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Rui Ueyama 406b469de4 Avoid doing binary search. 
MergedInputSection::getOffset is the busiest function in LLD if string
merging is enabled and input files have lots of mergeable sections.
It is usually the case when creating executable with debug info,
so it is pretty common.

The reason why it is slow is because it has to do faily complex
computations. For non-mergeable sections, section contents are
contiguous in output, so in order to compute an output offset,
we only have to add the output section's base address to an input
offset. But for mergeable strings, section contents are split for
merging, so they are not contigous. We've got to do some lookups.

We used to do binary search on the list of section pieces.
It is slow because I think it's hostile to branch prediction.

This patch replaces it with hash table lookup. Seems it's working
pretty well. Below is "perf stat -r10" output when linking clang
with debug info. In this case this patch speeds up about 4%.

Before:

       6584.153205 task-clock (msec)         #    1.001 CPUs utilized            ( +-  0.09% )
               238 context-switches          #    0.036 K/sec                    ( +-  6.59% )
                 0 cpu-migrations            #    0.000 K/sec                    ( +- 50.92% )
         1,067,675 page-faults               #    0.162 M/sec                    ( +-  0.15% )
    18,369,931,470 cycles                    #    2.790 GHz                      ( +-  0.09% )
     9,640,680,143 stalled-cycles-frontend   #   52.48% frontend cycles idle     ( +-  0.18% )
   <not supported> stalled-cycles-backend
    21,206,747,787 instructions              #    1.15  insns per cycle
                                             #    0.45  stalled cycles per insn  ( +-  0.04% )
     3,817,398,032 branches                  #  579.786 M/sec                    ( +-  0.04% )
       132,787,249 branch-misses             #    3.48% of all branches          ( +-  0.02% )

       6.579106511 seconds time elapsed                                          ( +-  0.09% )

After:

       6312.317533 task-clock (msec)         #    1.001 CPUs utilized            ( +-  0.19% )
               221 context-switches          #    0.035 K/sec                    ( +-  4.11% )
                 1 cpu-migrations            #    0.000 K/sec                    ( +- 45.21% )
         1,280,775 page-faults               #    0.203 M/sec                    ( +-  0.37% )
    17,611,539,150 cycles                    #    2.790 GHz                      ( +-  0.19% )
    10,285,148,569 stalled-cycles-frontend   #   58.40% frontend cycles idle     ( +-  0.30% )
   <not supported> stalled-cycles-backend
    18,794,779,900 instructions              #    1.07  insns per cycle
                                             #    0.55  stalled cycles per insn  ( +-  0.03% )
     3,287,450,865 branches                  #  520.799 M/sec                    ( +-  0.03% )
        72,259,605 branch-misses             #    2.20% of all branches          ( +-  0.01% )

       6.307411828 seconds time elapsed                                          ( +-  0.19% )

Differential Revision: http://reviews.llvm.org/D20645

llvm-svn: 270999
 		2016-05-27 14:39:13 +00:00
..
COFF Update for llvm change. 2016-05-26 20:31:06 +00:00
ELF Avoid doing binary search. 2016-05-27 14:39:13 +00:00
cmake/modules
…
docs ELF: New symbol table design. 2016-05-01 04:55:03 +00:00
include/lld Fix some Include What You Use warnings; other minor fixes. 2016-04-29 19:47:09 +00:00
lib Fix some Include What You Use warnings; other minor fixes. 2016-04-29 19:47:09 +00:00
test Avoid having to check in a binary. 2016-05-27 12:27:21 +00:00
tools/lld Remove unused include. NFC. 2016-05-15 19:36:23 +00:00
unittests Fix Clang-tidy misc-unused-using-decls and Include What You Use warnings. 2016-04-21 17:14:10 +00:00
.arcconfig
…
.clang-format
…
.gitignore
…
CMakeLists.txt Inline a trivial CMakeLists.txt. 2016-03-03 01:56:23 +00:00
CODE_OWNERS.TXT
…
LICENSE.TXT Update copyright year to 2016. 2016-03-30 22:40:59 +00:00
README.md
…

README.md

LLVM Linker (lld)

This directory and its subdirectories contain source code for the LLVM Linker, a modular cross platform linker which is built as part of the LLVM compiler infrastructure project.

lld is open source software. You may freely distribute it under the terms of the license agreement found in LICENSE.txt.