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Re: [PATCH] Come up with -flto=auto option.


On 7/23/19 8:23 AM, Martin Liška wrote:
> On 7/23/19 3:57 PM, Jeff Law wrote:
>> On 7/23/19 7:50 AM, Michael Matz wrote:
>>> Hi,
>>>
>>> On Tue, 23 Jul 2019, Jeff Law wrote:
>>>
>>>>> great you found time to make this. It should become the default for
>>>>> -flto IMO.
>>>> I was going to hack it into the rpm configury bits since we have access
>>>> to the # cores there.  But an auto-selector within GCC is even better.
>>>>
>>>> BTW, isn't this all going to wreck havoc with reproducible builds since 
>>>> partitioning can affect code generation?  That's one of our open 
>>>> questions...
>>>
>>> See Richi for this, but the reason for -flto=auto (or just -flto, or 
>>> whatever the endresult will be) is _also_ reproducible builds, because 
>>> some packages like to encode the compile flags into their binaries and 
>>> hence would change depending on build host just because of "-flto=32" vs. 
>>> "-flto=64" even when the code remains exactly the same.
>> Makes sense.
>>
>> What did you end up doing with old autoconf scripts that aren't LTO
>> safe?  Many of the older style tests to see if a function exists are
>> broken by LTO.  I've seen more issues with this than anything in the LTO
>> space so far.
> 
> Well, I've seen some of these failures, but only a few.
Many appear to be silent, possibly not really affecting anything (like
all the packages that test for doprnt, but really don't care about it in
the end).    But there were enough real failures that I put in auditing
to detect any cases where we get different config.h files with LTO vs
non-LTO and that is tripping often enough to have my concerns about how
much work it's going to be to get everything fixed.


But still, overall we're moving forward.  Next step is to get everything
classified into buckets and start iterating.  Presumably you'd be open
to a google doc of some kind where we can coordinate any such efforts?

jeff

ps.  I'm on PTO July 25 to Aug 5, so not much is going to happen in the
next couple weeks :-)


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