[libc] fix build errors

[SchrodingerZhu]

Bitfield conversion problem tested at: https://godbolt.org/z/dxjhs5Ghr

[llvmbot]

@llvm/pr-subscribers-libc

Author: Schrodinger ZHU Yifan (SchrodingerZhu)

Changes

---

Full diff: https://github.com/llvm/llvm-project/pull/95600.diff

2 Files Affected:

• (modified) libc/src/__support/macros/attributes.h (+1-1)
• (modified) libc/src/__support/threads/linux/rwlock.h (+3-2)

diff --git a/libc/src/__support/macros/attributes.h b/libc/src/__support/macros/attributes.h
index 7e8e2ddfac9b1..c6474673de85a 100644
--- a/libc/src/__support/macros/attributes.h
+++ b/libc/src/__support/macros/attributes.h
@@ -42,7 +42,7 @@
 #define LIBC_CONSTINIT
 #endif
 
-#ifdef __clang__
+#if defined(__clang__) && __has_attribute(preferred_type)
 #define LIBC_PREFERED_TYPE(TYPE) [[clang::preferred_type(TYPE)]]
 #else
 #define LIBC_PREFERED_TYPE(TYPE)
diff --git a/libc/src/__support/threads/linux/rwlock.h b/libc/src/__support/threads/linux/rwlock.h
index 5db0590fd6aa0..1778995cf3766 100644
--- a/libc/src/__support/threads/linux/rwlock.h
+++ b/libc/src/__support/threads/linux/rwlock.h
@@ -370,8 +370,9 @@ class RwLock {
 public:
   LIBC_INLINE constexpr RwLock(Role preference = Role::Reader,
                                bool is_pshared = false)
-      : is_pshared(is_pshared), preference(static_cast<unsigned>(preference)),
-        state(0), writer_tid(0), queue() {}
+      : is_pshared(is_pshared),
+        preference(static_cast<unsigned>(preference) & 1u), state(0),
+        writer_tid(0), queue() {}
 
   [[nodiscard]]
   LIBC_INLINE LockResult try_read_lock() {

[SchrodingerZhu]

Should be good now. Sorry for the noise.

[SchrodingerZhu]

I am now merging this without waiting for ARM/RISCV build, since RwLock are not enabled on ARM(32) and RISC-V anyway.