Toolchain management

Toolchains are comprised by a GNAT compiler and a gprbuild project file processor. Alire strives to simplify the availability of GNAT releases, which are packaged to be retrieved and used with ease.

The user can now select a preferred compiler via alr toolchain --select. Releases may still override this selection (for example to use a cross-compiler). Several compilers can be installed simultaneously, but only the last one --selected will be used by default. The rest will be used in preference to uninstalled compilers, if the default one does not match some crate’s dependencies.

There are two kinds of dependencies on GNAT compilers: generic dependencies on the gnat crate, which apply to every compiler, and dependencies on a precise native or cross-compiler.

The native compiler for each platform is always gnat_native, whereas cross-compilers follow the naming convention gnat_<target> (e.g., gnat_riscv_elf). Alire will only offer to install valid cross-compilers for the host platform.

Identifying available compilers

Compilers available for download can be listed with alr search --full --external-detect gnat_. They will also be shown by the selection assistant, alr toolchain --select.

Running alr toolchain without arguments will show the installed compilers and the preferred compiler, if any.

Automatic compiler installation

If a crate requires a target-specific compiler via its dependencies, alr will attempt to use first a matching installed compiler. Otherwise, a matching compiler will be automatically downloaded.

Automatic compiler selection by Alire

When a build is launched, or alr printenv is run to obtain a build environment, alr will use the available compilers as follows:

1. A target-specific compiler needed due to dependencies will be selected.
2. Otherwise, if the user has defined a preferred compiler, it will be selected.
3. If no preferred compiler has been defined, Alire will rely on the existing user environment.

Within these conditions, a compiler already downloaded will be preferred. Downloading a new compiler will be attempted only if no matching compiler is already available.

Specifying dependencies on GNAT compilers for crate publishing

From the point of view of a user preparing a release for publication, these are the considerations to bear in mind:

• Do not use any dependency on a compiler if you do not have a reason to do so.
• Use dependencies on gnat to specify known compiler version restrictions.
• Use dependencies on a precise gnat cross-compiler (e.g., gnat_riscv_elf) when your crate is purposely only available for such a target.
• It is normally not necessary to specify a dependency on the native compiler (gnat_native) as that would unnecessarily limit the availability of a library crate that might be useful to dependent cross-target crates.
• It may be useful to depend on gnat_native in cases where a crate builds a tool to be used always in the host platform, for example to be used in some action during the build process.

Note about cross-compilation

Independently of the compiler made available by alr in the environment, the crate project file still must define an appropriate Target attribute for the desired compiler. At the moment, Alire does not examine project file contents to identify necessary compilers, and relies only on regular depends-on dependencies.