[clang] [clang]Implement the c23 stdc bit builtins (PR #185978)

[Erich Keane via cfe-commits]

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@@ -2345,6 +2345,47 @@ static bool BuiltinPopcountg(Sema &S, CallExpr *TheCall) {
   return false;
 }
 
+/// Checks stdc bit-utility builtins (__builtin_stdc_*):
+/// bit_ceil, bit_floor, bit_width, count_ones, count_zeros,
+/// first_leading_one, first_leading_zero, first_trailing_one,
+/// first_trailing_zero, has_single_bit, leading_ones, leading_zeros,
+/// trailing_ones, trailing_zeros. They all take a single unsigned integer
+/// argument and return either int, bool, or the argument type depending on the
+/// specific builtin.
+static bool BuiltinStdCBuiltin(Sema &S, CallExpr *TheCall) {
+  if (S.checkArgCount(TheCall, 1))
+    return true;
+
+  ExprResult ArgRes = S.DefaultLvalueConversion(TheCall->getArg(0));
+  if (ArgRes.isInvalid())
+    return true;
+
+  Expr *Arg = ArgRes.get();
+  TheCall->setArg(0, Arg);
+
+  QualType ArgTy = Arg->getType();
+  if (!ArgTy->isUnsignedIntegerType() && !ArgTy->isExtVectorBoolType()) {
+    S.Diag(Arg->getBeginLoc(), diag::err_builtin_invalid_arg_type)
+        << 1 << /* scalar */ 1 << /* unsigned integer ty */ 3 << /* no fp */ 0
+        << ArgTy;
+    return true;
+  }
+
+  switch (TheCall->getBuiltinCallee()) {
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erichkeane wrote:

Instead of this switch, make the 'return' type an argument, either via an enum or by passing something.  We've already done a switch on these below, doing it again seems silly.

https://github.com/llvm/llvm-project/pull/185978