Suspecting a frontend bug in LLVM compiler

Greetings to all.

I am facing a problem with LLVM 3.5 and suspect it is a bug. Here is my observation.

I am trying to support a target specific function attribute for a particular target machine.

Having followed the steps specified in the compiler documentation, I see the attribute is passed to the backend if we include the attribute in the function definition. But this attribute is not propagate to the backend if I include it only in the function’s prototype/declaration (in the C program).

This virtual function (given below) invocation in clang/lib/CodeGen/CodeGenModule.cpp in clang is responsible for attaching the target specific attributes to the backend CodeGen objects (functions, variables etc.)

getTargetCodeGenInfo().SetTargetAttributes(D, GO, *this)

I found that this virtual function is getting invoked only for function definitions and not for function declarations.

Actual requirement:

Along with regular C functions we have special functions supported in the target and its definition and invocation will always come in different compilation units. There must be a way to recognize these special functions at the call-sites during codegen in the backend. By attaching some info in the function prototype, we can recognize these special functions at the call-site during backend codegen. That’s why I thought of a new target specific attribute to achieve this purpose.

Is it really a bug?

When I compile a program having function declarations/prototypes with generic attributes like attribute((const)), clang will add it to that function’s attributelist in the IR and it will be available in the backend.

Similarly I expected the target specific attribute should also be added to the attributelist available in the backend and I should be able to identify it using F.hasFnAttribute(“XYZ”), where XYZ is the new attribute name.

Since I work with LLVM 3.5 code-base, I am not sure whether this issue (if it is really a bug) has already been fixed with recent compiler releases.

Please write to me.



Please give a remark on this.


LLVM 3.5 is 2+ years old, and 4 releases old now :slight_smile:
The immediate response would be “try it with a newer version, and see if it works”.