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. So the developer default should perhaps be to keep PCH, and we
should only configure the CI builds to do without PCH.
/Magnus
-Aleksey
