[patch PR middle-end/39326 - compile time and memory explosion in combine

[Steven Bosscher]

On Mon, Mar 11, 2013 at 10:23 AM, Richard Biener wrote:
> On Sat, Mar 9, 2013 at 11:23 PM, Steven Bosscher wrote:
>> The attached patch fixes one of the (at least) three scalability
>> problems reported in PR middle-end/39326. This problem is that combine
>> takes forever and increases the memory footprint from ~650MB to >7GB.
>> The cause is DSE creating a lot of new registers in replace_read, and
>> those registers are not copy-prop'd out between dse1 and combine. The
>> result is many overlapping live ranges and single-set-single-use
>> registers that combine is made to work on.
>>
>> The fix is to just not create so many new registers in DSE in the
>> first place. It is wasteful and unnecessary if an existing register
>> can be re-used.
>>
>> With this patch, for the test case of the PR the combine time in
>> combine goes down from ~350s to ~4.5s, and the memory footprint
>> explosion is avoided. For my set of cc1-i files this also helps reduce
>> compile time a modest amount, especially for the larger files of
>> course.
>>
>> Bootstrapped&tested on {x86_64,powerpc64}-unknown-linux-gnu.
>> OK for trunk?
>
> Not sure on the patch details - but in general I wonder why _DSE_
> performs full redundancy elimination at all ... which replace_read
> seems to be.

Yes. No idea why, it's always been like that.

> Anyway, for this one I'd say we wait for stage1 and consider backporting
> for 4.8.1 given we want to do a 4.8 RC1 soon.

OK. I've found a buglet in the patch, too, so I'll post an updated
patch for trunk GCC 4.9.

Ciao!
Steven