[cfe-dev] suspecting a bug in Clang

[Akira Hatanaka via cfe-dev]

> On Sep 7, 2016, at 9:08 PM, Christudasan D via cfe-dev <cfe-dev at lists.llvm.org> wrote:
> 
> All,
> 
> 
> 
> I am facing a problem with LLVM 3.5 Compiler and suspect it is a bug in the clang. I earlier posted this query to llvm-dev list. 
> 
> 
> 
> 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.
> 
>  
> I understand that a look-up in the prototype for function-attrs is very rare at codegen. But I was keen to ask this when I found that the standard attributes ('const', for example) are actually propagated to the backend through function prototype.
> 
> In the sample program below I have declared function foo with attribute 'const' and you can see 'readnone' is added to the attribute list.
> 
> Sample Program:
> 
> extern void __attribute__((const)) foo (int);
> 
> int main ()
> {
> 	int  my_var = 45;
> 
> 	foo(my_var);
> 	return 0;
> }
> -----------------------------------------------------------------
> Generated IR:
> 
> ; Function Attrs: nounwind
> define i16 @main() #0 {
>   %1 = alloca i16, align 2
>   %my_var = alloca i16, align 2
>   store i16 0, i16* %1
>   call void @llvm.dbg.declare(metadata !{i16* %my_var}, metadata !13), !dbg !14
>   store i16 45, i16* %my_var, align 2, !dbg !15
>   %2 = load i16* %my_var, align 2, !dbg !16
>   call void @foo(i16 %2) #1, !dbg !16
>   ret i16 0, !dbg !17
> }
> 
> ; Function Attrs: nounwind readnone
> declare void @llvm.dbg.declare(metadata, metadata) #1
> 
> ; Function Attrs: nounwind readnone
> declare void @foo(i16) #2
> --------------------------------------------------------------------------------------------------------------------
> 
> 
I think you are right: SetTargetAttributes doesn’t get called for function declarations. GetOrCreateLLVMFunction is called to generate the LLVM IR for a function declaration, which  eventually calls ConstructAttributeList, in which readnone is added. If you want to add target attributes to function declarations, I believe you’ll have to somehow call SetTargetAttributes in ConstructAttributeList.
> 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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