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Subject: [Bug rtl-optimization/55177] missed optimizations with __builtin_bswap
Date: Fri, 02 Nov 2012 17:46:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55177

Eric Botcazou <ebotcazou at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
         AssignedTo|unassigned at gcc dot       |ebotcazou at gcc dot
                   |gnu.org                     |gnu.org

--- Comment #4 from Eric Botcazou <ebotcazou at gcc dot gnu.org> 2012-11-02 17:45:54 UTC ---
> The first example isn't *that* dumb, as a cut-down test case of real code 
> which may look more complex in reality.

OK, and I can indeed think of some usage patterns in real life.

> If the real code really *is* as simple as my test case, you're right that
> perhaps we *could* optimise it ourselves by eschewing the normal accessor
> macros for explicit-endian values, and manually byteswapping the constant
> instead.
> 
> But really, we shouldn't *have* to. The compiler can see what's happening, and
> it can deal with it a whole lot better than we can, especially when it comes
> to loads and stores. 

Another category would be comparisons then:

int compare1 (int x)
{
  return __builtin_bswap32 (x) == 0xdeadbeef;
}

int compare2 (int x, int y)
{
  return __builtin_bswap32 (x) == __builtin_bswap32 (y);
}

Will have a look shortly.