llvm-6502/include/llvm/Transforms
Peter Collingbourne 27821d7200 LowerBitSets: Use byte arrays instead of bit sets to represent in-memory bit 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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@231043 91177308-0d34-0410-b5e6-96231b3b80d8
2015-03-03 00:49:28 +00:00
..
InstCombine Removing LLVM_DELETED_FUNCTION, as MSVC 2012 was the last reason for requiring the macro. NFC; LLVM edition. 2015-02-15 22:54:22 +00:00
IPO LowerBitSets: Use byte arrays instead of bit sets to represent in-memory bit sets. 2015-03-03 00:49:28 +00:00
Scalar [PM] Clean up a stale comment that came from a differnt pass when 2015-02-01 11:35:56 +00:00
Utils Add missing includes. make_unique proliferated everywhere. 2015-03-01 21:28:53 +00:00
Instrumentation.h Add missing include. 2015-02-04 18:16:53 +00:00
IPO.h Introduce bitset metadata format and bitset lowering pass. 2015-02-20 20:30:47 +00:00
ObjCARC.h Reformat linefeeds. 2014-05-26 00:25:26 +00:00
Scalar.h Rewrite the global merge pass to be subprogram agnostic for now. 2015-02-23 19:28:45 +00:00
Vectorize.h Allow vectorization of bit intrinsics in BB Vectorizer. 2014-04-25 03:33:48 +00:00