[Build] Fix CUDA arch detection producing kernel-less builds on SM121 (#49904)

CMakeLists.txt

@@ -219,10 +219,8 @@ if(VLLM_GPU_LANG STREQUAL "CUDA")
   # the set of architectures we want to compile for and remove the from the
   # CMAKE_CUDA_FLAGS so that they are not applied globally.
   #
-  # `+PTX` in TORCH_CUDA_ARCH_LIST is not preserved here. It is emitted by torch
-  # as `code=compute_*`, while extract_unique_cuda_archs_ascending() records only
-  # `arch=compute_*`. If a kernel really needs PTX, add `+PTX` to that kernel's
-  # component-specific arch list below.
+  # `+PTX` in TORCH_CUDA_ARCH_LIST is not preserved here. If a kernel really
+  # needs PTX, add `+PTX` to that kernel's component-specific arch list below.
   #
   clear_cuda_arches(CUDA_ARCH_FLAGS)
   extract_unique_cuda_archs_ascending(CUDA_ARCHS "${CUDA_ARCH_FLAGS}")
@@ -232,6 +230,13 @@ if(VLLM_GPU_LANG STREQUAL "CUDA")
   cuda_archs_loose_intersection(CUDA_ARCHS
     "${CUDA_SUPPORTED_ARCHS}" "${CUDA_ARCHS}")
   message(STATUS "CUDA supported target architectures: ${CUDA_ARCHS}")
+  if(NOT CUDA_ARCHS)
+    message(FATAL_ERROR
+      "No supported CUDA architectures; the build would produce a binary "
+      "with no usable kernels. Detected gencode flags: ${CUDA_ARCH_FLAGS}; "
+      "supported: ${CUDA_SUPPORTED_ARCHS}. "
+      "Set TORCH_CUDA_ARCH_LIST for your GPU (e.g. 12.0).")
+  endif()
 else()
   #
   # For other GPU targets override the GPU architectures detected by cmake/torch

cmake/utils.cmake

@@ -241,14 +241,15 @@ endmacro()
 # `<major>.<minor>`, dedupes them and then sorts them in ascending order and 
 # stores them in `OUT_ARCHES`.
 #
-# Example:
-#   CUDA_ARCH_FLAGS="-gencode arch=compute_75,code=sm_75;...;-gencode arch=compute_90a,code=sm_90a" 
-#   extract_unique_cuda_archs_ascending(OUT_ARCHES CUDA_ARCH_FLAGS)
-#   OUT_ARCHES="7.5;...;9.0"
+# Prefer `code=sm_*`; fall back to `arch=compute_*` for PTX-only flags.
+# This handles mismatches such as `arch=compute_20,code=sm_121`.
 function(extract_unique_cuda_archs_ascending OUT_ARCHES CUDA_ARCH_FLAGS)
   set(_CUDA_ARCHES)
   foreach(_ARCH ${CUDA_ARCH_FLAGS})
-    string(REGEX MATCH "arch=compute_\([0-9]+[af]?\)" _COMPUTE ${_ARCH})
+    string(REGEX MATCH "code=sm_\([0-9]+[af]?\)" _COMPUTE ${_ARCH})
+    if (NOT _COMPUTE)
+      string(REGEX MATCH "arch=compute_\([0-9]+[af]?\)" _COMPUTE ${_ARCH})
+    endif()
     if (_COMPUTE)
       set(_COMPUTE ${CMAKE_MATCH_1})
     endif()

tests/test_cmake_utils.py

@@ -21,3 +21,28 @@ endif()
     )
 
     subprocess.run(["cmake", "-P", script], check=True)
+
+
+def test_extract_archs_prefers_sass_target_over_corrupted_virtual_arch(
+    tmp_path: Path,
+):
+    """torch's autodetection can emit a bogus arch=compute_* half (e.g.
+    capability 12.1 corrupted to arch=compute_20,code=sm_121); the SASS
+    target must win, while PTX-only entries keep the virtual arch."""
+    repo_root = Path(__file__).parents[1]
+    script = tmp_path / "test_extract_archs.cmake"
+    script.write_text(
+        f"""
+cmake_minimum_required(VERSION 3.26)
+include("{repo_root / "cmake" / "utils.cmake"}")
+extract_unique_cuda_archs_ascending(actual
+  "-gencode arch=compute_20,code=sm_121;\
+-gencode arch=compute_80,code=sm_80;\
+-gencode arch=compute_80,code=compute_80")
+if(NOT "${{actual}}" STREQUAL "8.0;12.1")
+  message(FATAL_ERROR "Expected '8.0;12.1', got '${{actual}}'")
+endif()
+"""
+    )
+
+    subprocess.run(["cmake", "-P", script], check=True)