lib/systems/inspect.nix: remove isPowerPC

Very confusingly, the `isPowerPC` predicate in
`lib/systems/inspect.nix` does *not* match `powerpc64le`!

This is because `isPowerPC` is defined as

  isPowerPC      = { cpu = cpuTypes.powerpc; };

Where `cpuTypes.powerpc` is:

  { bits = 32; significantByte = bigEndian; family = "power"; };

This means that the `isPowerPC` predicate actually only matches the
subset of machines marketed under this name which happen to be 32-bit
and running in big-endian mode which is equivalent to:

  with stdenv.hostPlatform; isPower && isBigEndian && is32bit

This seems like a sharp edge that people could easily cut themselves
on.  In fact, that has already happened: in
`linux/kernel/common-config.nix` there is a test which will always
fail:

  (stdenv.hostPlatform.isPowerPC && stdenv.hostPlatform.is64bit)

A more subtle case of the strict isPowerPC being used instead of the
moreg general isPower accidentally are the GHC expressions:

  Update pkgs/development/compilers/ghc/8.10.7.nix
  Update pkgs/development/compilers/ghc/8.8.4.nix
  Update pkgs/development/compilers/ghc/9.2.2.nix
  Update pkgs/development/compilers/ghc/9.0.2.nix
  Update pkgs/development/compilers/ghc/head.nix

Since the remaining legitimate use sites of isPowerPC are so few, remove
the isPowerPC predicate completely. The alternative expression above is
noted in the release notes as an alternative.

Co-authored-by: sternenseemann <sternenseemann@systemli.org>

3 files changed, 20 insertions, 2 deletions

diff --git a/nixos/doc/manual/from_md/release-notes/rl-2211.section.xml b/nixos/doc/manual/from_md/release-notes/rl-2211.section.xml
index 79268b398e60f..908d1ab46e8f1 100644
--- a/nixos/doc/manual/from_md/release-notes/rl-2211.section.xml
+++ b/nixos/doc/manual/from_md/release-notes/rl-2211.section.xml
@@ -51,7 +51,7 @@
   </section>
   <section xml:id="sec-release-22.11-incompatibilities">
     <title>Backward Incompatibilities</title>
-    <itemizedlist spacing="compact">
+    <itemizedlist>
       <listitem>
         <para>
           The <literal>isCompatible</literal> predicate checking CPU
@@ -69,6 +69,21 @@
           compatible</emphasis>.
         </para>
       </listitem>
+      <listitem>
+        <para>
+          The <literal>isPowerPC</literal> predicate, found on
+          <literal>platform</literal> attrsets
+          (<literal>hostPlatform</literal>,
+          <literal>buildPlatform</literal>,
+          <literal>targetPlatform</literal>, etc) has been removed in
+          order to reduce confusion. The predicate was was defined such
+          that it matches only the 32-bit big-endian members of the
+          POWER/PowerPC family, despite having a name which would imply
+          a broader set of systems. If you were using this predicate,
+          you can replace <literal>foo.isPowerPC</literal> with
+          <literal>(with foo; isPower &amp;&amp; is32bit &amp;&amp; isBigEndian)</literal>.
+        </para>
+      </listitem>
     </itemizedlist>
   </section>
   <section xml:id="sec-release-22.11-notable-changes">
diff --git a/nixos/doc/manual/release-notes/rl-2211.section.md b/nixos/doc/manual/release-notes/rl-2211.section.md
index 7d2eacce57fed..87fdf4e77f250 100644
--- a/nixos/doc/manual/release-notes/rl-2211.section.md
+++ b/nixos/doc/manual/release-notes/rl-2211.section.md
@@ -37,6 +37,9 @@ In addition to numerous new and upgraded packages, this release has the followin
   `lib.systems.parse.isCompatible` still exists, but has changed semantically:
   Architectures with differing endianness modes are *no longer considered compatible*.
 
+- The `isPowerPC` predicate, found on `platform` attrsets (`hostPlatform`, `buildPlatform`, `targetPlatform`, etc) has been removed in order to reduce confusion.  The predicate was was defined such that it matches only the 32-bit big-endian members of the POWER/PowerPC family, despite having a name which would imply a broader set of systems.  If you were using this predicate, you can replace `foo.isPowerPC` with `(with foo; isPower && is32bit && isBigEndian)`.
+
+
 <!-- To avoid merge conflicts, consider adding your item at an arbitrary place in the list instead. -->
 
 ## Other Notable Changes {#sec-release-22.11-notable-changes}
diff --git a/nixos/modules/installer/cd-dvd/iso-image.nix b/nixos/modules/installer/cd-dvd/iso-image.nix
index 1eaa940afb1fc..d1ccc6c2072f7 100644
--- a/nixos/modules/installer/cd-dvd/iso-image.nix
+++ b/nixos/modules/installer/cd-dvd/iso-image.nix
@@ -479,7 +479,7 @@ in
                 + lib.optionalString (isx86_32 || isx86_64) "-Xbcj x86"
                 # Untested but should also reduce size for these platforms
                 + lib.optionalString (isAarch32 || isAarch64) "-Xbcj arm"
-                + lib.optionalString (isPowerPC) "-Xbcj powerpc"
+                + lib.optionalString (isPower && is32bit && isBigEndian) "-Xbcj powerpc"
                 + lib.optionalString (isSparc) "-Xbcj sparc";
       description = ''
         Compression settings to use for the squashfs nix store.