[cfe-dev] output metadata for extern declared functions?

[David Blaikie]

Do you care about generating debug info for declarations of functions that
aren't even called? If so, then the approach you're taking will be
insufficient (since we won't even emit an IR declaration for such a
function)

If not, then you might want to take a look at where the IR for the call is
constructed (I don't know where this is, but you seem to be gaining some
proficiency tracing through Clang/LLVM internals that will serve you well
here) and then see how the target of the call is built and passed in to
that.


On Mon, Nov 11, 2013 at 3:44 PM, Lewis Burns <lewisurn at gmail.com> wrote:

>  I ran Clang in a debugger and traced how debug info metadata was emitted.
> It's a part of code generation of functions.
>
> I have a question about when the declaration of an extern function is
> emitted. For example, I have very simple code:
>
> extern int convert(unsigned u);
>
> void foo() {
>   int x = convert(0);
> }
>
> The corresponding LLVM code is:
>
> ...
> ; Function Attrs: nounwind uwtable
> define void @foo() #0 {
> entry:
>   %x = alloca i32, align 4
>   call void @llvm.dbg.declare(metadata !{i32* %x}, metadata !8), !dbg !10
>   %call = call i32 @convert(i32 0), !dbg !10
>   store i32 %call, i32* %x, align 4, !dbg !10
>   ret void, !dbg !11
> }
> ...
> declare i32 @convert(i32) #2  // when this line is emitted
>
> My question is where the "declare i32 @convert(i32) #2" line is emitted.
> I tried many breakpoints in EmitXXX family of functions in CodeGenModule
> and noticed that this piece of code
>
>   // Ignore declarations, they will be emitted on their first use.
>   if (const FunctionDecl *FD = dyn_cast<FunctionDecl>(Global)) {
>     // Forward declarations are emitted lazily on first use.
>     if (!FD->doesThisDeclarationHaveABody()) {
>       if (!FD->doesDeclarationForceExternallyVisibleDefinition())
>         return;
>
> causes the postpone of emission of the convert function declaration, but I
> couldn't figure out where and when the declaration is emitted. I set a
> breakpoint in the CodeGenModule::EmitDeferred() function, but nothing was
> done in that function.
>
> Any help is really appreciated.
>
>
> On 11/09/2013 04:14 PM, David Blaikie wrote:
>
> For those following this thread a critical detail would be that you want
> debug info metadata.
>
> There's no simple flag for this as we don't attach the function debug info
> metadata to every declaration, just to definitions (there's no filtering
> step)
>
> But why do you want this anyway? If you're performing
> optimizations/transformations based on debug info metadata, that's not
> really the desired approach. Debug info is not meant to affect code
> generation.
> On Nov 9, 2013 7:59 AM, "Lewis Burns" <lewisurn at gmail.com> wrote:
>
>>  Hi,
>>
>> I haven't worked on Clang before and have a simple question (supposedly)
>> for those who are familiar with metadata and LLVM bitcode generation.
>> Assume that I have a function which is declared as extern as the following:
>>
>> extern int convert(unsigned u);
>>
>> I want to have Clang generate metadata nodes for it by adding a metadata
>> node of subprogram into the list of subprograms defined in the current
>> compilation unit. The subprogram metadata node and its associated nodes
>> should have the info of the type signature. For example, I get the
>> following set of metadata nodes for function
>>
>> int convert(unsigned u) {return 0;}
>>
>> !3 = metadata !{metadata !4, metadata !10, metadata !33}
>> !4 = metadata !{i32 786478, metadata !1, metadata !5, metadata
>> !"convert", metadata !"convert", metadata !"", i32 23, metadata !6, i1
>> false, i1 true, i32 0, i32 0, null, i32 256, i1 false, i32 (i32)* @convert,
>> null, null, metadata !2, i32 23} ; [ DW_TAG_subprogram ] [line 23] [def]
>> [convert]
>> !7 = metadata !{metadata !8, metadata !9}
>> !8 = metadata !{i32 786468, null, null, metadata !"int", i32 0, i64 32,
>> i64 32, i64 0, i32 0, i32 5} ; [ DW_TAG_base_type ] [int] [line 0, size 32,
>> align 32, offset 0, enc DW_ATE_signed]
>> !9 = metadata !{i32 786468, null, null, metadata !"unsigned int", i32 0,
>> i64 32, i64 32, i64 0, i32 0, i32 7} ; [ DW_TAG_base_type ] [unsigned int]
>> [line 0, size 32, align 32, offset 0, enc DW_ATE_unsigned]
>>
>> which allows me to extract the source-level type signature for the
>> function by using LLVM debug info APIs. I'd like to get the source-level
>> type signature of the extern declared function, but Clang does not produce
>> metadata for it.
>>
>> By looking at the Clang AST for the extern declared function
>>
>> |-FunctionDecl 0x70598c0 <line:23:1, col:30> convert 'int (unsigned int)'
>> extern
>> | |-ParmVarDecl 0x7059800 <col:20, col:29> u 'unsigned int'
>>
>> I know that Clang has the information I need, and I just need to turn off
>> or remove the filter that ignores functions whose bodies are not available
>> during metadata node or/and code generation. Are there simple switches that
>> do this? If not, can anyone please explain how to do it by pointing me to
>> the right code snippets?
>>
>> Thanks very much,
>>
>>
>> --
>> Lewis
>>
>>
>> _______________________________________________
>> cfe-dev mailing list
>> cfe-dev at cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev
>>
>>
> --
> Lewis
>
>
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