fix(nccl-tests): pin /usr/local/cuda→12.8 symlink, auto-detect gencode by nvcc version

iso/builder/Dockerfile

@@ -37,7 +37,8 @@ https://developer.download.nvidia.com/compute/cuda/repos/debian12/x86_64/ /" \
         > /etc/apt/sources.list.d/cuda.list \
     && apt-get update -qq \
     && apt-get install -y cuda-nvcc-12-8 \
-    && rm -rf /var/lib/apt/lists/*
+    && rm -rf /var/lib/apt/lists/* \
+    && ln -sfn /usr/local/cuda-12.8 /usr/local/cuda
 
 RUN arch="$(dpkg --print-architecture)" \
     && case "$arch" in \

iso/builder/build-nccl-tests.sh

@@ -101,12 +101,23 @@ SRC_DIR=$(ls -d nccl-tests-* 2>/dev/null | head -1)
 cd "$SRC_DIR"
 
 echo "=== building all_reduce_perf ==="
-# CUDA 12.8 supports Volta through Blackwell (sm_70..sm_100).
-GENCODE="-gencode=arch=compute_70,code=sm_70 \
-         -gencode=arch=compute_80,code=sm_80 \
-         -gencode=arch=compute_86,code=sm_86 \
-         -gencode=arch=compute_90,code=sm_90 \
-         -gencode=arch=compute_100,code=sm_100"
+# Pick gencode based on the actual nvcc version:
+#   CUDA 12.x — Volta..Blackwell (sm_70..sm_100)
+#   CUDA 13.x — Hopper..Blackwell (sm_90..sm_100, Pascal/Volta/Ampere dropped)
+NVCC_MAJOR=$("$NVCC" --version 2>/dev/null | grep -oE 'release [0-9]+' | awk '{print $2}' | head -1)
+echo "nvcc major version: ${NVCC_MAJOR:-unknown}"
+if [ "${NVCC_MAJOR:-0}" -ge 13 ] 2>/dev/null; then
+    GENCODE="-gencode=arch=compute_90,code=sm_90 \
+             -gencode=arch=compute_100,code=sm_100"
+    echo "gencode: sm_90 sm_100 (CUDA 13+)"
+else
+    GENCODE="-gencode=arch=compute_70,code=sm_70 \
+             -gencode=arch=compute_80,code=sm_80 \
+             -gencode=arch=compute_86,code=sm_86 \
+             -gencode=arch=compute_90,code=sm_90 \
+             -gencode=arch=compute_100,code=sm_100"
+    echo "gencode: sm_70..sm_100 (CUDA 12)"
+fi
 make MPI=0 \
     NVCC="$NVCC" \
     CUDA_HOME="$CUDA_HOME" \