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+# Debug Info Module
+
+This module serves the purpose of generating debug symbols. We use LLVM's
+[source level debugging](https://llvm.org/docs/SourceLevelDebugging.html)
+features for generating the debug information. The general principle is
+this:
+
+Given the right metadata in the LLVM IR, the LLVM code generator is able to
+create DWARF debug symbols for the given code. The
+[metadata](https://llvm.org/docs/LangRef.html#metadata-type) is structured
+much like DWARF *debugging information entries* (DIE), representing type
+information such as datatype layout, function signatures, block layout,
+variable location and scope information, etc. It is the purpose of this
+module to generate correct metadata and insert it into the LLVM IR.
+
+As the exact format of metadata trees may change between different LLVM
+versions, we now use LLVM
+[DIBuilder](https://llvm.org/docs/doxygen/html/classllvm_1_1DIBuilder.html)
+to create metadata where possible. This will hopefully ease the adaption of
+this module to future LLVM versions.
+
+The public API of the module is a set of functions that will insert the
+correct metadata into the LLVM IR when called with the right parameters.
+The module is thus driven from an outside client with functions like
+`debuginfo::create_local_var_metadata(bx: block, local: &ast::local)`.
+
+Internally the module will try to reuse already created metadata by
+utilizing a cache. The way to get a shared metadata node when needed is
+thus to just call the corresponding function in this module:
+
+    let file_metadata = file_metadata(cx, file);
+
+The function will take care of probing the cache for an existing node for
+that exact file path.
+
+All private state used by the module is stored within either the
+CrateDebugContext struct (owned by the CodegenCx) or the
+FunctionDebugContext (owned by the FunctionCx).
+
+This file consists of three conceptual sections:
+1. The public interface of the module
+2. Module-internal metadata creation functions
+3. Minor utility functions
+
+
+## Recursive Types
+
+Some kinds of types, such as structs and enums can be recursive. That means
+that the type definition of some type X refers to some other type which in
+turn (transitively) refers to X. This introduces cycles into the type
+referral graph. A naive algorithm doing an on-demand, depth-first traversal
+of this graph when describing types, can get trapped in an endless loop
+when it reaches such a cycle.
+
+For example, the following simple type for a singly-linked list...
+
+```
+struct List {
+    value: i32,
+    tail: Option<Box<List>>,
+}
+```
+
+will generate the following callstack with a naive DFS algorithm:
+
+```
+describe(t = List)
+  describe(t = i32)
+  describe(t = Option<Box<List>>)
+    describe(t = Box<List>)
+      describe(t = List) // at the beginning again...
+      ...
+```
+
+To break cycles like these, we use "forward declarations". That is, when
+the algorithm encounters a possibly recursive type (any struct or enum), it
+immediately creates a type description node and inserts it into the cache
+*before* describing the members of the type. This type description is just
+a stub (as type members are not described and added to it yet) but it
+allows the algorithm to already refer to the type. After the stub is
+inserted into the cache, the algorithm continues as before. If it now
+encounters a recursive reference, it will hit the cache and does not try to
+describe the type anew.
+
+This behavior is encapsulated in the 'RecursiveTypeDescription' enum,
+which represents a kind of continuation, storing all state needed to
+continue traversal at the type members after the type has been registered
+with the cache. (This implementation approach might be a tad over-
+engineered and may change in the future)
+
+
+## Source Locations and Line Information
+
+In addition to data type descriptions the debugging information must also
+allow to map machine code locations back to source code locations in order
+to be useful. This functionality is also handled in this module. The
+following functions allow to control source mappings:
+
++ `set_source_location()`
++ `clear_source_location()`
++ `start_emitting_source_locations()`
+
+`set_source_location()` allows to set the current source location. All IR
+instructions created after a call to this function will be linked to the
+given source location, until another location is specified with
+`set_source_location()` or the source location is cleared with
+`clear_source_location()`. In the later case, subsequent IR instruction
+will not be linked to any source location. As you can see, this is a
+stateful API (mimicking the one in LLVM), so be careful with source
+locations set by previous calls. It's probably best to not rely on any
+specific state being present at a given point in code.
+
+One topic that deserves some extra attention is *function prologues*. At
+the beginning of a function's machine code there are typically a few
+instructions for loading argument values into allocas and checking if
+there's enough stack space for the function to execute. This *prologue* is
+not visible in the source code and LLVM puts a special PROLOGUE END marker
+into the line table at the first non-prologue instruction of the function.
+In order to find out where the prologue ends, LLVM looks for the first
+instruction in the function body that is linked to a source location. So,
+when generating prologue instructions we have to make sure that we don't
+emit source location information until the 'real' function body begins. For
+this reason, source location emission is disabled by default for any new
+function being codegened and is only activated after a call to the third
+function from the list above, `start_emitting_source_locations()`. This
+function should be called right before regularly starting to codegen the
+top-level block of the given function.
+
+There is one exception to the above rule: `llvm.dbg.declare` instruction
+must be linked to the source location of the variable being declared. For
+function parameters these `llvm.dbg.declare` instructions typically occur
+in the middle of the prologue, however, they are ignored by LLVM's prologue
+detection. The `create_argument_metadata()` and related functions take care
+of linking the `llvm.dbg.declare` instructions to the correct source
+locations even while source location emission is still disabled, so there
+is no need to do anything special with source location handling here.
+
+## Unique Type Identification
+
+In order for link-time optimization to work properly, LLVM needs a unique
+type identifier that tells it across compilation units which types are the
+same as others. This type identifier is created by
+`TypeMap::get_unique_type_id_of_type()` using the following algorithm:
+
+1. Primitive types have their name as ID
+
+2. Structs, enums and traits have a multipart identifier
+
+  1. The first part is the SVH (strict version hash) of the crate they
+     were originally defined in
+
+  2. The second part is the ast::NodeId of the definition in their
+     original crate
+
+  3. The final part is a concatenation of the type IDs of their concrete
+     type arguments if they are generic types.
+
+3. Tuple-, pointer-, and function types are structurally identified, which
+   means that they are equivalent if their component types are equivalent
+   (i.e., `(i32, i32)` is the same regardless in which crate it is used).
+
+This algorithm also provides a stable ID for types that are defined in one
+crate but instantiated from metadata within another crate. We just have to
+take care to always map crate and `NodeId`s back to the original crate
+context.
+
+As a side-effect these unique type IDs also help to solve a problem arising
+from lifetime parameters. Since lifetime parameters are completely omitted
+in debuginfo, more than one `Ty` instance may map to the same debuginfo
+type metadata, that is, some struct `Struct<'a>` may have N instantiations
+with different concrete substitutions for `'a`, and thus there will be N
+`Ty` instances for the type `Struct<'a>` even though it is not generic
+otherwise. Unfortunately this means that we cannot use `ty::type_id()` as
+cheap identifier for type metadata -- we have done this in the past, but it
+led to unnecessary metadata duplication in the best case and LLVM
+assertions in the worst. However, the unique type ID as described above
+*can* be used as identifier. Since it is comparatively expensive to
+construct, though, `ty::type_id()` is still used additionally as an
+optimization for cases where the exact same type has been seen before
+(which is most of the time).