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path: root/llvm/unittests/Transforms/IPO/LowerBitSets.cpp
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* Add explicit type to empty std::set initializer to fix the libc++ build.Peter Collingbourne2015-03-031-1/+1
| | | | llvm-svn: 231047
* LowerBitSets: Use byte arrays instead of bit sets to represent in-memory bit ↵Peter Collingbourne2015-03-031-15/+75
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | sets. By loading from indexed offsets into a byte array and applying a mask, a program can test bits from the bit set with a relatively short instruction sequence. For example, suppose we have 15 bit sets to lay out: A (16 bits), B (15 bits), C (14 bits), D (13 bits), E (12 bits), F (11 bits), G (10 bits), H (9 bits), I (7 bits), J (6 bits), K (5 bits), L (4 bits), M (3 bits), N (2 bits), O (1 bit) These bits can be laid out in a 16-byte array like this: Byte Offset 0123456789ABCDEF Bit 7 HHHHHHHHHIIIIIII 6 GGGGGGGGGGJJJJJJ 5 FFFFFFFFFFFKKKKK 4 EEEEEEEEEEEELLLL 3 DDDDDDDDDDDDDMMM 2 CCCCCCCCCCCCCCNN 1 BBBBBBBBBBBBBBBO 0 AAAAAAAAAAAAAAAA For example, to test bit X of A, we evaluate ((bits[X] & 1) != 0), or to test bit X of I, we evaluate ((bits[9 + X] & 0x80) != 0). This can be done in 1-2 machine instructions on x86, or 4-6 instructions on ARM. This uses the LPT multiprocessor scheduling algorithm to lay out the bits efficiently. Saves ~450KB of instructions in a recent build of Chromium. Differential Revision: http://reviews.llvm.org/D7954 llvm-svn: 231043
* LowerBitSets: Align referenced globals.Peter Collingbourne2015-02-251-12/+16
| | | | | | | | | | | | | | | | This change aligns globals to the next highest power of 2 bytes, up to a maximum of 128. This makes it more likely that we will be able to compress bit sets with a greater alignment. In many more cases, we can now take advantage of a new optimization also introduced in this patch that removes bit set checks if the bit set is all ones. The 128 byte maximum was found to provide the best tradeoff between instruction overhead and data overhead in a recent build of Chromium. It allows us to remove ~2.4MB of instructions at the cost of ~250KB of data. Differential Revision: http://reviews.llvm.org/D7873 llvm-svn: 230540
* LowerBitSets: Introduce global layout builder.Peter Collingbourne2015-02-241-0/+27
| | | | | | | | | | The builder is based on a layout algorithm that tries to keep members of small bit sets together. The new layout compresses Chromium's bit sets to around 15% of their original size. Differential Revision: http://reviews.llvm.org/D7796 llvm-svn: 230394
* Introduce bitset metadata format and bitset lowering pass.Peter Collingbourne2015-02-201-0/+64
This patch introduces a new mechanism that allows IR modules to co-operatively build pointer sets corresponding to addresses within a given set of globals. One particular use case for this is to allow a C++ program to efficiently verify (at each call site) that a vtable pointer is in the set of valid vtable pointers for the class or its derived classes. One way of doing this is for a toolchain component to build, for each class, a bit set that maps to the memory region allocated for the vtables, such that each 1 bit in the bit set maps to a valid vtable for that class, and lay out the vtables next to each other, to minimize the total size of the bit sets. The patch introduces a metadata format for representing pointer sets, an '@llvm.bitset.test' intrinsic and an LTO lowering pass that lays out the globals and builds the bitsets, and documents the new feature. Differential Revision: http://reviews.llvm.org/D7288 llvm-svn: 230054
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