RFR (tedious) 8216022: Use #pragma once

[coleen.phillimore at oracle.com]

On 1/3/19 11:09 AM, Kim Barrett wrote:
>> On Jan 3, 2019, at 9:55 AM, coleen.phillimore at oracle.com wrote:
>> On 1/3/19 2:48 AM, Kim Barrett wrote:
>>> I think we shouldn't make this change without considering the impact
>>> of the following bug:
>>>
>>> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58770
>>> GCC very slow compiling with #pragma once
>>>
>>> According to comment 3, using "#pragma once" introduces N^2 behavior
>>> on the number of included files, because the duplicate check uses a
>>> list rather than a hashtable.
>>>
>>> At the very least, a performance comparison should be made to find out
>>> what the impact of that bug is.
>> I did a performance test of the build with and without this change and it took the same time to build hotspot on my machine (fastdebug) with and without.  Both without precompiled headers.
>>
>> without:
>> STARTED:Thu Jan  3 08:52:45 EST 2019
>>       DONE:Thu Jan  3 08:57:20 EST 2019
>> with pragma once:
>> STARTED:Thu Jan  3 08:58:44 EST 2019
>>       DONE:Thu Jan  3 09:03:19 EST 2019
>>
>>
>> It may be that more than 1700 include files might cause a slower build.  We've done a good job fixing our includes to not include everything.
> I think the precompiled headers case needs to be checked.  The recent heavy trimming
> of precompiled.hpp probably helps a lot, but it would be unfortunate to lose all and then
> some of the speedup obtained from that trimming.
>

configure + make hotspot fastdebug with precompiled headers on linux-x64.

before:
STARTED:Thu Jan  3 12:39:34 EST 2019
    DONE:Thu Jan  3 12:43:18 EST 2019

with pragma once:
STARTED:Thu Jan  3 12:44:22 EST 2019
    DONE:Thu Jan  3 12:48:02 EST 2019

Coleen