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Add some more documentation. 

llvm-svn: 117070
This commit is contained in:
Dan Gohman 2010-10-21 21:55:35 +00:00
parent eff79f13a6
commit 8512270dbc
1 changed files with 21 additions and 15 deletions
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36
llvm/lib/Analysis/TypeBasedAliasAnalysis.cpp
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@ -12,27 +12,33 @@
//
// In LLVM IR, memory does not have types, so LLVM's own type system is not
// suitable for doing TBAA. Instead, metadata is added to the IR to describe
// a type system of a higher level language.
// a type system of a higher level language. This can be used to implement
// typical C/C++ TBAA, but it can also be used to implement custom alias
// analysis behavior for other languages.
//
// This pass is language-independent. The type system is encoded in
// metadata. This allows this pass to support typical C and C++ TBAA, but
// it can also support custom aliasing behavior for other languages.
// The current metadata format is very simple. TBAA MDNodes have up to
// three fields, e.g.:
// !0 = metadata !{ metadata !"an example type tree" }
// !1 = metadata !{ metadata !"int", metadata !0 }
// !2 = metadata !{ metadata !"float", metadata !0 }
// !3 = metadata !{ metadata !"const float", metadata !2, i64 1 }
//
// This is a work-in-progress. It doesn't work yet, and the metadata
// format isn't stable.
// The first field is an identity field. It can be any value, usually
// an MDString, which uniquely identifies the type. The most important
// name in the tree is the name of the root node. Two trees with
// different root node names are entirely disjoint, even if they
// have leaves with common names.
//
// The second field identifies the type's parent node in the tree, or
// is null or omitted for a root node. A type is considered to alias
// all of its decendents and all of its ancestors in the tree.
//
// The current metadata format is very simple. MDNodes have up to three
// fields, e.g.:
// !0 = metadata !{ !"name", !1, 0 }
// The first field is an identity field. It can be any MDString which
// uniquely identifies the type. The second field identifies the type's
// parent node in the tree, or is null or omitted for a root node.
// If the third field is present, it's an integer which if equal to 1
// indicates that the type is "constant" (meaning
// pointsToConstantMemory should return true; see
// indicates that the type is "constant" (meaning pointsToConstantMemory
// should return true; see
// http://llvm.org/docs/AliasAnalysis.html#OtherItfs).
//
// TODO: The current metadata encoding scheme doesn't support struct
// TODO: The current metadata format doesn't support struct
// fields. For example:
// struct X {
// double d;