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Re: [PATCH 4/4] Convert lto streamer out hashing to inchash


On Tue, Jul 22, 2014 at 09:40:15PM -0600, Jeff Law wrote:
> On 07/15/14 23:31, Andi Kleen wrote:
> >From: Andi Kleen <ak@linux.intel.com>
> >
> >No substantial changes, although the hash values will be slightly
> >different.
> >
> >gcc/:
> >
> >2014-07-10  Andi Kleen  <ak@linux.intel.com>
> >
> >	* lto-streamer-out.c (hash_tree): Convert to inchash.
> >	(add_flag): New macro.
> So my question here, does this make any existing LTO objects no longer
> usable?  If so, what, if any policy do we have when we make that kind of
> change?

It breaks the format because a few type hashes change

But breaks happen all the time during development. Currently just adding
a new command option breaks the format. Or pretty much any change to
gimple.

Generally in my experience LTO object files don't stay usable more
than a few days in phase 1.

Currently the format is even broken for patch level releases, usually
due to some option change.

The policy is just that for a given release that is unchanged the 
object format stays compatible.

I submitted some patches to improve this slightly for the options, 
but they were not accepted because of all the other issues.
https://gcc.gnu.org/ml/gcc-patches/2010-10/msg00099.html

Given that reality I assume the patch is ok for you?

-Andi

ak@linux.intel.com -- Speaking for myself only


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