We REALLY nead a NON-PCH build in JPRT NOW!

[Kim Barrett]

On Apr 2, 2015, at 12:06 PM, Volker Simonis <volker.simonis at gmail.com> wrote:
> 
> while precompiled headers (PCH) are a nice and effective way of
> improving hotspot build times

Are we sure about that?

Earlier today I ran into a local build failure that appeared to me to
be possibly PCH-related.

So I decided to try disabling PCH, since I now know about the option
for doing so, from Volker's email.  But I wanted to see how much
slower my build times would be.

Imagine my surprise when disabling PCH made my clean builds 10-15%
*faster*.  This is for a full jdk root "make images" build, which
makes that much of a difference even more astonishing, because there's
a lot more going on there than just compiling Hotspot.

So how sure are we that PCH (or perhaps the way we are using it)
provides any positive benefit at all?  I'm pretty sure it's not doing
anything good for me.

For reference, my build configuration is Ubuntu 14.04 x86_64,
reporting 24 processors.  My configure options are

  --enable-ccache
  --with-debug-level=release
  --disable-zip-debug-info
  --disable-precompiled-headers     -- new
  --with-jobs=24

I made sure to clear the ccache between each build.