x86lint

x86lint examines x86-64 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

• missing LOCK prefix on CMPXCHG and XADD
• oversized ADD 128
  • 05 80000000 instead of 83E8 80 (ADD EAX, 128 -> SUB EAX, -128)
• oversized branch displacement
  • E9 00000000 instead of EB 03 (JMP rel32 that fits in rel8)
• oversized immediates
  • 81C0 01000000 instead of 83C0 01 (ADD EAX, 1)
• oversized MOV encoding
  • C7C0 01000000 instead of B8 01000000 (MOV EAX, 1)
• redundant ADD/SUB zero
  • 83C0 00 (ADD EAX, 0) -- use TEST or remove
• redundant MOV reg, reg
  • 4889C0 (MOV RAX, RAX)
• suboptimal AND immediate
  • 83E0 FF (AND EAX, 0xFF) -- use MOVZBL
• suboptimal CMP zero
  • 83F8 00 instead of 85C0 (CMP EAX, 0 -> TEST EAX, EAX)
• suboptimal IMUL constant
  • 6BC0 03 (IMUL EAX, EAX, 3) -- use LEA or SHL
• suboptimal NOP sequence, see #9
  • multiple 90 instead of a single 66 90, etc.
• suboptimal SUB reg, reg
  • 29C0 (SUB EAX, EAX) -- use XOR for dependency-breaking
• suboptimal zero register, see #7
  • MOV EAX, 0 instead of XOR EAX, EAX
• unneeded explicit immediate
  • C1D0 01 instead of D1D0 (RCL EAX, 1)
• unneeded explicit register
  • 81C0 00010000 instead of 05 00010000 (ADD EAX, 0x100)
• unneeded LOCK prefix on XCHG
• unneeded MOVSXD
  • 48 63 C0 instead of 48 98 (MOVSXD RAX, EAX -> CDQE)
• unneeded REX prefix
  • XOR RAX, RAX 4831C0 instead of XOR EAX, EAX 31C0
  • 40C9 instead of C9 (LEAVE)
• unneeded SIB byte
  • C64465 04 05 instead of C645 04 05 (MOV byte [RBP+4], 5)
• unneeded zero displacement
  • 017E 00 instead of 013E (ADD [RSI], EDI)

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