[Bug c++/14455] New: Inner Altivec loop contains load/stores but not all registers being used

["tjw at omnigroup dot com" <gcc-bugzilla at gcc dot gnu dot org>]

Often in inner loops (no child functions called, big loop count), there will be several (many!) Altivec 
registers left unused.

The attached file demonstrates the problem, when built with tree-ssa with:

%PREFIX/bin/g++ -Winline -mdynamic-no-pic -fno-exceptions -O3 -maltivec -fstrict-aliasing 
-finline-functions -finline-limit=1000000000 -falign-loops=16 --param large-function-
growth=1000000 --param inline-unit-growth=1000 iterator_10.cpp -S -o /tmp/iterator_10.s

In my real world code, this is a big problem since I'm have code that does:

    - long computation for some answer
    - compute some address to ADD to the answer to (and the address is almost never in cache)
    - load from answer address
    - start on next loop
    - add answer and old answer and store

The problem is that the compiler totally blows the approach since it immediately stores the loaded 
old answer to the stack, causing a stall waiting for the load to complete (and thus preventing any 
asynchrony with the load and the computation in the next loop).

-- 
           Summary: Inner Altivec loop contains load/stores but not all
                    registers being used
           Product: gcc
           Version: tree-ssa
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: tjw at omnigroup dot com
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: powerpc-apple-darwin7.2.0
  GCC host triplet: powerpc-apple-darwin7.2.0
GCC target triplet: powerpc-apple-darwin7.2.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14455