ocamlPackages.ocaml: build fails on aarch64-apple-darwin

[lilyball]

Describe the bug

Building OCaml 4.12.0 fails on aarch64-apple-darwin (Apple Silicon). It fails most of the way through the build process. According to the build output, it runs the following:

> clang -c -O2 -fno-strict-aliasing -fwrapv -Wall -Wdeclaration-after-statement -fno-common -g  -D_FILE_OFFSET_BITS=64 -D_REENTRANT -DCAML_NAME_SPACE   -DCAMLDLLIMPORT= -DNATIVE_CODE -DTARGET_arm64 -DMODEL_default -DSYS_macosx   -o codefrag.n.o codefrag.c
> aarch64-apple-darwin-clang -c -Wno-trigraphs -DSYS_macosx -I../runtime -DMODEL_default -o arm64.o arm64.S
/nix/store/wbwcrf1g7ywmmw2lllc26r0zcdvassiq-bash-5.1-p8/bin/bash: line 1: aarch64-apple-darwin-clang: command not found

I checked the nix log of ocaml on my x86_64-apple-darwin machine, here's the corresponding commands there:

> clang -c -O2 -fno-strict-aliasing -fwrapv -Wall -Wdeclaration-after-statement -fno-common -g  -D_FILE_OFFSET_BITS=64 -D_REENTRANT -DCAML_NAME_SPACE   -DCAMLDLLIMPORT= -DNATIVE_CODE -DTARGET_amd64 -DMODEL_default -DSYS_macosx   -o codefrag.n.o codefrag.c
> clang -arch x86_64 -Wno-trigraphs -c -DSYS_macosx -I../runtime -DMODEL_default -o amd64.o amd64.S

So the x86-64 build runs clang -arch x86_64 as its assembler, but the aarch64 build runs aarch64-apple-darwin-clang instead, which doesn't exist. One might assume clang -arch aarch64 would have worked.

Running nix-shell -A ocaml '<nixpkgs>' --run env on both machines shows that the only reference to AS on the x86-64 machine is the line AS=as, but on the aarch64 machine it also has a preConfigure hook has an interesting definition:

nixpkgs/pkgs/development/compilers/ocaml/generic.nix

Lines 79 to 82 in 6f6f476

	'' + optionalString (stdenv.isDarwin && stdenv.isAarch64) ''
	# Do what upstream does by default now: https://github.com/ocaml/ocaml/pull/10176
	# This is required for aarch64-darwin, everything else works as is.
	AS="${stdenv.cc}/bin/cc -c" ASPP="${stdenv.cc}/bin/cc -c"

Of course, the build log shows aarch64-apple-darwin-clang is being invoked, not cc, and the x86-64 version isn't running as either, so I don't know how this leads to the attempted invocation of aarch64-apple-darwin-clang.

This preConfigure hook change was added in #114525, and I would assume it worked at that point, so I don't know why it's broken now.

Steps To Reproduce

Steps to reproduce the behavior:

1. Install Nix on an aarch64-apple-darwin machine
2. Update to current nixpkgs-unstable
3. Run nix-build -A ocaml '<nixpkgs>'

Expected behavior

It should build correctly

Notify maintainers

@bobrik

Metadata

 ❯ nix-shell -p nix-info --run "nix-info -m"
 - system: `"aarch64-darwin"`
 - host os: `Darwin 21.1.0, macOS 12.0.1`
 - multi-user?: `yes`
 - sandbox: `relaxed`
 - version: `nix-env (Nix) 2.4pre-rc1`
 - channels(lily): `"darwin, home-manager"`
 - channels(root): `"nixpkgs-21.11pre327990.4789953e5c1"`
 - nixpkgs: `/nix/var/nix/profiles/per-user/root/channels/nixpkgs`

Maintainer information:

# a list of nixpkgs attributes affected by the problem
attribute: ocamlPackages.ocaml
# a list of nixos modules affected by the problem
module:

[lilyball]

I reran the build with NIX_DEBUG=6 and attached the complete output.

ocaml build output.txt

[bobrik]

Last successful build: https://hydra.nixos.org/build/151577668
First broken build: https://hydra.nixos.org/build/153146299

The latter provides the range of changes: c12858..f16e0e. Bisecting with --first-parent points at:

• Staging next #135477

Which is great. we can find the first broken build there: https://hydra.nixos.org/build/157648264

This needs a bit more bisecting to figure out, but it definitely used to work.

[bobrik]

#133008 looks like it could be the reason, cc @symphorien

[bobrik]

It fails in master branch while configuring like this:

configure flags: --disable-static -prefix /nix/store/0w9n5af4m1nwps6qjxigxf6jc7h9bqww-ocaml-4.12.0 --host=aarch64-apple-darwin --target=aarch64-apple-darwin
configure: Configuring OCaml version 4.12.0
checking build system type... aarch64-apple-darwin21.1.0
checking host system type... aarch64-apple-darwin
checking target system type... aarch64-apple-darwin

With the following change it compiles:

diff --git a/pkgs/development/compilers/ocaml/generic.nix b/pkgs/development/compilers/ocaml/generic.nix
index e4cec4cf52e..ae6acfe0195 100644
--- a/pkgs/development/compilers/ocaml/generic.nix
+++ b/pkgs/development/compilers/ocaml/generic.nix
@@ -64,7 +64,6 @@ stdenv.mkDerivation (args // {
     "-target ${stdenv.targetPlatform.config}"
   ];
   dontAddStaticConfigureFlags = lib.versionOlder version "4.08";
-  configurePlatforms = lib.optionals (lib.versionAtLeast version "4.08") [ "host" "target" ];
   # x86_64-unknown-linux-musl-ld: -r and -pie may not be used together
   hardeningDisable = lib.optional (lib.versionAtLeast version "4.09" && stdenv.hostPlatform.isMusl) "pie";
 

configure flags: --disable-static -prefix /nix/store/s2m7f8vxa0i9d0vyr225y70chb4clr0p-ocaml-4.12.0
configure: Configuring OCaml version 4.12.0
checking build system type... aarch64-apple-darwin21.1.0
checking host system type... aarch64-apple-darwin21.1.0
checking target system type... aarch64-apple-darwin21.1.0

Hopefully this gives somebody a clue on how to fix this properly.

[symphorien]

I could build all of {pkgsStatic.,}ocaml-ng.ocamlPackages_4_{06,07,08,09,10,11,12}.dune_2 with this patch.

[symphorien]

cc @vbgl