How to make a new default config? I don’t see how to practically do it

[Avi Drissman]

Right now, ARC compilation for the Mac is a config, “//build/config/compiler:enable_arc”. It’s enabled for ~800 targets, and not enabled for 5 targets. That’s an understandable result of an incremental migration, but a bad situation to be in; for something that prevalent, we want to have it enabled globally and disabled for the handful of targets that don’t want it. That’s the situation that I want to end up at.

Plan A: Step 1: In a CL, add “enable_arc” to build/config/BUILDCONFIG.gn’s default_compiler_configs, and in 5 targets remove it. Step 2: take time to go through all the 800 targets that explicitly add “enable_arc”, and remove it as it’s redundant with the global config.

That doesn’t work. GN doesn’t like when you have the same config applied twice to a target.

OK, plan B: Step 1: In a CL, create an “enable_arc2” duplicate of “enable_arc”, and add that to the default config set while subtracting it from the config set of the handful of targets that don’t want it. Step 2: remove all the “enable_arc” configs. Step 3: rename “enable_arc2” to “enable_arc”.

That would work, but there are at least four different /third_party directories whose BUILD.gn files live in external repos (/third_party/angle, /third_party/dawn, /third_party/skia, /third_party/swiftshader). They depend on /build. Therefore I would need to do a “config -= [ “enable_arc2” ]” in each of them.

But there’s a problem. Angle, Dawn, Skia, and SwiftShader are autorolled. For each of them, I can prepare a CL in that external repo that depends on “enable_arc2” but if the roll happens before the change in the Chromium repo lands, then the build will fail because the rolled version of the external dep will be referring to a non-existent config. If the roll happens after the change in the Chromium repo lands, then the Chromium change will fail because we’d be building the dep in a configuration it doesn’t support. And I can’t hold up four different autorolled external dependencies to do a coordinated commit across five repos.

While gn’s enforcement of not allowing a config to be added twice to a list is a pain that I can work around, I don’t see how to work around its enforcement of not allowing a config to be removed from a list that doesn’t contain it. I don’t see how to do large scale migrations in situations where the BUILD.gn file is deps-ed in from the outside.

Is it possible to do “remove this config from the list of configs if it exists and do nothing if not”? Is it possible to add that capability to gn, even if it’s immediately removed afterwards? What I’m doing is not unreasonable: I’m trying to make the Mac/iOS build much cleaner, and gn’s strictness seems to make it impossible to get to the cleaner future we all want.

Thanks,

Avi

[Avi Drissman]

I got a suggestion from sun...@google.com (thanks) of a Plan C:

Step 1: Add an empty "enable_arc2" config to the global configs. Step 2: Wait for it to roll into all external deps. Step 3: Add "deps -= [ "enable_arc2"] to all the external deps. Step 4: Wait for all of them to autoroll. Step 5: Redefine "enable_arc2" to enable ARC. Step 6: etc...

With a similar set of extra steps during the rename from "enable_arc2" to "enable_arc".

I guess that would work? Thoughts?

Avi

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[Tomasz Sniatowski]

Hi!

There's a filter_include trick that can -= a config if it's there, I found one existing usage at https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/core/BUILD.gn;l=420

    # Only attempt to remove the 'default_optimization' label if it is already

    # present in 'configs'.

    if (filter_include(configs,

                       [ "//build/config/compiler:default_optimization" ]) !=

        []) {

      configs -= [ "//build/config/compiler:default_optimization" ]

    }

Seems like it could save some steps.

--

Tomasz Śniatowski

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From: Avi Drissman <a...@chromium.org>
Sent: Thursday, July 13, 2023 01:32
To: gn-dev <gn-...@chromium.org>
Subject: How to make a new default config? I don’t see how to practically do it

 

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[Avi Drissman]

!!!

That's a wonderful snippet! Thank you, this should make things a lot simpler.

Avi

[Basic email]

I hope u enjoy urself   I hope u have a valid warrent for 4 yrs of controlling my phone.  And filtering my calls. Emails browser  putting me in mentorship fb groups  and throwing me out of window s when I seek it and legal advice  

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[Basic email]

[Basic email]

 I have but u use Google lens to edit every video put me in a  mentorship  group I hope u have a warrent   to  silence regular citizens it really wasn't to u bc u  are just building the program it was to the  person who is building creating and fabricating information it was the person assuming my identity  and using Google docs  word games to post as my self   enjoy  and gob bless

[Avi Drissman]

An issue with Swiftshader.

I have an .mm file in a swiftshader_source_set, and if I play the filter_include game it can't find the configs:

ERROR at //third_party/swiftshader/src/WSI/BUILD.gn:79:24: Undefined identifier
    if (filter_include(configs,
                       ^------
See //third_party/swiftshader/src/Vulkan/BUILD.gn:218:5: which caused the file to be included.
    "../WSI",
    ^-------
FAILED: build.ninja.stamp

swiftshader_source_set seems to be using the old-style "configs to add/remove" scheme. Is there a way to query what configs there are so that I can optionally remove the ones I don't want?

Avi

[Tomasz Sniatowski]

Can you do the filter_include "remove new config if it's there" dance in template("swiftshader_source_set"), in the source_set there -- for all of swiftshader?

--

Tomasz Śniatowski

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From: Avi Drissman <a...@chromium.org>
Sent: Monday, July 17, 2023 23:53
To: Tomasz Sniatowski <tsnia...@vewd.com>
Cc: gn-dev <gn-...@chromium.org>
Subject: Re: How to make a new default config? I don’t see how to practically do it