---

yaml
---
r: 216357
b: refs/heads/stable
c: 5993ae8
h: refs/heads/master
i:
  216355: 2043aed
v: v3

Author: nrc

[refs]

@@ -29,5 +29,5 @@ refs/heads/tmp: 378a370ff2057afeb1eae86eb6e78c476866a4a6
 refs/tags/1.0.0-alpha.2: 4c705f6bc559886632d3871b04f58aab093bfa2f
 refs/tags/homu-tmp: a5286998df566e736b32f6795bfc3803bdaf453d
 refs/tags/1.0.0-beta: 8cbb92b53468ee2b0c2d3eeb8567005953d40828
-refs/heads/stable: 39e2e649cb0ef3da750d296af07d4cea6aadf51f
+refs/heads/stable: 5993ae86b85173d18fbc0cd620f61c011a8a7b03
 refs/tags/1.0.0: 55bd4f8ff2b323f317ae89e254ce87162d52a375

branches/stable/src/librustc_trans/trans/debuginfo/doc.rs

@@ -0,0 +1,189 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://!rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://!www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://!opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+//! # Debug Info Module
+//!
+//! This module serves the purpose of generating debug symbols. We use LLVM's
+//! [source level debugging](http://!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](http://!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](http://!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(bcx: 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(crate_context, path);
+//!
+//! 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 CrateContext) or the
+//! FunctionDebugContext (owned by the FunctionContext).
+//!
+//! 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: int,
+//!     tail: Option<Box<List>>,
+//! }
+//! ```
+//!
+//! will generate the following callstack with a naive DFS algorithm:
+//!
+//! ```
+//! describe(t = List)
+//!   describe(t = int)
+//!   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 behaviour 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 translated 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 translate 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
+//!          wereoriginally defined in
+//!
+//!     (2) The second part is the ast::NodeId of the definition in their
+//!          originalcrate
+//!
+//!     (3) The final part is a concatenation of the type IDs of their concrete
+//!          typearguments 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. (int, int) 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 node IDs 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).