Detect the host compiler instead of hardcoding gcc-15

CHANGELOG.md

@@ -18,6 +18,15 @@ and the project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.
   also unbreaks the case where the destination directory exists but is empty:
   the old code seeded a status file into it, and `git clone` then refused the
   now-non-empty directory.
+- **`check-env` passed on a machine that could not compile.** `cuda-env.sh`
+  hardcoded `CC=/usr/bin/gcc-15`, and `check-env` printed that path without ever
+  testing it — so a host without gcc 15 installed sailed through the gate and
+  died in cmake twelve minutes later, after a 1.4 GB clone and a full submodule
+  sync (`is not a full path to an existing compiler tool`). The host compiler is
+  now probed (`gcc-15`, `gcc-14`, `gcc-13`, then plain `gcc`), and `check-env`
+  fails if `CC`/`CXX` are not executable, or if the compiler is newer than the
+  cap in the toolkit's own `crt/host_config.h` — which is what a rolling distro
+  hands you by default, since CUDA 13.3 stops at gcc 15 and Manjaro is on 16.
 - **The pytorch preset was missing two build requirements.** Upstream builds
   through `scikit-build-core` now, and `--no-build-isolation` installs nothing on
   our behalf, so `numpy` and `scikit-build-core` have to be seeded into the venv

Makefile

@@ -108,6 +108,16 @@ check-env: ## Validate prerequisites: CUDA, Python, uv, GPU, disk
 	@[ -n "$(CUDA_HOME)" ] || { echo "ERROR: CUDA_HOME not set. Run: source scripts/cuda-env.sh"; exit 1; }
 	@[ -d "$(CUDA_HOME)" ] || { echo "ERROR: CUDA_HOME=$(CUDA_HOME) does not exist"; exit 1; }
 	@$(call require_bins,nvidia-smi python)
+	@[ -x "$(CC)" ]  || { echo "ERROR: CC=$(CC) is not an executable compiler. Install a CUDA-supported gcc (pacman -S gcc15 | apt install gcc-15 g++-15), or export CC/CXX"; exit 1; }
+	@[ -x "$(CXX)" ] || { echo "ERROR: CXX=$(CXX) is not an executable compiler. Install a CUDA-supported g++ (pacman -S gcc15 | apt install gcc-15 g++-15), or export CC/CXX"; exit 1; }
+	@hdr="$(CUDA_HOME)/include/crt/host_config.h"; \
+	  max=$$(grep -oE '__GNUC__ > [0-9]+' "$$hdr" 2>/dev/null | head -1 | grep -oE '[0-9]+$$' || true); \
+	  have=$$("$(CXX)" -dumpversion 2>/dev/null | cut -d. -f1 || true); \
+	  if [ -n "$$max" ] && [ -n "$$have" ] && [ "$$have" -gt "$$max" ]; then \
+	    echo "ERROR: $(CXX) is gcc $$have, and this CUDA supports up to gcc $$max — nvcc will refuse to compile."; \
+	    echo "       Install gcc $$max and re-source scripts/cuda-env.sh, or export CC/CXX to one."; \
+	    exit 1; \
+	  fi
 	@[ -x "$(UV)" ] || { echo "ERROR: uv not found at $(UV). Install: curl -LsSf https://astral.sh/uv/install.sh | sh"; exit 1; }
 	@[ -n "$(TORCH_CUDA_ARCH_LIST)" ] || echo "WARN: TORCH_CUDA_ARCH_LIST unset — will compile every arch (~9x slower)"
 	@echo "python               $$(python --version 2>&1)"

README.md

@@ -75,15 +75,15 @@ Anything scoring 4–5 gets resolved before you compile. Every row cost a real b
 ```bash
 git clone https://github.com/TGPSKI/conceit.git
 cd conceit
-source scripts/cuda-env.sh   # CUDA_HOME, TORCH_CUDA_ARCH_LIST=12.0+PTX, gcc-15
+source scripts/cuda-env.sh   # CUDA_HOME, TORCH_CUDA_ARCH_LIST=12.0+PTX, host compiler
 make check-env               # fails fast on anything missing
 ```
 
 `make help` lists every target. Nothing is installed system-wide; the source trees and venvs live under `src/` in the checkout.
 
 **Tested on:** Blackwell `sm_120` (RTX PRO 4500), CUDA 13.3, Python 3.14.6 via asdf, gcc/g++ 15, x86_64 Manjaro. Hopper and Ampere work by setting `TORCH_CUDA_ARCH_LIST`. Budget ~200 GB of disk and 32 GB of RAM for parallel nvcc.
 
-**You need:** the CUDA 13.3 toolkit (`scripts/install-cuda-toolkit.sh` handles the gcc-15 shim), `uv`, and — for the full torchvision/torchaudio builds — `libjpeg-turbo libpng ffmpeg sox`.
+**You need:** the CUDA 13.3 toolkit (`scripts/install-cuda-toolkit.sh` handles the gcc-15 shim), a gcc no newer than your CUDA supports (13.3 caps at 15 — `pacman -S gcc15`, `apt install gcc-15 g++-15`), `uv`, and — for the full torchvision/torchaudio builds — `libjpeg-turbo libpng ffmpeg sox`. `cuda-env.sh` picks the newest supported `gcc-*` it finds; `make check-env` fails if the result is missing or too new for the toolkit.
 
 ## Configure
 

scripts/cuda-env.sh

@@ -56,8 +56,22 @@ unset _root
 # Leaving this unset compiles every supported arch — roughly 9x the nvcc time.
 export TORCH_CUDA_ARCH_LIST="${TORCH_CUDA_ARCH_LIST:-12.0+PTX}"
 
-export CC="${CC:-/usr/bin/gcc-15}"
-export CXX="${CXX:-/usr/bin/g++-15}"
+# nvcc refuses a host compiler newer than the cap in its own host_config.h —
+# CUDA 13.3 stops at gcc 15 — and a rolling distro's default gcc runs ahead of
+# that (Manjaro is on 16). So prefer an explicitly versioned binary, newest
+# first, and fall back to the unversioned one only when no versioned gcc is
+# installed. Hardcoding a path here is what makes `make check-env` pass on a
+# machine that cannot compile: check-env now verifies whatever this resolves to
+# against the toolkit's own cap.
+_host_cc() {
+  local c
+  for c in "$1-15" "$1-14" "$1-13" "$1"; do
+    command -v "$c" 2>/dev/null && return 0
+  done
+}
+
+export CC="${CC:-$(_host_cc gcc)}"
+export CXX="${CXX:-$(_host_cc g++)}"
 export CUDAHOSTCXX="${CUDAHOSTCXX:-$CXX}"
 export CMAKE_C_COMPILER="${CMAKE_C_COMPILER:-$CC}"
 export CMAKE_CXX_COMPILER="${CMAKE_CXX_COMPILER:-$CXX}"