Stop using precompiled headers for Linux?

Magnus Ihse Bursie magnus.ihse.bursie at oracle.com
Thu Nov 1 15:17:10 UTC 2018


On 2018-11-01 15:53, Ioi Lam wrote:
> Just a stupid question. Does GCC have actual support for PCH? I know windows can load pre-compiled information from a special binary file. Does GCC support that kind of functionality?
Yes.

https://gcc.gnu.org/onlinedocs/gcc/Precompiled-Headers.html

/Magnus

>
> Thanks
> Ioi
>
>> On Nov 1, 2018, at 5:09 AM, Magnus Ihse Bursie <magnus.ihse.bursie at oracle.com> wrote:
>>
>>
>>
>>> On 2018-11-01 12:51, Thomas Stüfe wrote:
>>> On Thu, Nov 1, 2018 at 12:05 PM Magnus Ihse Bursie
>>> <magnus.ihse.bursie at oracle.com> wrote:
>>>> On 2018-11-01 11:54, Aleksey Shipilev wrote:
>>>>>> On 11/01/2018 11:43 AM, Magnus Ihse Bursie wrote:
>>>>>> But then again, it might just signal that the list of headers included in the PCH is no longer
>>>>>> optimal. If it used to be the case that ~100 header files were so interlinked, that changing any of
>>>>>> them caused recompilation of all files that included it and all the other 100 header files on the
>>>>>> PCH list, then there was a net gain for using PCH and no "punishment".
>>>>>>
>>>>>> But nowadays this list might be far too large. Perhaps there's just only a core set of say 20 header
>>>>>> files that are universally (or almost universally) included, and that's all that should be in the
>>>>>> PCH list then. My guess is that, with a proper selection of header files, PCH will still be a benefit.
>>>>> I agree. This smells like inefficient PCH list. We can improve that, but I think that would be a
>>>>> lower priority, given the abundance of CPU power we use to compile Hotspot. In my mind, the decisive
>>>>> factor for disabling PCH is to keep proper includes at all times, without masking it with PCH. Half
>>>>> of the trivial bugs I submit against hotspot are #include differences that show up in CI that builds
>>>>> without PCH.
>>>>>
>>>>> So this is my ideal world:
>>>>>    a) Efficient PCH list enabled by default for development pleasure;
>>>>>    b) CIs build without PCH all the time (jdk-submit tier1 included!);
>>>>>
>>>>> Since we don't yet have (a), and (b) seems to be tedious, regardless how many times both Red Hat and
>>>>> SAP people ask for it, disabling PCH by default feels like a good fallback.
>>>> Should just CI builds default to non-PCH, or all builds (that is, should
>>>> "configure" default to non-PCH on linux)? Maybe the former is better --
>>>> one thing that the test numbers here has not shown is if incremental
>>>> recompiles are improved by PCH. My gut feeling is that they really
>>>> should -- once you've created your PCH, subsequent recompiles will be
>>>> faster.
>>> That would only be true as long as you just change cpp files, no? As
>>> soon as you touch a header which is included in precompiled.hpp you
>>> are worse off than without pch.
>>>
>>>> So the developer default should perhaps be to keep PCH, and we
>>>> should only configure the CI builds to do without PCH.
>>> CI without pch would be better than nothing. But seeing how clunky and
>>> slow jdk-submit is (and how often there are problems), I rather fail
>>> early in my own build than waiting for jdk-submit to tell me something
>>> went wrong (well, that is why I usually build nonpch, like Ioi does).
>>>
>>> Just my 5 cent.
>> I hear you, loud and clear. :) I've created https://bugs.openjdk.java.net/browse/JDK-8213241 to disable PCH by default, for all builds, on gcc. (I'm interpreting "linux" in this case as "gcc", since this is compiler-dependent, and not OS dependent).
>>
>> /Magnus
>>
>>> ..Thomas
>>>> /Magnus
>>>>
>>>>
>>>>> -Aleksey
>>>>>



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