Stop using precompiled headers for Linux?

Ioi Lam ioi.lam at oracle.com
Thu Nov 1 14:53:47 UTC 2018


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?

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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