How to Get the Field Name of members in struct

What is Debug Info

Read Source Level Debugging with LLVM for more details. If you do not have enough time, a few examples from the documentation should be enough.

How to Generate Debug Info

Pass -g or --debug option to clang.

How to Access Debug Info

LLVM uses several intrinsic functions (name prefixed with llvm.dbg) to track source local variables through optimization and code generation.^[1]

Debug Info is arranged like a directed graph. However, LLVM does not provide a convenient way to traverse all llvm::DINodes. Therefore, we need an entry into debug info. Here we mainly focus on llvm.dbg.declare intrinsic, aka llvm::DbgDeclareInst, as the entry.

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This intrinsic provides information about a local element (e.g., variable). The first argument is metadata holding the address of variable, typically a static alloca in the function entry block. The second argument is a local variable containing a description of the variable. The third argument is a complex expression. An llvm.dbg.addr intrinsic describes the address of a source variable.

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This intrinsic is identical to llvm.dbg.addr, except that there can only be one call to llvm.dbg.declare for a given concrete local variable.

dbg_declare_inst->getAddress() gives the address (a llvm::Value*) of the variable. dbg_declare_inst->getVariable() returns a llvm::DILocalVariable*. The name of the variable can be obtained through debug_info_local_variable->getName(). The type of the variable can be found in debug_info_local_variable->getType(), which returns a llvm::DIType*.

Type System in Debug Info

Besides llvm::Type, LLVM provides a similar type system with more details in debug info. llvm::StructType does not contain information about field names. However, debug info contains information about variable name, typedef, field names and more.

Inheritance diagram for llvm::DIType

llvm::DIType is arranged like a directed graph. You can use getBaseType() or getElements() to dive into the graph. getTag() method can be used to quickly determine the type of the llvm::DIType. For example, getTag() may return llvm::dwarf::DW_TAG_pointer_type, llvm::dwarf::DW_TAG_structure_type, etc. Most of the tags can be found in
file llvm/BinaryFormat/Dwarf.def.

null

Nothing to say. Be careful with nullptr!

pointer type

pointer types are llvm::DIDerivedType. getBaseType() returns its pointee type. For example, if derived_type represents int*, then derived_type->getBaseType() should be int. Under most circumstances, it will return a llvm::DIType*.

Warning

getBaseType() can return nullptr!

member type

member types are llvm::DIDerivedType with tag llvm::dwarf::DW_TAG_member. Usually they are elements of a struct or a class. The field name can be obtained through getName() method.

structure type

structure types are llvm::DICompositeType with tag llvm::dwarf::DW_TAG_structure_type. getName() returns its name.

Note

The return value of getName() may defer from that of llvm::StructType::getStructName(). If you declare multiple structures with the same name but in different scopes (for example, two different structures named A in function f and g), llvm::StructType::getStructName() would produce something like %struct.A and %struct.A.0 respectively, while llvm::DICompositeType::getName() will always give a A.

getElements() returns all its fields. Under most circumstances, a field should be a llvm::DIDerivedType with tag llvm::dwarf::DW_TAG_member.

Warning

The tag of an element can be something else from llvm::dwarf::DW_TAG_member! Especially in c++.

typedef

llvm::DIDerivedType with tag llvm::dwarf::DW_TAG_typedef. getBaseType() returns the type of the alias.

How to Obtain Field Name in struct

Note

struct Elements in StructType may defer from those in DICompositeType because of alignment. Compiler may add additional elements to StructType for alignment, but not for DICompositeType. Therefore, the better way to find corresponding elements in StructType and DICompositeType is by offset.

The key point is to maintain mapping between llvm::Type and llvm::DIType, llvm::Value and llvm::DINode.

See lib-monitor/inserter/field_name_extractor at main · liblaf/lib-monitor for implementation details.

Strange Things about getelementptr

Invalid element idx!

const struct __attribute__((packed)) { uint32_t v; } *q = p;
uint32_t v;
__asm__ ("ldr  %0, %1" : "=r"(v) : "m"(*q));

%struct.anon.0 = type { i32 }
...
%89 = load %struct.anon.0*, %struct.anon.0** %13, align 4, !dbg !3598
%90 = getelementptr inbounds %struct.anon.0, %struct.anon.0* %89, i32 1, !dbg !3598
%91 = call i64 asm "ldr   ${0:Q}, $1  \0A\09ldr   ${0:R}, $2  \0A\09", "=&r,*m,*m"(%struct.anon.0* %88, %struct.anon.0* %90) #7, !dbg !3599, !srcloc !3600

Warning

Pay attention to the GetElementPtrInst!

Reference

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1. https://llvm.org/docs/SourceLevelDebugging.html ↩︎ ↩︎ ↩︎