clang set metadata to allocainst - clang

Question

First I'm real noob with clang/llvm.
BUT I'm trying to modify clang for some purpose.
I'd like to add metadata whenever an Alloca instruction is emitted in IR code for a variable which has some annotation.
I noticed this function in CGDecl.cpp:
CodeGenFunction::AutoVarEmission
CodeGenFunction::EmitAutoVarAlloca(const VarDecl &D)
which contains the nice line in the end:
if (D.hasAttr<AnnotateAttr>())
EmitVarAnnotations(&D, emission.Address);
this looks like the condition I need, so I modified it to
if (D.hasAttr<AnnotateAttr>()) {
AnnotateAttr* attr = D.getAttr<AnnotateAttr>();
if(attr->getAnnotation() == "_my_custom_annotation_") {
// set metadata...
}
EmitVarAnnotations(&D, emission.Address);
}
my Issue is I don't know how to add metadata at this point, because I can't find a way to access the instruction
In CGExp.cpp, however, I see where the AllocaInstr is built, but at this point I don't have access to the VarDecl, so I don't know if the annotation is there.
I tried anyway to add metadata (unconditionaly) in this function:
llvm::AllocaInst *CodeGenFunction::CreateIRTemp(QualType Ty,
const Twine &Name) {
llvm::AllocaInst *Alloc = CreateTempAlloca(ConvertType(Ty), Name);
// FIXME: Should we prefer the preferred type alignment here?
CharUnits Align = getContext().getTypeAlignInChars(Ty);
// how to put it conditionaly on the annotation?
llvm::MDNode* node = getRangeForLoadFromType(Ty);
Alloc->setMetadata("_my_custom_metadata", node);
Alloc->setAlignment(Align.getQuantity());
return Alloc;
}
by adding the setMetadata call.
However I don't see the metadata attached in the generated IR.
I compile with clang -g -S -target i686-pc-win32 -emit-llvm main.cpp -o output.ll
Maybe I'm totally wrong, but the thing is I don't master the code generation in clang :)
PS: here is the code I compile
int main() {
__attribute__ ((annotate("_my_custom_annotation_"))) float a[12];
}
Any help is appreciated!
Thanks

Answer 1

if (D.hasAttr<AnnotateAttr>()) {
AnnotateAttr* attr = D.getAttr<AnnotateAttr>();
if(attr->getAnnotation() == "_my_custom_annotation_") {
// set metadata...
}
EmitVarAnnotations(&D, emission.Address);
}
Looks like you are at the right place. In fact all EmitAutoVarAlloca has special handling for different kinds of variable declarations, but all end with the "address" (i.e., the instruction) in emission.Address.
So what you want to do is:
if (D.hasAttr<AnnotateAttr>()) {
AnnotateAttr* attr = D.getAttr<AnnotateAttr>();
if(attr->getAnnotation() == "_my_custom_annotation_") {
emission.Address->setMetadata(...); // <--- your MDNode goes here
}
EmitVarAnnotations(&D, emission.Address);
}
However, I would recommend a special attribute for adding metadata to instructions. If you read further through the code you will see that the AnnotateAttr has a special meaning and your emitted IR may not be as expected. You can add a custom attribute in the Attr.td file. I suggest a copy of the Annotate entry. Then you can follow the AnnotateAttr through the code and add code for your Attribute at the right places to get it recognized and handled by clang.