RFR(XS): 7175914: Usage of gcc with precompiled headers produces wrong build dependencies

[Bengt Rutisson]

Hi Volker,

I am in no way an expert on precompiled headers, but for my own 
understanding I have a question regarding this change.

In hotspot there should be no files that rely on dependencies only being 
included through precompiled headers. On Solaris we don't support 
precompiled headers, so unless there are issues with Linux specific 
files, there should not be any files that only rely on includes through 
precompiled headers.

You mention that the documentation for -fpch-deps is a bit fuzzy and I 
have to agree. My question is: if gcc resolves a dependency through 
precompiled headers will it then skip listing that dependency even if it 
is explicitly present in the source file as an #include? Is this what 
-fpch-deps fixes?

Thanks,
Bengt

On 2012-06-12 11:34, Frederic Parain wrote:
> Looks good.
>
> Could you just fix copyright year in make/bsd/makefiles/gcc.make
> and make/solaris/makefiles/gcc.make.
>
> Thanks,
>
> Fred
>
> On 06/11/12 06:32 PM, Volker Simonis wrote:
>> Hi,
>>
>> please review and push this little change which fixes the 
>> dependencies for
>> gcc builds which use precompiled headers:
>>
>> http://cr.openjdk.java.net/~simonis/7175914/
>>
>> Below is some background:
>>
>> Dependencies which are resolved from the precompiled headers file during
>> compilation are not by default listed as dependencies in the 
>> dependencies file
>> generated by gcc with the '-MMD' option. This leads to incomplete 
>> dependencies
>> and potentially bogus incremental builds. To fix the problem, we have to
>> additionally use the '-fpch-deps' option which instructs gcc to include
>> dependencies from the precompiled headers file as well.
>>
>> Notice that the gcc documentation (at least for gcc 4.1 and 4.4) is 
>> imprecise
>> with respect to precompiled headers and dependencies genereation. The
>> description of the flags for the generation of the dependency-files 
>> (-M, -MM,
>> -MMD, ..) don't even mention that these flags may be impacted by the 
>> use of
>> precompiled headers. The description of '-fpch-deps' is wrong in the 
>> sense that
>> it claims that if it is not set, a dependency for the precompiled 
>> header file
>> will be generated. This is not true for gcc 4.1 and 4.4. If 
>> '-fpch-deps' is not
>> set, the generated dependencies file only contains dependencies which 
>> are not
>> resolved trough the precompiled headers file. If '-fpch-deps' is set, 
>> the
>> dependencies file contains all the dependencies, some of them even 
>> several
>> times! While this is not nice and unnecessarily bloats the 
>> dependencies files,
>> it is at least correct.
>>
>> Another undocumented pitfall is the fact that the precompiled header 
>> file has
>> to be prepared with '-M' as well in order to make '-fpch-deps' work. 
>> This is at
>> least suggested by the gcc bug
>> "28483 - Dependency tracking should be on by default when copiling 
>> .gchs" [1].
>> Fortunately, the command line used by the HotSpot build to create the
>> precompiled headers file already contains these parameters, so we 
>> don't have to
>> do anything in this case.
>>
>> And there's also another unresolved gcc bug
>> "14933 - missing pre-compiled header depends with -MD" [2]
>> which seems to be obsoleted by the '-fpch-deps' option but which I
>> want to mention
>> here for reference.
>>
>> Regards,
>> Volker
>>
>> [1] http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28483
>> [2] http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14933
>