gaul / x86lint

Examine x86 machine code to find suboptimal encodings and sequences

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

• Agner Fog optimization guide
• Intel instruction set reference
• Intel optimization manual
• Intel x86 encoder decoder - library to parse instructions

License

Copyright (C) 2018 Andrew Gaul

Licensed under the Apache License, Version 2.0