[Bug target/50883] [ARM] Suboptimal optimization for small structures

["rearnsha at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50883

Richard Earnshaw <rearnsha at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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             Status|WAITING                     |NEW
                 CC|                            |rearnsha at gcc dot gnu.org

--- Comment #6 from Richard Earnshaw <rearnsha at gcc dot gnu.org> 2011-10-27 16:53:07 UTC ---
So for PPC, this seems to be a lucky side-effect of the way the PPC ABI is
defined.  On PPC the argument is passed by reference and the compiler generates
an initial load expression of X from memory (a callee copy).  The RTL
optimization passes are then (in combination with the subreg splitting code)
able to notice, somehow, that this is equivalent to the final result and to
make use of it.

On ARM the structure is passed by value and the argument expanding code
immediately tries to store a copy into the stack.  Then, rather than seeing
that this could be used from a register value it tries to work solely with the
stack copy.

I notice that neither architecture optimizes this if the struct is passed as an
address rather than by value.