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Diffstat (limited to 'lib/CodeGen/SelectionDAG/FastISel.cpp')
| -rw-r--r-- | lib/CodeGen/SelectionDAG/FastISel.cpp | 28 |
1 files changed, 28 insertions, 0 deletions
diff --git a/lib/CodeGen/SelectionDAG/FastISel.cpp b/lib/CodeGen/SelectionDAG/FastISel.cpp index dcb30ad..9f70bc9 100644 --- a/lib/CodeGen/SelectionDAG/FastISel.cpp +++ b/lib/CodeGen/SelectionDAG/FastISel.cpp @@ -9,6 +9,34 @@ // // This file contains the implementation of the FastISel class. // +// "Fast" instruction selection is designed to emit very poor code quickly. +// Also, it is not designed to be able to do much lowering, so most illegal +// types (e.g. i64 on 32-bit targets) and operations (e.g. calls) are not +// supported. It is also not intended to be able to do much optimization, +// except in a few cases where doing optimizations reduces overall compile +// time (e.g. folding constants into immediate fields, because it's cheap +// and it reduces the number of instructions later phases have to examine). +// +// "Fast" instruction selection is able to fail gracefully and transfer +// control to the SelectionDAG selector for operations that it doesn't +// support. In many cases, this allows us to avoid duplicating a lot of +// the complicated lowering logic that SelectionDAG currently has. +// +// The intended use for "fast" instruction selection is "-O0" mode +// compilation, where the quality of the generated code is irrelevant when +// weighed against the speed at which the code can be generated. Also, +// at -O0, the LLVM optimizers are not running, and this makes the +// compile time of codegen a much higher portion of the overall compile +// time. Despite its limitations, "fast" instruction selection is able to +// handle enough code on its own to provide noticeable overall speedups +// in -O0 compiles. +// +// Basic operations are supported in a target-independent way, by reading +// the same instruction descriptions that the SelectionDAG selector reads, +// and identifying simple arithmetic operations that can be directly selected +// from simple operators. More complicated operations currently require +// target-specific code. +// //===----------------------------------------------------------------------===// #include "llvm/Function.h" |