dev/nixpkgs
1
0
Fork 0
Code Issues Pull Requests Actions 2 Packages Projects Releases Wiki Activity

doc/cross-compilation: fixup

Browse Source 
More cleanups and stuff. May need to be split up.
This commit is contained in:
Matthew Bauer 2018-11-18 23:23:22 -06:00
parent ee58ab3cb9
commit 9d3108c3ae
2 changed files with 98 additions and 98 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

188
doc/cross-compilation.xml
View File

@ -6,17 +6,17 @@
<title>Introduction</title>
<para>
"Cross-compilation" means compiling a program on one machine for another
type of machine. For example, a typical use of cross compilation is to
compile programs for embedded devices. These devices often don't have the
computing power and memory to compile their own programs. One might think
that cross-compilation is a fairly niche concern, but there are advantages
to being rigorous about distinguishing build-time vs run-time environments
even when one is developing and deploying on the same machine. Nixpkgs is
increasingly adopting the opinion that packages should be written with
cross-compilation in mind, and nixpkgs should evaluate in a similar way (by
minimizing cross-compilation-specific special cases) whether or not one is
cross-compiling.
"Cross-compilation" means compiling a program on one machine for another type
of machine. For example, a typical use of cross-compilation is to compile
programs for embedded devices. These devices often don't have the computing
power and memory to compile their own programs. One might think that
cross-compilation is a fairly niche concern. However, there are significant
advantages to rigorously distinguishing between build-time and run-time
environments! This applies even when one is developing and deploying on the
same machine. Nixpkgs is increasingly adopting the opinion that packages
should be written with cross-compilation in mind, and nixpkgs should evaluate
in a similar way (by minimizing cross-compilation-specific special cases)
whether or not one is cross-compiling.
</para>
<para>
@ -34,15 +34,15 @@
<title>Platform parameters</title>
<para>
Nixpkgs follows the
<link xlink:href="https://gcc.gnu.org/onlinedocs/gccint/Configure-Terms.html">common
historical convention of GNU autoconf</link> of distinguishing between 3
types of platform: <wordasword>build</wordasword>,
<wordasword>host</wordasword>, and <wordasword>target</wordasword>. In
summary, <wordasword>build</wordasword> is the platform on which a package
is being built, <wordasword>host</wordasword> is the platform on which it
is to run. The third attribute, <wordasword>target</wordasword>, is
relevant only for certain specific compilers and build tools.
Nixpkgs follows the <link
xlink:href="https://gcc.gnu.org/onlinedocs/gccint/Configure-Terms.html">conventions
of GNU autoconf</link>. We distinguish between 3 types of platforms when
building a derivation: <wordasword>build</wordasword>,
<wordasword>host</wordasword>, and <wordasword>target</wordasword>. In
summary, <wordasword>build</wordasword> is the platform on which a package
is being built, <wordasword>host</wordasword> is the platform on which it
will run. The third attribute, <wordasword>target</wordasword>, is relevant
only for certain specific compilers and build tools.
</para>
<para>
@ -64,7 +64,7 @@
<para>
The "build platform" is the platform on which a package is built. Once
someone has a built package, or pre-built binary package, the build
platform should not matter and be safe to ignore.
platform should not matter and can be ignored.
</para>
</listitem>
</varlistentry>
@ -94,11 +94,11 @@
<para>
The build process of certain compilers is written in such a way that the
compiler resulting from a single build can itself only produce binaries
for a single platform. The task specifying this single "target platform"
is thus pushed to build time of the compiler. The root cause of this
mistake is often that the compiler (which will be run on the host) and
the the standard library/runtime (which will be run on the target) are
built by a single build process.
for a single platform. The task of specifying this single "target
platform" is thus pushed to build time of the compiler. The root cause of
this that the compiler (which will be run on the host) and the standard
library/runtime (which will be run on the target) are built by a single
build process.
</para>
<para>
There is no fundamental need to think about a single target ahead of
@ -135,8 +135,10 @@
<para>
This is a two-component shorthand for the platform. Examples of this
would be "x86_64-darwin" and "i686-linux"; see
<literal>lib.systems.doubles</literal> for more. This format isn't very
standard, but has built-in support in Nix, such as the
<literal>lib.systems.doubles</literal> for more. The first component
corresponds to the CPU architecture of the platform and the second to the
operating system of the platform (<literal>[cpu]-[os]</literal>). This
format has built-in support in Nix, such as the
<varname>builtins.currentSystem</varname> impure string.
</para>
</listitem>
@ -147,12 +149,13 @@
</term>
<listitem>
<para>
This is a 3- or 4- component shorthand for the platform. Examples of
this would be "x86_64-unknown-linux-gnu" and "aarch64-apple-darwin14".
This is a standard format called the "LLVM target triple", as they are
pioneered by LLVM and traditionally just used for the
<varname>targetPlatform</varname>. This format is strictly more
informative than the "Nix host double", as the previous format could
This is a 3- or 4- component shorthand for the platform. Examples of this
would be <literal>x86_64-unknown-linux-gnu</literal> and
<literal>aarch64-apple-darwin14</literal>. This is a standard format
called the "LLVM target triple", as they are pioneered by LLVM. In the
4-part form, this corresponds to
<literal>[cpu]-[vendor]-[os]-[abi]</literal>. This format is strictly
more informative than the "Nix host double", as the previous format could
analogously be termed. This needs a better name than
<varname>config</varname>!
</para>
@ -164,12 +167,11 @@
</term>
<listitem>
<para>
This is a nix representation of a parsed LLVM target triple with
white-listed components. This can be specified directly, or actually
parsed from the <varname>config</varname>. [Technically, only one need
be specified and the others can be inferred, though the precision of
inference may not be very good.] See
<literal>lib.systems.parse</literal> for the exact representation.
This is a Nix representation of a parsed LLVM target triple
with white-listed components. This can be specified directly,
or actually parsed from the <varname>config</varname>. See
<literal>lib.systems.parse</literal> for the exact
representation.
</para>
</listitem>
</varlistentry>
@ -249,17 +251,17 @@
</para>
<para>
Some examples will probably make this clearer. If a package is being built
with a <literal>(build, host, target)</literal> platform triple of
<literal>(foo, bar, bar)</literal>, then its build-time dependencies would
have a triple of <literal>(foo, foo, bar)</literal>, and <emphasis>those
packages'</emphasis> build-time dependencies would have triple of
<literal>(foo, foo, foo)</literal>. In other words, it should take two
"rounds" of following build-time dependency edges before one reaches a
fixed point where, by the sliding window principle, the platform triple no
longer changes. Indeed, this happens with cross compilation, where only
rounds of native dependencies starting with the second necessarily coincide
with native packages.
Some examples will make this clearer. If a package is being built with a
<literal>(build, host, target)</literal> platform triple of <literal>(foo,
bar, bar)</literal>, then its build-time dependencies would have a triple of
<literal>(foo, foo, bar)</literal>, and <emphasis>those packages'</emphasis>
build-time dependencies would have a triple of <literal>(foo, foo,
foo)</literal>. In other words, it should take two "rounds" of following
build-time dependency edges before one reaches a fixed point where, by the
sliding window principle, the platform triple no longer changes. Indeed,
this happens with cross-compilation, where only rounds of native
dependencies starting with the second necessarily coincide with native
packages.
</para>
<note>
@ -271,23 +273,23 @@
</note>
<para>
How does this work in practice? Nixpkgs is now structured so that
build-time dependencies are taken from <varname>buildPackages</varname>,
whereas run-time dependencies are taken from the top level attribute set.
For example, <varname>buildPackages.gcc</varname> should be used at build
time, while <varname>gcc</varname> should be used at run time. Now, for
most of Nixpkgs's history, there was no <varname>buildPackages</varname>,
and most packages have not been refactored to use it explicitly. Instead,
one can use the six (<emphasis>gasp</emphasis>) attributes used for
specifying dependencies as documented in
<xref linkend="ssec-stdenv-dependencies"/>. We "splice" together the
run-time and build-time package sets with <varname>callPackage</varname>,
and then <varname>mkDerivation</varname> for each of four attributes pulls
the right derivation out. This splicing can be skipped when not cross
compiling as the package sets are the same, but is a bit slow for cross
compiling. Because of this, a best-of-both-worlds solution is in the works
with no splicing or explicit access of <varname>buildPackages</varname>
needed. For now, feel free to use either method.
How does this work in practice? Nixpkgs is now structured so that build-time
dependencies are taken from <varname>buildPackages</varname>, whereas
run-time dependencies are taken from the top level attribute set. For
example, <varname>buildPackages.gcc</varname> should be used at build-time,
while <varname>gcc</varname> should be used at run-time. Now, for most of
Nixpkgs's history, there was no <varname>buildPackages</varname>, and most
packages have not been refactored to use it explicitly. Instead, one can use
the six (<emphasis>gasp</emphasis>) attributes used for specifying
dependencies as documented in <xref linkend="ssec-stdenv-dependencies"/>. We
"splice" together the run-time and build-time package sets with
<varname>callPackage</varname>, and then <varname>mkDerivation</varname> for
each of four attributes pulls the right derivation out. This splicing can be
skipped when not cross-compiling as the package sets are the same, but is a
bit slow for cross-compiling. Because of this, a best-of-both-worlds
solution is in the works with no splicing or explicit access of
<varname>buildPackages</varname> needed. For now, feel free to use either
method.
</para>
<note>
@ -305,11 +307,11 @@
<title>Cross packaging cookbook</title>
<para>
Some frequently problems when packaging for cross compilation are good to
just spell and answer. Ideally the information above is exhaustive, so this
section cannot provide any new information, but its ludicrous and cruel to
expect everyone to spend effort working through the interaction of many
features just to figure out the same answer to the same common problem.
Some frequently encountered problems when packaging for cross-compilation
should be answered here. Ideally, the information above is exhaustive, so
this section cannot provide any new information, but it is ludicrous and
cruel to expect everyone to spend effort working through the interaction of
many features just to figure out the same answer to the same common problem.
Feel free to add to this list!
</para>
@ -366,15 +368,14 @@
<note>
<para>
More information needs to moved from the old wiki, especially
<link xlink:href="https://nixos.org/wiki/CrossCompiling" />, for this
section.
More information needs to be moved from the old wiki, especially <link
xlink:href="https://nixos.org/wiki/CrossCompiling" />, for this section.
</para>
</note>
<para>
Nixpkgs can be instantiated with <varname>localSystem</varname> alone, in
which case there is no cross compiling and everything is built by and for
which case there is no cross-compiling and everything is built by and for
that system, or also with <varname>crossSystem</varname>, in which case
packages run on the latter, but all building happens on the former. Both
parameters take the same schema as the 3 (build, host, and target) platforms
@ -440,15 +441,14 @@ nix-build &lt;nixpkgs&gt; --arg crossSystem.config '&lt;arch&gt;-&lt;os&gt;-&lt;
build plan or package set. A simple "build vs deploy" dichotomy is adequate:
the sliding window principle described in the previous section shows how to
interpolate between the these two "end points" to get the 3 platform triple
for each bootstrapping stage. That means for any package a given package
set, even those not bound on the top level but only reachable via
dependencies or <varname>buildPackages</varname>, the three platforms will
be defined as one of <varname>localSystem</varname> or
<varname>crossSystem</varname>, with the former replacing the latter as one
traverses build-time dependencies. A last simple difference then is
<varname>crossSystem</varname> should be null when one doesn't want to
cross-compile, while the <varname>*Platform</varname>s are always non-null.
<varname>localSystem</varname> is always non-null.
for each bootstrapping stage. That means for any package a given package set,
even those not bound on the top level but only reachable via dependencies or
<varname>buildPackages</varname>, the three platforms will be defined as one
of <varname>localSystem</varname> or <varname>crossSystem</varname>, with the
former replacing the latter as one traverses build-time dependencies. A last
simple difference is that <varname>crossSystem</varname> should be null when
one doesn't want to cross-compile, while the <varname>*Platform</varname>s
are always non-null. <varname>localSystem</varname> is always non-null.
</para>
</section>
<!--============================================================-->
@ -461,14 +461,14 @@ nix-build &lt;nixpkgs&gt; --arg crossSystem.config '&lt;arch&gt;-&lt;os&gt;-&lt;
<note>
<para>
If one explores nixpkgs, they will see derivations with names like
<literal>gccCross</literal>. Such <literal>*Cross</literal> derivations is
a holdover from before we properly distinguished between the host and
target platforms —the derivation with "Cross" in the name covered the
<literal>build = host != target</literal> case, while the other covered the
<literal>host = target</literal>, with build platform the same or not based
on whether one was using its <literal>.nativeDrv</literal> or
<literal>.crossDrv</literal>. This ugliness will disappear soon.
If one explores Nixpkgs, they will see derivations with names like
<literal>gccCross</literal>. Such <literal>*Cross</literal> derivations is a
holdover from before we properly distinguished between the host and target
platforms—the derivation with "Cross" in the name covered the <literal>build
= host != target</literal> case, while the other covered the <literal>host =
target</literal>, with build platform the same or not based on whether one
was using its <literal>.nativeDrv</literal> or <literal>.crossDrv</literal>.
This ugliness will disappear soon.
</para>
</note>
</section>

8
doc/stdenv.xml
View File

@ -258,15 +258,15 @@ genericBuild
</para>
<para>
It is important to note dependencies are not necessarily propagated as the
same sort of dependency that they were before, but rather as the
It is important to note that dependencies are not necessarily propagated as
the same sort of dependency that they were before, but rather as the
corresponding sort so that the platform rules still line up. The exact rules
for dependency propagation can be given by assigning to each dependency two
integers based one how its host and target platforms are offset from the
depending derivation's platforms. Those offsets are given below in the
descriptions of each dependency list attribute. Algorithmically, we traverse
propagated inputs, accumulating every propagated dependency's propagated
dependenciess and adjusting them to account for the "shift in perspective"
dependencies and adjusting them to account for the "shift in perspective"
described by the current dependency's platform offsets. This results in sort
a transitive closure of the dependency relation, with the offsets being
approximately summed when two dependency links are combined. We also prune
@ -424,7 +424,7 @@ let f(h, h + 1, i) = i + h
target offset from the new derivation's platforms. These are programs used
at build time that produce code to run with code produced by the depending
package. Most commonly, these are tools used to build the runtime or
standard library taht the currently-being-built compiler will inject into
standard library that the currently-being-built compiler will inject into
any code it compiles. In many cases, the currently-being-built-compiler is
itself employed for that task, but when that compiler won't run (i.e. its
build and host platform differ) this is not possible. Other times, the