Stop using precompiled headers for Linux?

Magnus Ihse Bursie magnus.ihse.bursie at oracle.com
Thu Nov 1 12:09:28 UTC 2018



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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