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There are currently two schemes for mapping instruction operands to instruction-format variables for generating the instruction encoders and decoders for the assembler and disassembler respectively: a) to map by name and b) to map by position. In the long run, we'd like to remove the position-based scheme and use only name-based mapping. Unfortunately, the name-based scheme currently cannot deal with complex operands (those with suboperands), and so we currently must use the position-based scheme for those. On the other hand, the position-based scheme cannot deal with (register) variables that are split into multiple ranges. An upcoming commit to the PowerPC backend (adding VSX support) will require this capability. While we could teach the position-based scheme to handle that, since we'd like to move away from the position-based mapping generally, it seems silly to teach it new tricks now. What makes more sense is to allow for partial transitioning: use the name-based mapping when possible, and only use the position-based scheme when necessary. Now the problem is that mixing the two sensibly was not possible: the position-based mapping would map based on position, but would not skip those variables that were mapped by name. Instead, the two sets of assignments would overlap. However, I cannot currently change the current behavior, because there are some backends that rely on it [I think mistakenly, but I'll send a message to llvmdev about that]. So I've added a new TableGen bit variable: noNamedPositionallyEncodedOperands, that can be used to cause the position-based mapping to skip variables mapped by name. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203767 91177308-0d34-0410-b5e6-96231b3b80d8 |
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| README.txt | ||
Low Level Virtual Machine (LLVM) ================================ This directory and its subdirectories contain source code for the Low Level Virtual Machine, a toolkit for the construction of highly optimized compilers, optimizers, and runtime environments. LLVM is open source software. You may freely distribute it under the terms of the license agreement found in LICENSE.txt. Please see the documentation provided in docs/ for further assistance with LLVM, and in particular docs/GettingStarted.rst for getting started with LLVM and docs/README.txt for an overview of LLVM's documentation setup. If you're writing a package for LLVM, see docs/Packaging.rst for our suggestions.