Moving Compiler Dependency Checks to Kconfig

Zack Brown — Wed, 19 Sep 2018 12:00:00 +0000

The Linux kernel config system, Kconfig, uses a macro language very similar to the make build tool's macro language. There are a few differences, however. And of course, make is designed as a general-purpose build tool while Kconfig is Linux-kernel-specific. But, why would the kernel developers create a whole new macro language so closely resembling that of an existing general-purpose tool?

One reason became clear recently when Linus Torvalds asked developers to add an entirely new system of dependency checks to the Kconfig language, specifically testing the capabilities of the GCC compiler.

It's actually an important issue. The Linux kernel wants to support as many versions of GCC as possible—so long as doing so would not require too much insanity in the kernel code itself—but different versions of GCC support different features. The GCC developers always are tweaking and adjusting, and GCC releases also sometimes have bugs that need to be worked around. Some Linux kernel features can only be built using one version of the compiler or another. And, some features build better or faster if they can take advantage of various GCC features that exist only in certain versions.

Up until this year, the kernel build system has had to check all those compiler features by hand, using many hacky methods. The art of probing a tool to find out if it supports a given feature dates back decades and is filled with insanity. Imagine giving a command that you know will fail, but giving it anyway because the specific manner of failure will tell you what you need to know for a future command to work. Now imagine hundreds of hacks like that in the Linux kernel build system.

Part of the problem with having those hacky checks in the build system is that you find out about them only during the build—not during configuration. But since some kernel features require certain GCC versions, the proper place to learn about the GCC version is at config time. If the user's compiler doesn't support a given feature, there's no reason to show that feature in the config system. It should just silently not exist.

Linus requested that developers migrate those checks into the Kconfig system and regularize them into the macro language itself. This way, kernel features with particular GCC dependencies could identify those dependencies and then show up or not show up at config time, according to whether those dependencies had been met.

That's the reason simply using make wouldn't work. The config language had to represent the results of all those ugly hacks in a friendly way that developers could make use of.

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