Stop using precompiled headers for Linux?

[Erik Joelsson]

Hello,

On 2018-10-30 13:17, Aleksey Shipilev wrote:
> On 10/30/2018 06:26 PM, Ioi Lam wrote:
>> Is there any advantage of using precompiled headers on Linux?
> I have measured it recently on shenandoah repositories, and fastdebug/release build times have not
> improved with or without PCH. Actually, it gets worse when you touch a single header that is in PCH
> list, and you end up recompiling the entire Hotspot. I would be in favor of disabling it by default.
I just did a measurement on my local workstation (2x8 cores x2 ht Ubuntu 
18.04 using Oracle devkit GCC 7.3.0). I ran "time make hotspot" with 
clean build directories.

linux-x64:
real    4m6.657s
user    61m23.090s
sys    6m24.477s

linux-x64-npch
real    3m41.130s
user    66m11.824s
sys    4m19.224s

linux-x64-debug
real    4m47.117s
user    75m53.740s
sys    8m21.408s

linux-x64-debug-npch
real    4m42.877s
user    84m30.764s
sys    4m54.666s

linux-x64-slowdebug
real    3m54.564s
user    44m2.828s
sys    6m22.785s

linux-x64-slowdebug-npch
real    3m23.092s
user    55m3.142s
sys    4m10.172s

These numbers support your claim. Wall clock time is actually increased 
with PCH enabled, but total user time is decreased. Does not seem worth 
it to me.
>> It's on by default and we keep having
>> breakage where someone would forget to add #include. The latest instance is JDK-8213148.
> Yes, we catch most of these breakages in CIs. Which tells me adding it to jdk-submit would cover
> most of the breakage during pre-integration testing.
jdk-submit is currently running what we call "tier1". We do have builds 
of Linux slowdebug with precompiled headers disabled in tier2. We also 
build solaris-sparcv9 in tier1 which does not support precompiled 
headers at all, so to not be caught in jdk-submit you would have to be 
in Linux specific code. The example bug does not seem to be that. 
Mach5/jdk-submit was down over the weekend and yesterday so my suspicion 
is the offending code in this case was never tested.

That said, given that we get practically no benefit from PCH on 
Linux/GCC, we should probably just turn it off by default for Linux 
and/or GCC. I think we need to investigate Macos as well here.

/Erik
> -Aleksey
>