Re: Why does lower-subreg mark copied pseudos as "decomposable"?

[Richard Sandiford <rdsandiford at googlemail dot com>]

Andrew Stubbs <ams@codesourcery.com> writes:
> Hi all,
>
> I can see why copying from one pseudo-register to another would not be a 
> reason *not* to decompose a register, but I don't understand why this is 
> a reason to say it *should* be decomposed.

The idea is that, if a backend implements an N-word pseudo move using
N word-mode moves, it is better to expose those moves before register
allocation.  It's easier for RA to find N separate word-mode registers
than a single contiguous N-word one.

The problem is the "if a backend implements ..." bit: the current code
doesn't check.  This patch:

    http://gcc.gnu.org/ml/gcc-patches/2012-04/msg00094.html

should help.  It's still waiting for me to find a case where the two
possible ways of handling hot-cold partitioning behave differently.

Richard