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calls can be supported.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@57428 91177308-0d34-0410-b5e6-96231b3b80d8
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//
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// "Fast" instruction selection is designed to emit very poor code quickly.
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// Also, it is not designed to be able to do much lowering, so most illegal
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// types (e.g. i64 on 32-bit targets) and operations (e.g. calls) are not
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// supported. It is also not intended to be able to do much optimization,
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// except in a few cases where doing optimizations reduces overall compile
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// time (e.g. folding constants into immediate fields, because it's cheap
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// and it reduces the number of instructions later phases have to examine).
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// types (e.g. i64 on 32-bit targets) and operations are not supported. It is
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// also not intended to be able to do much optimization, except in a few cases
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// where doing optimizations reduces overall compile time. For example, folding
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// constants into immediate fields is often done, because it's cheap and it
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// reduces the number of instructions later phases have to examine.
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//
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// "Fast" instruction selection is able to fail gracefully and transfer
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// control to the SelectionDAG selector for operations that it doesn't
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// support. In many cases, this allows us to avoid duplicating a lot of
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// support. In many cases, this allows us to avoid duplicating a lot of
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// the complicated lowering logic that SelectionDAG currently has.
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//
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// The intended use for "fast" instruction selection is "-O0" mode
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// compilation, where the quality of the generated code is irrelevant when
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// weighed against the speed at which the code can be generated. Also,
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// weighed against the speed at which the code can be generated. Also,
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// at -O0, the LLVM optimizers are not running, and this makes the
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// compile time of codegen a much higher portion of the overall compile
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// time. Despite its limitations, "fast" instruction selection is able to
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// time. Despite its limitations, "fast" instruction selection is able to
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// handle enough code on its own to provide noticeable overall speedups
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// in -O0 compiles.
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//
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// Basic operations are supported in a target-independent way, by reading
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// the same instruction descriptions that the SelectionDAG selector reads,
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// and identifying simple arithmetic operations that can be directly selected
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// from simple operators. More complicated operations currently require
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// from simple operators. More complicated operations currently require
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// target-specific code.
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//
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//===----------------------------------------------------------------------===//
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