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

Volker Simonis volker.simonis at gmail.com
Thu Apr 9 09:34:41 UTC 2015

On Thu, Apr 9, 2015 at 10:10 AM, Andrew Dinn <adinn at redhat.com> wrote:
> On 09/04/15 09:02, Erik Joelsson wrote:
>> I think that as long as we claim to support building both with and
>> without PCH, the automatic testing should test both with and without
>> PCH. By adding one or two build targets, or perhaps change an existing
>> target, we could increase our test matrix to cover this easily.
> I think this restates Volker's original remarks in redux.

No, not at all! I don't necessarily want to test more build
configurations (that's another topic).

My point is that PCH changes the compilation semantics and can hide
real program errors. That's because with PCHs, every compilation unit
sees the full precompiled header database (i.e. all the headers which
are included in the "precompiled.hpp" PCH file). So if somebody
forgets to include a dependency X.hpp in A.cpp, A.cpp may still
compile with PCH because it includes the precompiled header file
"precompiled.hpp" which in turn includes X.hpp. But the compilation of
A.cpp will fail on platforms/configurations where we do not use
precompiled headers. The two references I gave in my original mail are
bugs that resulted from this problem.

Besides program errors, the use of PCH can also lead to behavioral
changes in the created binary when it comes to inlining. Because of
PCHs some compilation units may be able to inline methods even if they
do not explicitly include the files which contain the corresponding
implementations because the implementation files are included in the
PCH file. Without PCHs the compilers will simply emit calls to these
functions (and, depending on the toolchain, emit a warning).

I'm not familiar with ccache so I can not say if it has similar effects.

> So, given that we do need this (NOW! :-) is anyone able and willing to
> sponsor this?

Yes, this question remains to be answered :)


> regards,
> Andrew Dinn
> -----------

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