc dot gnu.org
2014-10-16 13:03 ` glisse at gcc dot gnu.org
2014-10-24 16:50 ` glisse at gcc dot gnu.org [this message]
2024-05-15 16:30 ` rguenth at gcc dot gnu.org

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