[libc] Keep headers compatible with -std=c89 mode

[frobtech]

C89 doesn't have the restrict keyword. When in -std=c89
mode, GNU-compatible compilers require the __restrict spelling.

[llvmbot]

@llvm/pr-subscribers-libc

Author: Roland McGrath (frobtech)

Changes

C89 doesn't have the restrict keyword. When in -std=c89
mode, GNU-compatible compilers require the __restrict spelling.

---

Full diff: https://github.com/llvm/llvm-project/pull/121981.diff

1 Files Affected:

• (modified) libc/include/__llvm-libc-common.h (+8-1)

diff --git a/libc/include/__llvm-libc-common.h b/libc/include/__llvm-libc-common.h
index d9d70aff771c08..c63eb134a5e5d7 100644
--- a/libc/include/__llvm-libc-common.h
+++ b/libc/include/__llvm-libc-common.h
@@ -50,7 +50,14 @@
 #define __END_C_DECLS
 
 #undef __restrict
-#define __restrict restrict // C99 and above support the restrict keyword.
+#if __STDC_VERSION__ >= 199901L
+// C99 and above support the restrict keyword.
+#define __restrict restrict
+#elif !defined(__GNUC__)
+// GNU-compatible compilers accept the __ spelling in all modes.
+// Otherwise, omit the qualifier for pure C89 compatibility.
+#define __restrict
+#endif
 
 #undef _Noreturn
 #if __STDC_VERSION__ >= 201112L

[jhuber6]

Do we need to do this for __inline__ as well? I forget if that shows up anywhere.

[frobtech]

Do we need to do this for __inline__ as well? I forget if that shows up anywhere.

We don't have any use of inline (by any spelling) in the headers today, except for one set that's #ifdef __cplusplus.