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author | Dan Gohman <gohman@apple.com> | 2008-09-30 20:48:29 +0000 |
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committer | Dan Gohman <gohman@apple.com> | 2008-09-30 20:48:29 +0000 |
commit | 5ec9efd61bc4214c787287409498e8b78f28c922 (patch) | |
tree | a4d734b78f499759b6a02ba25019e8faa5e07f8a /utils/TableGen/FastISelEmitter.cpp | |
parent | dd7c3e4e56f739f6b67a4f3d67d061f2c5bca029 (diff) | |
download | llvm-5ec9efd61bc4214c787287409498e8b78f28c922.tar.gz llvm-5ec9efd61bc4214c787287409498e8b78f28c922.tar.bz2 llvm-5ec9efd61bc4214c787287409498e8b78f28c922.tar.xz |
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
Diffstat (limited to 'utils/TableGen/FastISelEmitter.cpp')
-rw-r--r-- | utils/TableGen/FastISelEmitter.cpp | 30 |
1 files changed, 6 insertions, 24 deletions
diff --git a/utils/TableGen/FastISelEmitter.cpp b/utils/TableGen/FastISelEmitter.cpp index 0aa9dfc49d..5e3876369e 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" |