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LLVM backend for 6502
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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
autoconf Revert r230812. Do not break builds for no reason. 2015-02-28 19:43:20 +00:00
bindings Restore LLVMLinkModules C API until it is properly deprecated. 2015-03-02 18:59:38 +00:00
cmake cmake: Use MSVC atomics with clang-cl 2015-03-02 22:00:23 +00:00
docs [PerformanceTips] Document various items folks have suggested 2015-03-02 19:19:04 +00:00
examples llvm/examples: Add missing include according to r230907. 2015-03-02 01:04:34 +00:00
include LowerBitSets: Use byte arrays instead of bit sets to represent in-memory bit sets. 2015-03-03 00:49:28 +00:00
lib LowerBitSets: Use byte arrays instead of bit sets to represent in-memory bit sets. 2015-03-03 00:49:28 +00:00
projects [cmake] Use the external project machinery for libcxxabi so that it can 2014-07-25 10:27:40 +00:00
test LowerBitSets: Use byte arrays instead of bit sets to represent in-memory bit sets. 2015-03-03 00:49:28 +00:00
tools Add r230655 back with a fix. 2015-03-02 19:08:03 +00:00
unittests LowerBitSets: Use byte arrays instead of bit sets to represent in-memory bit sets. 2015-03-03 00:49:28 +00:00
utils lit: Add 'cd' support to the internal shell and port some tests 2015-03-02 21:33:18 +00:00
.arcconfig Updated phabricator server. 2014-04-07 03:57:04 +00:00
.clang-format Test commit. 2014-03-02 13:08:46 +00:00
.clang-tidy Enable display of compiler diagnostics in clang-tidy by default. 2014-10-29 17:29:38 +00:00
.gitignore Add one more vim swap file pattern 2015-01-30 21:59:28 +00:00
CMakeLists.txt Minor follow-ups to r229720 suggested on llvmdev 2015-02-27 18:22:46 +00:00
CODE_OWNERS.TXT Assume code ownership for the PS4 to ensure patches get reviewed, per the Developer Policy. 2015-01-28 18:33:39 +00:00
configure Revert r230812. Do not break builds for no reason. 2015-02-28 19:43:20 +00:00
CREDITS.TXT Rise from the dead and update personal info 2014-08-25 17:51:04 +00:00
LICENSE.TXT Remove projects/sample. 2014-03-12 22:40:22 +00:00
llvm.spec.in
LLVMBuild.txt Remove the very substantial, largely unmaintained legacy PGO 2013-10-02 15:42:23 +00:00
Makefile [configure/make] Propagate names of build host tools when making BuildTools 2014-03-25 21:45:41 +00:00
Makefile.common
Makefile.config.in Add a check for misbehaving -Wcomment from gcc-4.7 and add 2014-11-05 00:35:15 +00:00
Makefile.rules Add a check for misbehaving -Wcomment from gcc-4.7 and add 2014-11-05 00:35:15 +00:00
README.txt Revert test commit 2015-02-04 18:46:00 +00:00

README.txt 

Low Level Virtual Machine (LLVM)
================================

This directory and its subdirectories contain source code for LLVM,
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Please see the documentation provided in docs/ for further
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If you're writing a package for LLVM, see docs/Packaging.rst for our
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