On 08/01/2016 05:31 PM, Segher Boessenkool wrote:
Hi,
On Mon, Aug 01, 2016 at 06:52:54PM +0000, Bernd Edlinger wrote:
On 08/01/16 19:54, Jeff Law wrote:
Looks like you've probably nailed it. It'll be interesting see if
there's any fallout (though our RTL optimizer testing is pretty weak, so
even if there were, I doubt we'd catch it).
If there is, it will probably a performance regression...
I tested building Linux with and without the patch, on many archs.
The few that show differences are:
alpha 6148872 6148776
ia64 16946958 16946670
s390 12345770 12345850
tile 12016086 12016070
(left before, right after; arm and aarch64 did not build, kernel problems).
So all except s390 generate smaller code even.
They're all deep enough in the noise that I wouldn't care either way :-)
However I think there are more paradoxical subregs generated all over,
but the aarch64 insv code pattern did trigger more hidden bugs than
any other port. It is certainly unfortunate that the major source
of paradoxical subreg is in a target-dependent code path :(
It is certainly unfortunate that paradoxical subregs exist at all! :-)
Yea. It probably seemed like a good idea 25-30 years ago, but I always
cringe when I see them being used. Yea it gives the compiler some more
freedom, but more often than not I think we'd be better off with real
extensions.
jeff
