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

[Eric Christopher]

On Mon, Nov 11, 2013 at 4:13 PM, Lewis Burns <lewisurn at gmail.com> wrote:
> No, I don't care about those functions that aren't called.
>
> Okay, I walked through the EmitCall family of functions of CodeGenFunction,
> but didn't notice much. I guess that I'll have to trace them through more
> carefully to see when it is done.
>
> My thought is to copy the debug info metadata emission logic used during
> generating the function prolog to function declarations. Do you think if it
> works?
>

Easiest is looking at getOrCreateFunctionType. You can probably hook
into EmitCall if you want to emit a debug info declaration for the
function.

-eric

> Thanks,
>
>
> On 11/12/2013 07:49 AM, David Blaikie wrote:
>
> 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
>
>
>
> --
> Lewis
>
>
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