Add support for the llvm.is.constant.* intrinsics. (#185)

bcoppens · web-flow · commit 2c420066da0e · 2024-01-12T12:56:09.000-05:00
Add support for the llvm.is.constant.* intrinsics. According to the LLVM LangRef, this intrinsic will never return true when constant folding is not run, so always returning false is safe. Partially resolves issue #149.
diff --git a/lib/Target/CBackend/CBackend.cpp b/lib/Target/CBackend/CBackend.cpp
@@ -2573,6 +2573,7 @@ void CWriter::generateHeader(Module &M) {
       case Intrinsic::trunc:
       case Intrinsic::umax:
       case Intrinsic::umin:
+      case Intrinsic::is_constant:
         intrinsicsToDefine.push_back(&*I);
         continue;
       }
@@ -4643,6 +4644,9 @@ void CWriter::printIntrinsicDefinition(FunctionType *funT, unsigned Opcode,
     case Intrinsic::umin:
       Out << "  r = a < b ? a : b;\n";
       break;
+    case Intrinsic::is_constant:
+      Out << "  r = 0 /* llvm.is.constant */;\n";
+      break;
     }
 
   } else {
@@ -4764,6 +4768,7 @@ bool CWriter::lowerIntrinsics(Function &F) {
           case Intrinsic::dbg_declare:
           case Intrinsic::umax:
           case Intrinsic::umin:
+          case Intrinsic::is_constant:
             // We directly implement these intrinsics
             break;
 
@@ -5078,6 +5083,7 @@ bool CWriter::visitBuiltinCall(CallInst &I, Intrinsic::ID ID) {
   case Intrinsic::trap:
   case Intrinsic::umax:
   case Intrinsic::umin:
+  case Intrinsic::is_constant:
     return false; // these use the normal function call emission
   }
 }
diff --git a/test/cpp_tests/test_is_constant.cpp b/test/cpp_tests/test_is_constant.cpp
@@ -0,0 +1,8 @@
+int f(int i) {
+  if (__builtin_constant_p(i) &&
+      i == 1) /* generates the llvm.is.constant.i32 intrinsic at -O0 */
+    return i + 5;
+  return 6;
+}
+
+int main() { return f(1); }
