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x86lint

x86lint examines x86 machine code to find suboptimal encodings and sequences. For example, add eax, 1 can encode with either an 8- or 32-bit immediate:

83C0 01
81C0 01000000

Using the former can result in smaller and faster code. x86lint can help compiler writers generate better code and documents the complexity of x86.

Implemented analyses

  • implicit EAX
    • 81C0 00010000 instead of 05 00010000 (ADD EAX, 0x100)
  • missing LOCK prefix on CMPXCHG and XADD
  • oversized immediates
    • 81C0 01000000 instead of 83C0 01 (ADD EAX, 1)
  • strength-reduce AND with immediate to MOVZBL
  • suboptimal CMP 0 83FF 00 instead of TEST 85C0
  • suboptimal no-ops
    • multiple 90 instead of a single 60 90, etc.
  • suboptimal zero register, see #7
    • MOV EAX, 0 instead of XOR EAX, EAX
  • unnecessary REX prefix
    • XOR RAX, RAX 4831C0 instead of XOR EAX, EAX 31C0
    • 40C9 instead of C9 (LEAVE)
  • unnecessary immediate
    • C1D0 01 instead of D1D0 (RCL EAX, 1)
  • unneeded LOCK prefix on XCHG

Compilation

First install the Intel x86 encoder decoder:

git clone https://github.com/intelxed/xed.git xed
git clone https://github.com/intelxed/mbuild.git mbuild
cd xed
./mfile.py install --install-dir=kits/xed-install

Next build x86lint:

git clone https://github.com/gaul/x86lint.git x86lint
cd x86lint
XED_PATH=/path/to/xed make all

Usage

x86lint is intended to be part of compiler test suites which should #include "x86lint.h" and link libx86lint.a. It can also read arbitrary ELF executables via:

./x86lint /bin/ls

References

License

Copyright (C) 2018 Andrew Gaul

Licensed under the Apache License, Version 2.0

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Examine x86 machine code to find suboptimal encodings and sequences

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