[llvm-dev] llvm-objcopy --keep-symbols performance

[Brian Cain via llvm-dev]

While investigating an issue with android build performance, I found that llvm-objcopy performance is far from parity with GNU objcopy for the "--keep-symbols" case, when the keep symbols list is very large.

I can see that llvm objcopy creates a list of NameOrPattern-s to represent the keep symbol list, but it looks like it exhaustively compares each symbol in the keep list against each symbol in the input file.  Not sure what GNU objcopy does but merging the non-pattern symbol names into a sorted list would make searching much faster.

Below is a script to reproduce the problem and its output.  The test case below demonstrates a significant performance difference.  The android build failure case reported had a shared obj with ~700k symbols and the keep list was ~300k symbols and it took ~8 minutes to execute.

~~~
#!/bin/bash

set -euo pipefail

echo creating init file
echo <<EOF > input.S
.section .text
EOF
echo '' > keep_syms.txt
for i in $(seq 0 100000)
do
    echo -e "sym_${i}:\nnop\n" >> input.S
    if [[ ${i} -lt 30000 ]]; then
        echo "sym_${i}" >> keep_syms.txt
    fi
done
echo <<EOF >> input.S
.section .ballast
nop
EOF
echo creating obj file
llvm-mc -triple arm-linux-androideabi -filetype=obj input.S -o out.o
echo creating shared obj file
ld.lld  -shared out.o -o libtestcase.so

echo performing llvm objcopy
\time llvm-objcopy -S --remove-section .ballast  \
    --keep-symbols=keep_syms.txt \
    libtestcase.so libtestcase_smaller.so

echo performing GNU objcopy
\time arm-linux-androideabi-objcopy -S --remove-section .ballast  \
    --keep-symbols=keep_syms.txt \
    libtestcase.so libtestcase_smaller.so
~~~

Here's the output I get when I run it on ToT-within-last-week-or-so:

$ PATH=$PWD/bin:$PATH ../../tmp/qt66370/32/objcopy_perf.sh
creating init file
creating obj file
creating shared obj file
ld.lld: warning: lld uses blx instruction, no object with architecture supporting feature detected
performing llvm objcopy
12.34user 0.00system 0:12.35elapsed 99%CPU (0avgtext+0avgdata 20228maxresident)k
0inputs+2288outputs (0major+3815minor)pagefaults 0swaps
performing GNU objcopy
0.03user 0.00system 0:00.04elapsed 97%CPU (0avgtext+0avgdata 20396maxresident)k
0inputs+2288outputs (0major+5895minor)pagefaults 0swaps


Alternate access to the script above:
https://gist.github.com/androm3da/83560d92f3fe637b58aa115ba6b68456

-Brian
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