================
@@ -1818,8 +1819,21 @@ void Preprocessor::ExpandBuiltinMacro(Token &Tok) {
             // usual allocation and deallocation functions. Required by libc++
             return 201802;
           default:
+            // We may get here because of aux builtins which may not be
+            // supported on the default target, for example if we have an X86
+            // specific builtin and the current target is SPIR-V. Sometimes we
+            // rely on __has_builtin returning true when passed a builtin that
+            // is not supported on the default target due to LangOpts but is
+            // supported on the aux target. See
+            // test/Headers/__cpuidex_conflict.c for an example. If the builtin
+            // is an aux builtin and it can never be supported on the default
+            // target, __has_builtin should return false.
+            if (getBuiltinInfo().isAuxBuiltinID(BuiltinID) &&
+                
getBuiltinInfo().isAuxBuiltinIDAlwaysUnsupportedOnDefaultTarget(
----------------
sarnex wrote:

The change in question uses the `ifdef` to try to prevent double definitions 
[here](https://github.com/llvm/llvm-project/blob/main/clang/lib/Headers/cpuid.h#L345),
 so if we change the return value then it would break that code (which was 
caught by the LIT fail), and reading the above PR it seems it was hard to find 
a check that worked, so I'm not sure what's best.

https://github.com/llvm/llvm-project/pull/121839
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