llvmPackages_7.compiler-rt fails to produce output path on armv7l

[Atemu]

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Same issue as #66693 or #85622, but on armv7l

Additional context
Add any other context about the problem here.

On @thefloweringash's hydra, the clang build first started failing with this error after going from 84cf00f to 7e07846.
I diffed the range and found 74f3b25 to be the most likely culprit.

It enabled some cmakeFlags globally which used to only be enabled on darwin and useLLVM systems.
I don't know what exactly these do but, I was able to build llvmPackages_7.compiler-rt on my rpi2 by disabling them on aarch32:

diff --git a/pkgs/development/compilers/llvm/7/compiler-rt.nix b/pkgs/development/compilers/llvm/7/compiler-rt.nix
index 055fb01215f..e30a5171008 100644
--- a/pkgs/development/compilers/llvm/7/compiler-rt.nix
+++ b/pkgs/development/compilers/llvm/7/compiler-rt.nix
@@ -20,7 +20,7 @@ stdenv.mkDerivation {
     "-DSCUDO_DEFAULT_OPTIONS=DeleteSizeMismatch=0:DeallocationTypeMismatch=0"
   ];
 
-  cmakeFlags = [
+  cmakeFlags = stdenv.lib.optionals (!stdenv.isAarch32) [
     "-DCOMPILER_RT_DEFAULT_TARGET_ONLY=ON"
     "-DCMAKE_C_COMPILER_TARGET=${stdenv.hostPlatform.config}"
     "-DCMAKE_ASM_COMPILER_TARGET=${stdenv.hostPlatform.config}"

Since I have no idea why this works and what would be affected, I'd rather have someone look over this.

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@TravisWhitaker @matthewbauer

Metadata
Please run nix-shell -p nix-info --run "nix-info -m" and paste the result.

• system: "armv7l-linux"
• host os: Linux 5.4.25, NixOS, 20.09pre-git (Nightingale)
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.3.3
• channels(root): "nixos-unstable2, nixos-20.09pre223023.fce7562cf46, nixos-unstable"
• nixpkgs: /nix/var/nix/profiles/per-user/root/channels/nixos

Maintainer information:

# a list of nixpkgs attributes affected by the problem
attribute: llvmPackages
# a list of nixos modules affected by the problem
module:

[matthewbauer]

we probably need to add armv7l here:

https://github.com/llvm/llvm-project/blob/b5973d20b388ab7aa7590757177454e17fb2e59b/compiler-rt/cmake/builtin-config-ix.cmake#L27

[Atemu]

I tried but it fails with this error now:

CMake Error at cmake/Modules/AddCompilerRT.cmake:273 (add_library):
  No SOURCES given to target: clang_rt.builtins-armv7l
Call Stack (most recent call first):
  lib/builtins/CMakeLists.txt:602 (add_compiler_rt_runtime)

I guess it can't find a default target for armv7l somehow?

Not really familiar with cmake unfortunately

[Atemu]

I made some progress, it seems to be taking the target arch from the triplet given to CMAKE_C_COMPILER_TARGET, even if you override the target arch with a flag. Printf debugging showed me that it does this halfway through the configure phase, it's armhf in the beginning if I set the flag.

Setting the compiler target to armhf-unknown-linux-gnueabihf instead of armv7l-unknown-linux-gnueabihf makes it find its target and compile successfully.

Does CMAKE_C_COMPILER_TARGET have any importance outside the build process?

[matthewbauer]

Does CMAKE_C_COMPILER_TARGET have any importance outside the build process?

I don't think it does. It's possible it sets -marmhfin compiler calls, but that is considered equivalent to -marmv7l.