[libclc] [libclc][CMake] Allow empty CMAKE_C_COMPILER_ID when CMAKE_C_COMPILER_WORKS is set (PR #186717)

[Wenju He via cfe-commits]

https://github.com/wenju-he updated https://github.com/llvm/llvm-project/pull/186717

>From 42cb1aa23e196c64161419d646a7d31694fbeca8 Mon Sep 17 00:00:00 2001
From: Wenju He <wenju.he at intel.com>
Date: Mon, 16 Mar 2026 00:24:15 +0100
Subject: [PATCH] [libclc][CMake] Allow empty CMAKE_C_COMPILER_ID when
 CMAKE_C_COMPILER_WORKS is set

When LLVM_TARGETS_TO_BUILD does not contain the host target, runtime
build can not identify a compatible target triple for the host compiler.
CMAKE_C_COMPILER is set to clang (e.g., instead of clang-cl on Windows),
and CMAKE_C_COMPILER_ID is empty although the compiler is functional.

Update CMakeDetermineCLCCompiler to manually verify compiler is clang
via '--version'. This ensures valid configurations can proceed, e.g.:
LLVM_TARGETS_TO_BUILD=AMDGPU;LIBCLC_TARGETS_TO_BUILD=amdgcn-amd-amdhsa-llvm
---
 .../modules/CMakeDetermineCLCCompiler.cmake   | 28 ++++++++++++++++---
 1 file changed, 24 insertions(+), 4 deletions(-)

diff --git a/libclc/cmake/modules/CMakeDetermineCLCCompiler.cmake b/libclc/cmake/modules/CMakeDetermineCLCCompiler.cmake
index 2138ad85d0059..7aec85fdc9daa 100644
--- a/libclc/cmake/modules/CMakeDetermineCLCCompiler.cmake
+++ b/libclc/cmake/modules/CMakeDetermineCLCCompiler.cmake
@@ -1,9 +1,29 @@
 if(NOT CMAKE_CLC_COMPILER)
-  if(NOT CMAKE_C_COMPILER_ID MATCHES "Clang")
-    message(FATAL_ERROR
-      "The CLC language requires the C compiler (CMAKE_C_COMPILER) to be "
-      "Clang, but CMAKE_C_COMPILER_ID is '${CMAKE_C_COMPILER_ID}'.")
+  set(is_clang FALSE)
+  if(CMAKE_C_COMPILER_ID MATCHES "Clang")
+    set(is_clang TRUE)
+  elseif(NOT CMAKE_C_COMPILER_ID AND CMAKE_C_COMPILER_WORKS)
+    # When ID is empty but compiler works, verify it's actually clang.
+    execute_process(
+      COMMAND "${CMAKE_C_COMPILER}" --version
+      OUTPUT_VARIABLE version_output
+      ERROR_VARIABLE version_output
+      RESULT_VARIABLE res
+    )
+    if(res EQUAL 0 AND version_output MATCHES "[Cc]lang")
+      set(is_clang TRUE)
+    endif()
   endif()
+
+  if(NOT is_clang)
+    if(CMAKE_C_COMPILER_ID)
+      set(reason "Found '${CMAKE_C_COMPILER_ID}'")
+    else()
+      set(reason "Version check failed for '${CMAKE_C_COMPILER}'")
+    endif()
+    message(FATAL_ERROR "The CLC language requires Clang. ${reason}.")
+  endif()
+
   set(CMAKE_CLC_COMPILER "${CMAKE_C_COMPILER}" CACHE FILEPATH "CLC compiler")
 endif()