[llvm-dev] Adding a function attribute with an argument

[Tehila Mayzels via llvm-dev]

Hi,

 

I'm trying to add a function attribute to clang/llvm.

I need an attribute that will have an additional information of a variable name.

e.g. I would like to add support for something like:

 

void foo() __attribute__ ((xyz(v)) {…}

 

such that on the IR stage I can realize that the function foo has the attribute xyz and an argument v that  marks variable v (which is a variable inside foo) for my needs.

 

I was able to add a function attribute without an argument, but I don't understand how to parse/save/analyze the argument.

 

I would appreciate any ideas/examples/references.

 

Thanks a lot,

Tehila.

 

 

[Gaël Jobin via llvm-dev]

Hi Tehila,

I recently added new function attributes to clang/llvm with arguments.

What I did is the following:

1. Add your attributes to tools/clang/include/clang/Basic/Attr.td

def MYATTR : InheritableAttr {

  let Spellings = [GNU<"myattr">, CXX11<"gnu", "myattr">];

  let Args = [DefaultIntArgument<"myargument", 5>];

  let Subjects = SubjectList<[Function]>;

  let Documentation = [Undocumented];

}

2. Manage it in tools/clang/lib/CodeGen/CGCall.cpp...

void CodeGenModule::ConstructAttributeList(const CGFunctionInfo &FI,

                                           const Decl *TargetDecl,

                                           AttributeListType &PAL,

                                           unsigned &CallingConv,

                                           bool AttrOnCallSite) {

//other code...

if (TargetDecl) {

if (TargetDecl->hasAttr<MYEXAMPLEAttr>())

{

auto myexample = TargetDecl->getAttr<MYEXAMPLEAttr>();

FuncAttrs.addAttribute("myexample");

unsigned myargument = myexample->getMyargument();

FuncAttrs.addAttribute("myargument", llvm::utostr(myargument));

}

//other code...

}

//other code...

}

3. ...and in tools/clang/lib/Sema/SemaDeclAttr.cpp

static void ProcessDeclAttribute(Sema &S, Scope *scope, Decl *D,

                                 const AttributeList &Attr,

                                 bool IncludeCXX11Attributes) {

//other code...

switch (Attr.getKind()) {

case AttributeList::AT_MYEXAMPLE:

handleMYEXAMPLEAttr(S, D, Attr);

break;

//other code...

}

static void handleMYEXAMPLEAttr(Sema &S, Decl *D, const AttributeList &Attr) {

uint32_t myargument = AVMAttr::DefaultMyargument;

if (Attr.getNumArgs() && !checkUInt32Argument(S, Attr, Attr.getArgAsExpr(0), myargument))

return;

D->addAttr(::new (S.Context)

AVMAttr(Attr.getRange(), S.Context, myargument,

Attr.getAttributeSpellingListIndex()));

}

Then you should have access to it in LLVM IR.

I may have forgotten some stuff but it should help you get started.

Regards,

Gael

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[Tehila Mayzels via llvm-dev]

Hi Gael,

 

Thanks a lot for your response, it was very helpful!

How did you manage to access the argument value at the IR stage?

 

Thanks again,

Tehila.

 

Gaël Jobin via llvm-dev llvm-dev at lists.llvm.org 
Thu Oct 8 01:08:05 PDT 2015

>> llvm-dev at lists.llvm.org

> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

 

[Gaël Jobin via llvm-dev]

Hi Tehila,

Le mardi 13 octobre 2015 à 15:55 +0300, Tehila Mayzels a écrit :

How did you manage to access the argument value at the IR stage?

Something like this if I remember correctly:

Function *f = ...;

int myargument;

if(f->hasFnAttribute("myexample"))

{

if(f->hasFnAttribute("myargument"))

{

myargument = f->getFnAttribute("myargument").getValueAsInt();

}

}

//...

Regards,

Gaël