Re: [PATCH,rs6000] disable mfcr pattern for TARGET_ISEL

[David Edelsohn <dje.gcc@gmail.com>]

On Tue, Jul 20, 2010 at 4:45 PM, Nathan Froyd <froydnj@codesourcery.com> wrote:
> mfcr can be a slow instruction, particular for multiple-issue
> processors, where it requires serializing the instruction stream to
> ensure that CR has been properly updated.  The primary pattern where
> mfcr occurs deals with moving an individual bit from CR to a GPR.  This
> pattern not only uses mfcr, but also binds the mfcr and accompanying
> rlwinm together instead of permitting them to be scheduled separately.
>
> On TARGET_ISEL processors, however, we can improve the matter somewhat
> by using isel instead of mfcr.  With isel, there's no need to
> synchronize on CR as a whole, just an individual field.  The
> instructions that load zero and one can also be scheduled separately
> from the isel itself.  The patch below enables this improvement by
> turning off the aforementioned mfcr/rlwinm pattern for TARGET_ISEL
> processors.
>
> The patch could be improved; there's a TARGET_POWERPC64 pattern that's
> identical save for the modes of registers slightly further on in the
> file.  I don't have a power7 system to test the equivalent 64-bit path
> on, but if somebody was willing to test on such a system for me, I could
> tweak the patch accordingly.  (Such a tweaked patch could also use mode
> iterators, which would be an improvement.)
>
> Tested with cross to powerpc-eabispe.  OK to commit?
>
> -Nathan
>
>        * config/rs6000/rs6000.md (define_insn ""): Enable only for
>        !TARGET_ISEL targets.

First, the ChangeLog needs a better identifier.  define_insn "" ?  At
least add a "*" name to the unnamed pattern.

Do processors with TARGET_ISEL also implement MFCRF?  mfcrf does not
have the same expense as mfcr.  There may be other reasons to use ISEL
more aggressively, but I am surprised that this would be a problem on
embedded processors with ISEL.  Is this for old processors with ISEL
or did new processors not enable TARGET_MFCRF in GCC?

Thanks, David