RFR: 8367972: ZGC: Reduce ZBarrierSet includes

[Axel Boldt-Christmas]

On Fri, 19 Sep 2025 11:48:30 GMT, Francesco Andreuzzi <fandreuzzi at openjdk.org> wrote:

> Hi @xmas92, some more data about this change.

Thanks for checking @fandreuz.

Seems like a nice improvement. But it is unclear to me where we draw the line for inclusion in precompiled headers. Looking at that data we see that we reduced the number of times we include these files by 95% but also increased the average time we spent per inclusion by ~700%. Meaning that before the includes resulted in very little extra job for most compilation units (but still added up). But now most includes actually make use of the files and require more time. 

I believe @stefank has mentioned to me at some point the ability to have precompiled headers on a per directory basis. Because it looks like none of the gc/z includes really qualify for being in the `precompiled.hpp` file, but may belong in a hypothetical `gc/z/precompiled.hpp` file. 

I say this as someone that almost always configure my local builds without precompiled headers and have limited knowledge and experience of when they are beneficial.

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PR Comment: https://git.openjdk.org/jdk/pull/27386#issuecomment-3317120303