Disable _ExtInt by default

Since the _ExtInt type got into the repo, we've discovered that the ABI
implications weren't completely understood. The other architectures are
going to be audited (see D79118), however downstream targets aren't
going to benefit from this audit.

This patch disables the _ExtInt type by default and makes the
target-info an opt-in.  As it is audited, I'll re-enable these for all
of our default targets.

clang/include/clang/Basic/TargetInfo.h

@@ -547,6 +547,12 @@ class TargetInfo : public virtual TransferrableTargetInfo,
     return (getPointerWidth(0) >= 64) || getTargetOpts().ForceEnableInt128;
   } // FIXME
 
+  /// Determine whether the _ExtInt type is supported on this target. This
+  /// limitation is put into place for ABI reasons.
+  virtual bool hasExtIntType() const {
+    return false;
+  }
+
   /// Determine whether _Float16 is supported on this target.
   virtual bool hasLegalHalfType() const { return HasLegalHalfType; }
 

clang/lib/Basic/Targets/X86.h

@@ -435,6 +435,8 @@ class LLVM_LIBRARY_VISIBILITY X86_32TargetInfo : public X86TargetInfo {
   }
 
   ArrayRef<Builtin::Info> getTargetBuiltins() const override;
+
+  bool hasExtIntType() const override { return true; }
 };
 
 class LLVM_LIBRARY_VISIBILITY NetBSDI386TargetInfo
@@ -737,6 +739,8 @@ class LLVM_LIBRARY_VISIBILITY X86_64TargetInfo : public X86TargetInfo {
   }
 
   ArrayRef<Builtin::Info> getTargetBuiltins() const override;
+
+  bool hasExtIntType() const override { return true; }
 };
 
 // x86-64 Windows target

clang/lib/Sema/SemaType.cpp

@@ -1443,6 +1443,9 @@ static QualType ConvertDeclSpecToType(TypeProcessingState &state) {
     break;
   }
   case DeclSpec::TST_extint: {
+    if (!S.Context.getTargetInfo().hasExtIntType())
+      S.Diag(DS.getTypeSpecTypeLoc(), diag::err_type_unsupported)
+        << "_ExtInt";
     Result = S.BuildExtIntType(DS.getTypeSpecSign() == TSS_unsigned,
                                DS.getRepAsExpr(), DS.getBeginLoc());
     if (Result.isNull()) {

clang/test/Sema/ext-int-not-supported.c

@@ -0,0 +1,5 @@
+// RUN: %clang_cc1 -triple armv7 -fsyntax-only -verify %s
+
+void foo() {
+  _ExtInt(33) a; // expected-error{{_ExtInt is not supported on this target}}
+}