[clang] [CIR] Add proper handling for no prototype function calls (PR #150553)

[Sirui Mu via cfe-commits]

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@@ -1296,6 +1298,44 @@ RValue CIRGenFunction::emitCall(clang::QualType calleeTy,
   const CIRGenFunctionInfo &funcInfo =
       cgm.getTypes().arrangeFreeFunctionCall(args, fnType);
 
+  // C99 6.5.2.2p6:
+  //   If the expression that denotes the called function has a type that does
+  //   not include a prototype, [the default argument promotions are performed].
+  //   If the number of arguments does not equal the number of parameters, the
+  //   behavior is undefined. If the function is defined with a type that
+  //   includes a prototype, and either the prototype ends with an ellipsis (,
+  //   ...) or the types of the arguments after promotion are not compatible
+  //   with the types of the parameters, the behavior is undefined. If the
+  //   function is defined with a type that does not include a prototype, and
+  //   the types of the arguments after promotion are not compatible with those
+  //   of the parameters after promotion, the behavior is undefined [except in
+  //   some trivial cases].
+  // That is, in the general case, we should assume that a call through an
+  // unprototyped function type works like a *non-variadic* call. The way we
+  // make this work is to cast to the exxact type fo the promoted arguments.
+  if (isa<FunctionNoProtoType>(fnType)) {
+    assert(!cir::MissingFeatures::opCallChain());
+    assert(!cir::MissingFeatures::addressSpace());
+    cir::FuncType calleeTy = getTypes().getFunctionType(funcInfo);
+    // get non-variadic function type
+    calleeTy = cir::FuncType::get(calleeTy.getInputs(),
+                                  calleeTy.getReturnType(), false);
+    auto calleePtrTy = cir::PointerType::get(calleeTy);
+
+    mlir::Operation *fn = callee.getFunctionPointer();
+    mlir::Value addr;
+    if (auto funcOp = llvm::dyn_cast<cir::FuncOp>(fn)) {
+      addr = builder.create<cir::GetGlobalOp>(
+          getLoc(e->getSourceRange()),
+          cir::PointerType::get(funcOp.getFunctionType()), funcOp.getSymName());
+    } else {
+      addr = fn->getResult(0);
+    }
+
+    fn = builder.createBitcast(addr, calleePtrTy).getDefiningOp();
+    callee.setFunctionPointer(fn);
+  }
+
   assert(!cir::MissingFeatures::opCallNoPrototypeFunc());
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Lancern wrote:

I believe this assert and the `opCallNoPrototypeFunc` missing feature guard could be removed.

https://github.com/llvm/llvm-project/pull/150553