rs6000 has instructions that can do almost everything 32 bit
at least as efficiently as corresponding 64 bit things. The
mode promotion can be defered to when a wide mode is necessary.
So it helps a lot not promote mode for pseudos. SPECint test
shows that the overall performance improvement (by geomean) is
more than 2% with this patch.
testsuite/gcc.target/powerpc/not-promote-mode.c illustrates how
the patch eliminates the redundant extensions and do further
optimization by disabling mode promotion for pseduos.