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.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 >>>> >
