Add instructions & environment for LLVM devs (#25)

Author: khaser

README.md

@@ -1,29 +1,79 @@
 # LLVM fork to work on RISC-V V-ext vectorizing issues
 
-## Repository layout
-TODO
+## Working on LLVM
 
-## Getting dev shell
+### Getting dev shell
 * Install `nix`:
 ```
 sh <(curl --proto '=https' --tlsv1.2 -L https://nixos.org/nix/install) --no-daemon
 ```
 * Enable flakes experimental feature:
 ```
 mkdir ~/.config/nix/
-echo "experimental-features = nix-command flakes" > ~/.config/nix/nix.conf
+echo "experimental-features = nix-command flakes impure-derivations ca-derivations" > ~/.config/nix/nix.conf
 ```
 * Enter to nix shell
 ```
-nix develop
+nix develop '.#toolchain'
 ```
 * (Optional) Setup [direnv](https://github.com/direnv/direnv) to enter into nix shell automatically
 
+### Setup connection to binary cache
+#### Why is that necessary?
+Every nix package have unique hash, which can be computed before building process.
+When you build any thing, nix firstly tries to find built package across connected binary caches.
+Cross LLVM build process requires a lot of tools,
+not all of which are available as binary package in public package repositories.
+After first LLVM build (which I'am already done) all of this packages become available in our binary package cache.
 
-## Building
+#### Setup ssh access
+Firstly, you need setup ssh access to specified user **for `root` user in your system**.
+```
+host vityaman-nix-storage
+    user nix-storage
+    hostname 62.84.116.90
+    port 22
+    identityfile ~/.ssh/nix-storage_id_rsa
+```
+Private key can be found in telegram developers chat.
+#### Setup nix substituters
+Next, you need add our host into *substituters*.
+Usually, it can be done by adding following line into `~/.config/nix/nix.conf`:
+```
+substituters = ssh://vityaman-nix-storage
+```
+
+### Building LLVM
+Enter into nix shell:
+```
+nix develop
+```
 
-Inside nix shell:
+Then build libllvm in normal way:
 ```
 cmake $cmakeFlags -S llvm -B build
 ninja -C build
 ```
+
+After that, you can use built libllvm with upstream clang, which preinstalled in nix shell.
+Minimal example to compile code for Linux/RISC-V environment.
+```
+// inside `nix develop` shell
+riscv64-unknown-linux-gnu-clang++ hello.cpp -emit-llvm -S -o hello.ll
+../build/bin/llc -march=riscv64 --relocation-model=pic hello.ll -o hello.s
+riscv64-unknown-linux-gnu-clang++ hello.s hello.o
+```
+For details see [LLVM docs](https://llvm.org/docs/GettingStarted.html).
+
+### Running performance testing
+TODO[khaser]: not implemented yet
+
+### Running regression testing
+TODO[khaser]: not implemented yet
+
+### Misc
+Original LLVM readme can be found in [./README-llvm.md](./README-llvm.md)
+
+### Infra notes
+TODO[khaser]: write notes
+

flake.nix

@@ -14,14 +14,15 @@
       circt = (import nixpkgs-circt { inherit system; }).circt;
       pkgsRV = pkgs.pkgsCross.riscv64;
       targetLlvmLibraries = pkgsRV.llvmPackages_21;
+      patched-libllvm = (targetLlvmLibraries.libllvm.override { src = ./.; });
     in rec {
 
       defaultPackage = packages.toolchain;
 
       packages.toolchain = with pkgsRV; (wrapCCWith rec {
         cc = (targetLlvmLibraries.clang-unwrapped.override {
                 src = ./.;
-                libllvm = (targetLlvmLibraries.libllvm.override { src = ./.; });
+                libllvm = patched-libllvm;
               });
         # libstdcxx is taken from gcc in an ad-hoc way in cc-wrapper.
         libcxx = null;
@@ -98,11 +99,18 @@
         '';
       };
 
-      devShell = (defaultPackage.overrideAttrs (oldAttrs: {
-        name = "llvm-env";
-        buildInputs = oldAttrs.buildInputs;
-      }));
+      devShells.default = targetLlvmLibraries.stdenv.mkDerivation {
+        name = "devShell";
+        # buildInputs = [];
+        # ++ (with patched-libllvm; nativeBuildInputs ++ buildInputs ++ propagatedBuildInputs);
 
+        cmakeFlags = [
+          "-GNinja"
+          "-DCMAKE_BUILD_TYPE=Debug"
+          "-DLLVM_TARGETS_TO_BUILD=RISCV"
+        ];
+
+      };
 
     });
 }