From 5ec9efd61bc4214c787287409498e8b78f28c922 Mon Sep 17 00:00:00 2001 From: Dan Gohman Date: Tue, 30 Sep 2008 20:48:29 +0000 Subject: [PATCH] Move the primary fast-isel top-level comments to FastISel.cpp, where they'll be a little more visible. Also, update and reword them a bit. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@56877 91177308-0d34-0410-b5e6-96231b3b80d8 --- lib/CodeGen/SelectionDAG/FastISel.cpp | 28 +++++++++++++++++++++++++ utils/TableGen/FastISelEmitter.cpp | 30 ++++++--------------------- 2 files changed, 34 insertions(+), 24 deletions(-) diff --git a/lib/CodeGen/SelectionDAG/FastISel.cpp b/lib/CodeGen/SelectionDAG/FastISel.cpp index dcb30ad1882..9f70bc998c9 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" diff --git a/utils/TableGen/FastISelEmitter.cpp b/utils/TableGen/FastISelEmitter.cpp index 0aa9dfc49d4..5e3876369ee 100644 --- a/utils/TableGen/FastISelEmitter.cpp +++ b/utils/TableGen/FastISelEmitter.cpp @@ -7,32 +7,14 @@ // //===----------------------------------------------------------------------===// // -// This tablegen backend emits a "fast" instruction selector. +// This tablegen backend emits code for use by the "fast" instruction +// selection algorithm. See the comments at the top of +// lib/CodeGen/SelectionDAG/FastISel.cpp for background. // -// This instruction selection method is designed to emit very poor code -// quickly. Also, it is not designed to do much lowering, so most illegal -// types (e.g. i64 on 32-bit targets) and operations (e.g. calls) are not -// supported and cannot easily be added. Blocks containing operations -// that are not supported need to be handled by a more capable selector, -// such as the SelectionDAG selector. +// This file scans through the target's tablegen instruction-info files +// and extracts instructions with obvious-looking patterns, and it emits +// code to look up these instructions by type and operator. // -// 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. -// -// If compile time is so important, you might wonder why we don't just -// skip codegen all-together, emit LLVM bytecode files, and execute them -// with an interpreter. The answer is that it would complicate linking and -// debugging, and also because that isn't how a compiler is expected to -// work in some circles. -// -// If you need better generated code or more lowering than what this -// instruction selector provides, use the SelectionDAG (DAGISel) instruction -// selector instead. If you're looking here because SelectionDAG isn't fast -// enough, consider looking into improving the SelectionDAG infastructure -// instead. At the time of this writing there remain several major -// opportunities for improvement. -// //===----------------------------------------------------------------------===// #include "FastISelEmitter.h" -- 2.34.1