[libc] Handle the unknown default target in CMake

[petrhosek]

When the backend for the host target isn't enabled, Clang would report the default target as unknown. This currently breaks the libc CMake build, but shouldn't in the case where we're cross-compiling since we're given an explicit target and the default one isn't being used.

[llvmbot]

@llvm/pr-subscribers-libc

Author: Petr Hosek (petrhosek)

Changes

When the backend for the host target isn't enabled, Clang would report the default target as unknown. This currently breaks the libc CMake build, but shouldn't in the case where we're cross-compiling since we're given an explicit target and the default one isn't being used.

---

Full diff: https://github.com/llvm/llvm-project/pull/115122.diff

1 Files Affected:

• (modified) libc/cmake/modules/LLVMLibCArchitectures.cmake (+31-34)

diff --git a/libc/cmake/modules/LLVMLibCArchitectures.cmake b/libc/cmake/modules/LLVMLibCArchitectures.cmake
index 7711127c1a81e1..e1eba073b7808c 100644
--- a/libc/cmake/modules/LLVMLibCArchitectures.cmake
+++ b/libc/cmake/modules/LLVMLibCArchitectures.cmake
@@ -92,17 +92,6 @@ if(NOT libc_compiler_target_info)
 endif()
 string(STRIP ${libc_compiler_target_info} libc_compiler_target_info)
 string(SUBSTRING ${libc_compiler_target_info} 8 -1 libc_compiler_triple)
-get_arch_and_system_from_triple(${libc_compiler_triple}
-                                compiler_arch compiler_sys)
-if(NOT compiler_arch)
-  message(FATAL_ERROR
-          "libc build: Invalid or unknown libc compiler target triple: "
-          "${libc_compiler_triple}")
-endif()
-
-set(LIBC_TARGET_ARCHITECTURE ${compiler_arch})
-set(LIBC_TARGET_OS ${compiler_sys})
-set(LIBC_CROSSBUILD FALSE)
 
 # One should not set LLVM_RUNTIMES_TARGET and LIBC_TARGET_TRIPLE
 if(LLVM_RUNTIMES_TARGET AND LIBC_TARGET_TRIPLE)
@@ -126,12 +115,40 @@ endif()
 # architecture.
 if(explicit_target_triple)
   get_arch_and_system_from_triple(${explicit_target_triple} libc_arch libc_sys)
-  if(NOT libc_arch)
+  if(NOT libc_arch OR NOT libc_sys)
     message(FATAL_ERROR
             "libc build: Invalid or unknown triple: ${explicit_target_triple}")
   endif()
   set(LIBC_TARGET_ARCHITECTURE ${libc_arch})
   set(LIBC_TARGET_OS ${libc_sys})
+  # If the compiler target triple is not the same as the triple specified by
+  # LIBC_TARGET_TRIPLE or LLVM_RUNTIMES_TARGET, we will add a --target option
+  # if the compiler is clang. If the compiler is GCC we just error out as there
+  # is no equivalent of an option like --target.
+  if(NOT libc_compiler_triple STREQUAL explicit_target_triple)
+    set(LIBC_CROSSBUILD TRUE)
+    if(CMAKE_COMPILER_IS_GNUCXX)
+      message(FATAL_ERROR
+              "GCC target triple (${libc_compiler_triple}) and the explicity "
+              "specified target triple (${explicit_target_triple}) do not match.")
+    else()
+      list(APPEND
+           LIBC_COMPILE_OPTIONS_DEFAULT "--target=${explicit_target_triple}")
+    endif()
+  else()
+    set(LIBC_CROSSBUILD FALSE)
+  endif()
+else()
+  get_arch_and_system_from_triple(${libc_compiler_triple}
+                                  compiler_arch compiler_sys)
+  if(NOT compiler_arch OR NOT compiler_sys)
+    message(FATAL_ERROR
+            "libc build: Unknown compiler default target triple: "
+            "${libc_compiler_triple}")
+  endif()
+  set(LIBC_TARGET_ARCHITECTURE ${compiler_arch})
+  set(LIBC_TARGET_OS ${compiler_sys})
+  set(LIBC_CROSSBUILD FALSE)
 endif()
 
 if((LIBC_TARGET_OS STREQUAL "unknown") OR (LIBC_TARGET_OS STREQUAL "none"))
@@ -188,31 +205,11 @@ else()
           "Unsupported libc target operating system ${LIBC_TARGET_OS}")
 endif()
 
-
-# If the compiler target triple is not the same as the triple specified by
-# LIBC_TARGET_TRIPLE or LLVM_RUNTIMES_TARGET, we will add a --target option
-# if the compiler is clang. If the compiler is GCC we just error out as there
-# is no equivalent of an option like --target.
-if(explicit_target_triple AND
-   (NOT (libc_compiler_triple STREQUAL explicit_target_triple)))
-  set(LIBC_CROSSBUILD TRUE)
-  if(CMAKE_COMPILER_IS_GNUCXX)
-    message(FATAL_ERROR
-            "GCC target triple (${libc_compiler_triple}) and the explicity "
-            "specified target triple (${explicit_target_triple}) do not match.")
-  else()
-    list(APPEND
-         LIBC_COMPILE_OPTIONS_DEFAULT "--target=${explicit_target_triple}")
-  endif()
-endif()
-
-
 # Windows does not support full mode build.
 if (LIBC_TARGET_OS_IS_WINDOWS AND LLVM_LIBC_FULL_BUILD)
   message(FATAL_ERROR "Windows does not support full mode build.")
 endif ()
 
-
 message(STATUS
-        "Building libc for ${LIBC_TARGET_ARCHITECTURE} on ${LIBC_TARGET_OS} with
-        LIBC_COMPILE_OPTIONS_DEFAULT: ${LIBC_COMPILE_OPTIONS_DEFAULT}")
+        "Building libc for ${LIBC_TARGET_ARCHITECTURE} on ${LIBC_TARGET_OS} with "
+        "LIBC_COMPILE_OPTIONS_DEFAULT: ${LIBC_COMPILE_OPTIONS_DEFAULT}")

[lntue]

libc_compiler_triple is not obtained before this anymore, so it means that it is always crossbuild whenever target triple is specified?

[petrhosek]

libc_compiler_triple is set on line 96.

[petrhosek]

@lntue Would it be possible to take another look? This is a prerequisite for #113267.