[llvm-dev] Suspecting a frontend bug in LLVM compiler

[Christudasan D via llvm-dev]

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.



Thanks,

Christu
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