> 1 nov. 2018 kl. 17:49 skrev Erik Joelsson <erik.joels...@oracle.com>: > >> On 2018-11-01 08:17, Magnus Ihse Bursie wrote: >>> 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 > But the Visual Studio compiler seems to be able to gain much more build > performance from it. I don't have fresh numbers but I definitely remember a > non PCH build on Windows taking more than double the time, if not triple or > quadruple.
Could that be due to a lower starting point? I mean, if the windows compilation takes more time in the base case, it's easier to improve times with PCH. /Magnus > > /Erik >> /Magnus >> >>> >>> Thanks >>> Ioi >>> >>>> On Nov 1, 2018, at 5:09 AM, Magnus Ihse Bursie >>>> <magnus.ihse.bur...@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.bur...@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 >
