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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/61619] Benefits from -ftree-vectorize lost easily when changing unrelated code
Date: Thu, 26 Jun 2014 14:48:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61619

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
                 CC|                            |jamborm at gcc dot gnu.org
           Assignee|unassigned at gcc dot gnu.org      |rguenth at gcc dot gnu.org

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
._94/3491 (constexpr const long unsigned int ._94 [1]) @0x7ffff1d5ca90
  Type: variable definition analyzed
  Visibility: prevailing_def_ironly artificial
  References:
  Referring: main/3232 (read)
  Availability: available
  Varpool flags: initialized read-only const-value-known

Ok, for the above SRA fails to do a scalar replacement and value-numbering
doesn't even reach the aggregate copy from the constant initializer.
That's because only PRE exposes it and PRE doesn't do all the fancy VN
tricks.

I suppose I have to look closer at some point.  Interesting testcase
showing possible issues with C++ constexpr initializers.

Note that we also have

  MEM[(char * {ref-all})&S] = MEM[(char * {ref-all})&._94];   

but this aggregate copy has sizeof (long) so we could have optimized it
(it's ref-all already) to use an integer type.

This is lowered from

  array<long unsigned int, 1ul>::array (&S, (const struct initializer_list &)
&TARGET_EXPR <D.64552, {._M_array=(const long unsigned int *) &._94,
._M_len=1}>)

to

        D.64552._M_array = &._94;
        D.64552._M_len = 1;
        try
          {
            array<long unsigned int, 1ul>::array (&S, &D.64552);
          }

which eventually ends up calling memmove which we optimize to the aggregate
assignment.