[llvm] [Runtimes][libc] Include kernel headers when using RUNIMES_USE_LIBC (PR #165114)

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 created https://github.com/llvm/llvm-project/pull/165114

When setting RUNTIMES_USE_LIBC=libc we pass -nostdlibinc if the compiler
supports it. This causes compilation failures when trying to build
libc++ as libc will try and reference kernel headers in its own headers
that it now cannot find. Explicitly reference the kernel headers so the
compiler can find them.


>From 5d5ff26e2d72f42938115dee06828ba282393680 Mon Sep 17 00:00:00 2001
From: Aiden Grossman <aidengrossman at google.com>
Date: Sat, 25 Oct 2025 19:39:23 +0000
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Created using spr 1.3.7
---
 runtimes/cmake/Modules/HandleLibC.cmake | 1 +
 1 file changed, 1 insertion(+)

diff --git a/runtimes/cmake/Modules/HandleLibC.cmake b/runtimes/cmake/Modules/HandleLibC.cmake
index 51fbf04df7e3b..01da5b260d3d4 100644
--- a/runtimes/cmake/Modules/HandleLibC.cmake
+++ b/runtimes/cmake/Modules/HandleLibC.cmake
@@ -30,6 +30,7 @@ elseif (RUNTIMES_USE_LIBC STREQUAL "llvm-libc")
   check_cxx_compiler_flag(-nostdlibinc CXX_SUPPORTS_NOSTDLIBINC_FLAG)
   if(CXX_SUPPORTS_NOSTDLIBINC_FLAG)
     target_compile_options(runtimes-libc-headers INTERFACE "-nostdlibinc")
+    target_compile_options(runtimes-libc-headers INTERFACE "-idirafter${LIBC_KERNEL_HEADERS}")
   endif()
 
   add_library(runtimes-libc-static INTERFACE)